xref: /titanic_52/usr/src/uts/common/fs/zfs/arc.c (revision 8d2b0ea9d2a9f0e441345ec63219b0f8b91021f8)

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*
 * DVA-based Adjustable Replacement Cache
 *
 * While much of the theory of operation used here is
 * based on the self-tuning, low overhead replacement cache
 * presented by Megiddo and Modha at FAST 2003, there are some
 * significant differences:
 *
 * 1. The Megiddo and Modha model assumes any page is evictable.
 * Pages in its cache cannot be "locked" into memory.  This makes
 * the eviction algorithm simple: evict the last page in the list.
 * This also make the performance characteristics easy to reason
 * about.  Our cache is not so simple.  At any given moment, some
 * subset of the blocks in the cache are un-evictable because we
 * have handed out a reference to them.  Blocks are only evictable
 * when there are no external references active.  This makes
 * eviction far more problematic:  we choose to evict the evictable
 * blocks that are the "lowest" in the list.
 *
 * There are times when it is not possible to evict the requested
 * space.  In these circumstances we are unable to adjust the cache
 * size.  To prevent the cache growing unbounded at these times we
 * implement a "cache throttle" that slows the flow of new data
 * into the cache until we can make space available.
 *
 * 2. The Megiddo and Modha model assumes a fixed cache size.
 * Pages are evicted when the cache is full and there is a cache
 * miss.  Our model has a variable sized cache.  It grows with
 * high use, but also tries to react to memory pressure from the
 * operating system: decreasing its size when system memory is
 * tight.
 *
 * 3. The Megiddo and Modha model assumes a fixed page size. All
 * elements of the cache are therefor exactly the same size.  So
 * when adjusting the cache size following a cache miss, its simply
 * a matter of choosing a single page to evict.  In our model, we
 * have variable sized cache blocks (rangeing from 512 bytes to
 * 128K bytes).  We therefor choose a set of blocks to evict to make
 * space for a cache miss that approximates as closely as possible
 * the space used by the new block.
 *
 * See also:  "ARC: A Self-Tuning, Low Overhead Replacement Cache"
 * by N. Megiddo & D. Modha, FAST 2003
 */

/*
 * The locking model:
 *
 * A new reference to a cache buffer can be obtained in two
 * ways: 1) via a hash table lookup using the DVA as a key,
 * or 2) via one of the ARC lists.  The arc_read() interface
 * uses method 1, while the internal arc algorithms for
 * adjusting the cache use method 2.  We therefor provide two
 * types of locks: 1) the hash table lock array, and 2) the
 * arc list locks.
 *
 * Buffers do not have their own mutexs, rather they rely on the
 * hash table mutexs for the bulk of their protection (i.e. most
 * fields in the arc_buf_hdr_t are protected by these mutexs).
 *
 * buf_hash_find() returns the appropriate mutex (held) when it
 * locates the requested buffer in the hash table.  It returns
 * NULL for the mutex if the buffer was not in the table.
 *
 * buf_hash_remove() expects the appropriate hash mutex to be
 * already held before it is invoked.
 *
 * Each arc state also has a mutex which is used to protect the
 * buffer list associated with the state.  When attempting to
 * obtain a hash table lock while holding an arc list lock you
 * must use: mutex_tryenter() to avoid deadlock.  Also note that
 * the active state mutex must be held before the ghost state mutex.
 *
 * Arc buffers may have an associated eviction callback function.
 * This function will be invoked prior to removing the buffer (e.g.
 * in arc_do_user_evicts()).  Note however that the data associated
 * with the buffer may be evicted prior to the callback.  The callback
 * must be made with *no locks held* (to prevent deadlock).  Additionally,
 * the users of callbacks must ensure that their private data is
 * protected from simultaneous callbacks from arc_buf_evict()
 * and arc_do_user_evicts().
 *
 * Note that the majority of the performance stats are manipulated
 * with atomic operations.
 *
 * The L2ARC uses the l2arc_buflist_mtx global mutex for the following:
 *
 *	- L2ARC buflist creation
 *	- L2ARC buflist eviction
 *	- L2ARC write completion, which walks L2ARC buflists
 *	- ARC header destruction, as it removes from L2ARC buflists
 *	- ARC header release, as it removes from L2ARC buflists
 */

#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/zio_checksum.h>
#include <sys/zfs_context.h>
#include <sys/arc.h>
#include <sys/refcount.h>
#include <sys/vdev.h>
#ifdef _KERNEL
#include <sys/vmsystm.h>
#include <vm/anon.h>
#include <sys/fs/swapnode.h>
#include <sys/dnlc.h>
#endif
#include <sys/callb.h>
#include <sys/kstat.h>

static kmutex_t		arc_reclaim_thr_lock;
static kcondvar_t	arc_reclaim_thr_cv;	/* used to signal reclaim thr */
static uint8_t		arc_thread_exit;

extern int zfs_write_limit_shift;
extern uint64_t zfs_write_limit_max;
extern kmutex_t zfs_write_limit_lock;

#define	ARC_REDUCE_DNLC_PERCENT	3
uint_t arc_reduce_dnlc_percent = ARC_REDUCE_DNLC_PERCENT;

typedef enum arc_reclaim_strategy {
	ARC_RECLAIM_AGGR,		/* Aggressive reclaim strategy */
	ARC_RECLAIM_CONS		/* Conservative reclaim strategy */
} arc_reclaim_strategy_t;

/* number of seconds before growing cache again */
static int		arc_grow_retry = 60;

/* shift of arc_c for calculating both min and max arc_p */
static int		arc_p_min_shift = 4;

/* log2(fraction of arc to reclaim) */
static int		arc_shrink_shift = 5;

/*
 * minimum lifespan of a prefetch block in clock ticks
 * (initialized in arc_init())
 */
static int		arc_min_prefetch_lifespan;

static int arc_dead;

/*
 * The arc has filled available memory and has now warmed up.
 */
static boolean_t arc_warm;

/*
 * These tunables are for performance analysis.
 */
uint64_t zfs_arc_max;
uint64_t zfs_arc_min;
uint64_t zfs_arc_meta_limit = 0;
int zfs_mdcomp_disable = 0;
int zfs_arc_grow_retry = 0;
int zfs_arc_shrink_shift = 0;
int zfs_arc_p_min_shift = 0;

/*
 * Note that buffers can be in one of 6 states:
 *	ARC_anon	- anonymous (discussed below)
 *	ARC_mru		- recently used, currently cached
 *	ARC_mru_ghost	- recentely used, no longer in cache
 *	ARC_mfu		- frequently used, currently cached
 *	ARC_mfu_ghost	- frequently used, no longer in cache
 *	ARC_l2c_only	- exists in L2ARC but not other states
 * When there are no active references to the buffer, they are
 * are linked onto a list in one of these arc states.  These are
 * the only buffers that can be evicted or deleted.  Within each
 * state there are multiple lists, one for meta-data and one for
 * non-meta-data.  Meta-data (indirect blocks, blocks of dnodes,
 * etc.) is tracked separately so that it can be managed more
 * explicitly: favored over data, limited explicitly.
 *
 * Anonymous buffers are buffers that are not associated with
 * a DVA.  These are buffers that hold dirty block copies
 * before they are written to stable storage.  By definition,
 * they are "ref'd" and are considered part of arc_mru
 * that cannot be freed.  Generally, they will aquire a DVA
 * as they are written and migrate onto the arc_mru list.
 *
 * The ARC_l2c_only state is for buffers that are in the second
 * level ARC but no longer in any of the ARC_m* lists.  The second
 * level ARC itself may also contain buffers that are in any of
 * the ARC_m* states - meaning that a buffer can exist in two
 * places.  The reason for the ARC_l2c_only state is to keep the
 * buffer header in the hash table, so that reads that hit the
 * second level ARC benefit from these fast lookups.
 */

typedef struct arc_state {
	list_t	arcs_list[ARC_BUFC_NUMTYPES];	/* list of evictable buffers */
	uint64_t arcs_lsize[ARC_BUFC_NUMTYPES];	/* amount of evictable data */
	uint64_t arcs_size;	/* total amount of data in this state */
	kmutex_t arcs_mtx;
} arc_state_t;

/* The 6 states: */
static arc_state_t ARC_anon;
static arc_state_t ARC_mru;
static arc_state_t ARC_mru_ghost;
static arc_state_t ARC_mfu;
static arc_state_t ARC_mfu_ghost;
static arc_state_t ARC_l2c_only;

typedef struct arc_stats {
	kstat_named_t arcstat_hits;
	kstat_named_t arcstat_misses;
	kstat_named_t arcstat_demand_data_hits;
	kstat_named_t arcstat_demand_data_misses;
	kstat_named_t arcstat_demand_metadata_hits;
	kstat_named_t arcstat_demand_metadata_misses;
	kstat_named_t arcstat_prefetch_data_hits;
	kstat_named_t arcstat_prefetch_data_misses;
	kstat_named_t arcstat_prefetch_metadata_hits;
	kstat_named_t arcstat_prefetch_metadata_misses;
	kstat_named_t arcstat_mru_hits;
	kstat_named_t arcstat_mru_ghost_hits;
	kstat_named_t arcstat_mfu_hits;
	kstat_named_t arcstat_mfu_ghost_hits;
	kstat_named_t arcstat_deleted;
	kstat_named_t arcstat_recycle_miss;
	kstat_named_t arcstat_mutex_miss;
	kstat_named_t arcstat_evict_skip;
	kstat_named_t arcstat_hash_elements;
	kstat_named_t arcstat_hash_elements_max;
	kstat_named_t arcstat_hash_collisions;
	kstat_named_t arcstat_hash_chains;
	kstat_named_t arcstat_hash_chain_max;
	kstat_named_t arcstat_p;
	kstat_named_t arcstat_c;
	kstat_named_t arcstat_c_min;
	kstat_named_t arcstat_c_max;
	kstat_named_t arcstat_size;
	kstat_named_t arcstat_hdr_size;
	kstat_named_t arcstat_data_size;
	kstat_named_t arcstat_other_size;
	kstat_named_t arcstat_l2_hits;
	kstat_named_t arcstat_l2_misses;
	kstat_named_t arcstat_l2_feeds;
	kstat_named_t arcstat_l2_rw_clash;
	kstat_named_t arcstat_l2_read_bytes;
	kstat_named_t arcstat_l2_write_bytes;
	kstat_named_t arcstat_l2_writes_sent;
	kstat_named_t arcstat_l2_writes_done;
	kstat_named_t arcstat_l2_writes_error;
	kstat_named_t arcstat_l2_writes_hdr_miss;
	kstat_named_t arcstat_l2_evict_lock_retry;
	kstat_named_t arcstat_l2_evict_reading;
	kstat_named_t arcstat_l2_free_on_write;
	kstat_named_t arcstat_l2_abort_lowmem;
	kstat_named_t arcstat_l2_cksum_bad;
	kstat_named_t arcstat_l2_io_error;
	kstat_named_t arcstat_l2_size;
	kstat_named_t arcstat_l2_hdr_size;
	kstat_named_t arcstat_memory_throttle_count;
} arc_stats_t;

static arc_stats_t arc_stats = {
	{ "hits",			KSTAT_DATA_UINT64 },
	{ "misses",			KSTAT_DATA_UINT64 },
	{ "demand_data_hits",		KSTAT_DATA_UINT64 },
	{ "demand_data_misses",		KSTAT_DATA_UINT64 },
	{ "demand_metadata_hits",	KSTAT_DATA_UINT64 },
	{ "demand_metadata_misses",	KSTAT_DATA_UINT64 },
	{ "prefetch_data_hits",		KSTAT_DATA_UINT64 },
	{ "prefetch_data_misses",	KSTAT_DATA_UINT64 },
	{ "prefetch_metadata_hits",	KSTAT_DATA_UINT64 },
	{ "prefetch_metadata_misses",	KSTAT_DATA_UINT64 },
	{ "mru_hits",			KSTAT_DATA_UINT64 },
	{ "mru_ghost_hits",		KSTAT_DATA_UINT64 },
	{ "mfu_hits",			KSTAT_DATA_UINT64 },
	{ "mfu_ghost_hits",		KSTAT_DATA_UINT64 },
	{ "deleted",			KSTAT_DATA_UINT64 },
	{ "recycle_miss",		KSTAT_DATA_UINT64 },
	{ "mutex_miss",			KSTAT_DATA_UINT64 },
	{ "evict_skip",			KSTAT_DATA_UINT64 },
	{ "hash_elements",		KSTAT_DATA_UINT64 },
	{ "hash_elements_max",		KSTAT_DATA_UINT64 },
	{ "hash_collisions",		KSTAT_DATA_UINT64 },
	{ "hash_chains",		KSTAT_DATA_UINT64 },
	{ "hash_chain_max",		KSTAT_DATA_UINT64 },
	{ "p",				KSTAT_DATA_UINT64 },
	{ "c",				KSTAT_DATA_UINT64 },
	{ "c_min",			KSTAT_DATA_UINT64 },
	{ "c_max",			KSTAT_DATA_UINT64 },
	{ "size",			KSTAT_DATA_UINT64 },
	{ "hdr_size",			KSTAT_DATA_UINT64 },
	{ "data_size",			KSTAT_DATA_UINT64 },
	{ "other_size",			KSTAT_DATA_UINT64 },
	{ "l2_hits",			KSTAT_DATA_UINT64 },
	{ "l2_misses",			KSTAT_DATA_UINT64 },
	{ "l2_feeds",			KSTAT_DATA_UINT64 },
	{ "l2_rw_clash",		KSTAT_DATA_UINT64 },
	{ "l2_read_bytes",		KSTAT_DATA_UINT64 },
	{ "l2_write_bytes",		KSTAT_DATA_UINT64 },
	{ "l2_writes_sent",		KSTAT_DATA_UINT64 },
	{ "l2_writes_done",		KSTAT_DATA_UINT64 },
	{ "l2_writes_error",		KSTAT_DATA_UINT64 },
	{ "l2_writes_hdr_miss",		KSTAT_DATA_UINT64 },
	{ "l2_evict_lock_retry",	KSTAT_DATA_UINT64 },
	{ "l2_evict_reading",		KSTAT_DATA_UINT64 },
	{ "l2_free_on_write",		KSTAT_DATA_UINT64 },
	{ "l2_abort_lowmem",		KSTAT_DATA_UINT64 },
	{ "l2_cksum_bad",		KSTAT_DATA_UINT64 },
	{ "l2_io_error",		KSTAT_DATA_UINT64 },
	{ "l2_size",			KSTAT_DATA_UINT64 },
	{ "l2_hdr_size",		KSTAT_DATA_UINT64 },
	{ "memory_throttle_count",	KSTAT_DATA_UINT64 }
};

#define	ARCSTAT(stat)	(arc_stats.stat.value.ui64)

#define	ARCSTAT_INCR(stat, val) \
	atomic_add_64(&arc_stats.stat.value.ui64, (val));

#define	ARCSTAT_BUMP(stat) 	ARCSTAT_INCR(stat, 1)
#define	ARCSTAT_BUMPDOWN(stat)	ARCSTAT_INCR(stat, -1)

#define	ARCSTAT_MAX(stat, val) {					\
	uint64_t m;							\
	while ((val) > (m = arc_stats.stat.value.ui64) &&		\
	    (m != atomic_cas_64(&arc_stats.stat.value.ui64, m, (val))))	\
		continue;						\
}

#define	ARCSTAT_MAXSTAT(stat) \
	ARCSTAT_MAX(stat##_max, arc_stats.stat.value.ui64)

/*
 * We define a macro to allow ARC hits/misses to be easily broken down by
 * two separate conditions, giving a total of four different subtypes for
 * each of hits and misses (so eight statistics total).
 */
#define	ARCSTAT_CONDSTAT(cond1, stat1, notstat1, cond2, stat2, notstat2, stat) \
	if (cond1) {							\
		if (cond2) {						\
			ARCSTAT_BUMP(arcstat_##stat1##_##stat2##_##stat); \
		} else {						\
			ARCSTAT_BUMP(arcstat_##stat1##_##notstat2##_##stat); \
		}							\
	} else {							\
		if (cond2) {						\
			ARCSTAT_BUMP(arcstat_##notstat1##_##stat2##_##stat); \
		} else {						\
			ARCSTAT_BUMP(arcstat_##notstat1##_##notstat2##_##stat);\
		}							\
	}

kstat_t			*arc_ksp;
static arc_state_t 	*arc_anon;
static arc_state_t	*arc_mru;
static arc_state_t	*arc_mru_ghost;
static arc_state_t	*arc_mfu;
static arc_state_t	*arc_mfu_ghost;
static arc_state_t	*arc_l2c_only;

/*
 * There are several ARC variables that are critical to export as kstats --
 * but we don't want to have to grovel around in the kstat whenever we wish to
 * manipulate them.  For these variables, we therefore define them to be in
 * terms of the statistic variable.  This assures that we are not introducing
 * the possibility of inconsistency by having shadow copies of the variables,
 * while still allowing the code to be readable.
 */
#define	arc_size	ARCSTAT(arcstat_size)	/* actual total arc size */
#define	arc_p		ARCSTAT(arcstat_p)	/* target size of MRU */
#define	arc_c		ARCSTAT(arcstat_c)	/* target size of cache */
#define	arc_c_min	ARCSTAT(arcstat_c_min)	/* min target cache size */
#define	arc_c_max	ARCSTAT(arcstat_c_max)	/* max target cache size */

static int		arc_no_grow;	/* Don't try to grow cache size */
static uint64_t		arc_tempreserve;
static uint64_t		arc_meta_used;
static uint64_t		arc_meta_limit;
static uint64_t		arc_meta_max = 0;

typedef struct l2arc_buf_hdr l2arc_buf_hdr_t;

typedef struct arc_callback arc_callback_t;

struct arc_callback {
	void			*acb_private;
	arc_done_func_t		*acb_done;
	arc_buf_t		*acb_buf;
	zio_t			*acb_zio_dummy;
	arc_callback_t		*acb_next;
};

typedef struct arc_write_callback arc_write_callback_t;

struct arc_write_callback {
	void		*awcb_private;
	arc_done_func_t	*awcb_ready;
	arc_done_func_t	*awcb_done;
	arc_buf_t	*awcb_buf;
};

struct arc_buf_hdr {
	/* protected by hash lock */
	dva_t			b_dva;
	uint64_t		b_birth;
	uint64_t		b_cksum0;

	kmutex_t		b_freeze_lock;
	zio_cksum_t		*b_freeze_cksum;

	arc_buf_hdr_t		*b_hash_next;
	arc_buf_t		*b_buf;
	uint32_t		b_flags;
	uint32_t		b_datacnt;

	arc_callback_t		*b_acb;
	kcondvar_t		b_cv;

	/* immutable */
	arc_buf_contents_t	b_type;
	uint64_t		b_size;
	uint64_t		b_spa;

	/* protected by arc state mutex */
	arc_state_t		*b_state;
	list_node_t		b_arc_node;

	/* updated atomically */
	clock_t			b_arc_access;

	/* self protecting */
	refcount_t		b_refcnt;

	l2arc_buf_hdr_t		*b_l2hdr;
	list_node_t		b_l2node;
};

static arc_buf_t *arc_eviction_list;
static kmutex_t arc_eviction_mtx;
static arc_buf_hdr_t arc_eviction_hdr;
static void arc_get_data_buf(arc_buf_t *buf);
static void arc_access(arc_buf_hdr_t *buf, kmutex_t *hash_lock);
static int arc_evict_needed(arc_buf_contents_t type);
static void arc_evict_ghost(arc_state_t *state, uint64_t spa, int64_t bytes);

#define	GHOST_STATE(state)	\
	((state) == arc_mru_ghost || (state) == arc_mfu_ghost ||	\
	(state) == arc_l2c_only)

/*
 * Private ARC flags.  These flags are private ARC only flags that will show up
 * in b_flags in the arc_hdr_buf_t.  Some flags are publicly declared, and can
 * be passed in as arc_flags in things like arc_read.  However, these flags
 * should never be passed and should only be set by ARC code.  When adding new
 * public flags, make sure not to smash the private ones.
 */

#define	ARC_IN_HASH_TABLE	(1 << 9)	/* this buffer is hashed */
#define	ARC_IO_IN_PROGRESS	(1 << 10)	/* I/O in progress for buf */
#define	ARC_IO_ERROR		(1 << 11)	/* I/O failed for buf */
#define	ARC_FREED_IN_READ	(1 << 12)	/* buf freed while in read */
#define	ARC_BUF_AVAILABLE	(1 << 13)	/* block not in active use */
#define	ARC_INDIRECT		(1 << 14)	/* this is an indirect block */
#define	ARC_FREE_IN_PROGRESS	(1 << 15)	/* hdr about to be freed */
#define	ARC_L2_WRITING		(1 << 16)	/* L2ARC write in progress */
#define	ARC_L2_EVICTED		(1 << 17)	/* evicted during I/O */
#define	ARC_L2_WRITE_HEAD	(1 << 18)	/* head of write list */
#define	ARC_STORED		(1 << 19)	/* has been store()d to */

#define	HDR_IN_HASH_TABLE(hdr)	((hdr)->b_flags & ARC_IN_HASH_TABLE)
#define	HDR_IO_IN_PROGRESS(hdr)	((hdr)->b_flags & ARC_IO_IN_PROGRESS)
#define	HDR_IO_ERROR(hdr)	((hdr)->b_flags & ARC_IO_ERROR)
#define	HDR_PREFETCH(hdr)	((hdr)->b_flags & ARC_PREFETCH)
#define	HDR_FREED_IN_READ(hdr)	((hdr)->b_flags & ARC_FREED_IN_READ)
#define	HDR_BUF_AVAILABLE(hdr)	((hdr)->b_flags & ARC_BUF_AVAILABLE)
#define	HDR_FREE_IN_PROGRESS(hdr)	((hdr)->b_flags & ARC_FREE_IN_PROGRESS)
#define	HDR_L2CACHE(hdr)	((hdr)->b_flags & ARC_L2CACHE)
#define	HDR_L2_READING(hdr)	((hdr)->b_flags & ARC_IO_IN_PROGRESS &&	\
				    (hdr)->b_l2hdr != NULL)
#define	HDR_L2_WRITING(hdr)	((hdr)->b_flags & ARC_L2_WRITING)
#define	HDR_L2_EVICTED(hdr)	((hdr)->b_flags & ARC_L2_EVICTED)
#define	HDR_L2_WRITE_HEAD(hdr)	((hdr)->b_flags & ARC_L2_WRITE_HEAD)

/*
 * Other sizes
 */

#define	HDR_SIZE ((int64_t)sizeof (arc_buf_hdr_t))
#define	L2HDR_SIZE ((int64_t)sizeof (l2arc_buf_hdr_t))

/*
 * Hash table routines
 */

#define	HT_LOCK_PAD	64

struct ht_lock {
	kmutex_t	ht_lock;
#ifdef _KERNEL
	unsigned char	pad[(HT_LOCK_PAD - sizeof (kmutex_t))];
#endif
};

#define	BUF_LOCKS 256
typedef struct buf_hash_table {
	uint64_t ht_mask;
	arc_buf_hdr_t **ht_table;
	struct ht_lock ht_locks[BUF_LOCKS];
} buf_hash_table_t;

static buf_hash_table_t buf_hash_table;

#define	BUF_HASH_INDEX(spa, dva, birth) \
	(buf_hash(spa, dva, birth) & buf_hash_table.ht_mask)
#define	BUF_HASH_LOCK_NTRY(idx) (buf_hash_table.ht_locks[idx & (BUF_LOCKS-1)])
#define	BUF_HASH_LOCK(idx)	(&(BUF_HASH_LOCK_NTRY(idx).ht_lock))
#define	HDR_LOCK(buf) \
	(BUF_HASH_LOCK(BUF_HASH_INDEX(buf->b_spa, &buf->b_dva, buf->b_birth)))

uint64_t zfs_crc64_table[256];

/*
 * Level 2 ARC
 */

#define	L2ARC_WRITE_SIZE	(8 * 1024 * 1024)	/* initial write max */
#define	L2ARC_HEADROOM		2		/* num of writes */
#define	L2ARC_FEED_SECS		1		/* caching interval secs */
#define	L2ARC_FEED_MIN_MS	200		/* min caching interval ms */

#define	l2arc_writes_sent	ARCSTAT(arcstat_l2_writes_sent)
#define	l2arc_writes_done	ARCSTAT(arcstat_l2_writes_done)

/*
 * L2ARC Performance Tunables
 */
uint64_t l2arc_write_max = L2ARC_WRITE_SIZE;	/* default max write size */
uint64_t l2arc_write_boost = L2ARC_WRITE_SIZE;	/* extra write during warmup */
uint64_t l2arc_headroom = L2ARC_HEADROOM;	/* number of dev writes */
uint64_t l2arc_feed_secs = L2ARC_FEED_SECS;	/* interval seconds */
uint64_t l2arc_feed_min_ms = L2ARC_FEED_MIN_MS;	/* min interval milliseconds */
boolean_t l2arc_noprefetch = B_TRUE;		/* don't cache prefetch bufs */
boolean_t l2arc_feed_again = B_TRUE;		/* turbo warmup */
boolean_t l2arc_norw = B_TRUE;			/* no reads during writes */

/*
 * L2ARC Internals
 */
typedef struct l2arc_dev {
	vdev_t			*l2ad_vdev;	/* vdev */
	spa_t			*l2ad_spa;	/* spa */
	uint64_t		l2ad_hand;	/* next write location */
	uint64_t		l2ad_write;	/* desired write size, bytes */
	uint64_t		l2ad_boost;	/* warmup write boost, bytes */
	uint64_t		l2ad_start;	/* first addr on device */
	uint64_t		l2ad_end;	/* last addr on device */
	uint64_t		l2ad_evict;	/* last addr eviction reached */
	boolean_t		l2ad_first;	/* first sweep through */
	boolean_t		l2ad_writing;	/* currently writing */
	list_t			*l2ad_buflist;	/* buffer list */
	list_node_t		l2ad_node;	/* device list node */
} l2arc_dev_t;

static list_t L2ARC_dev_list;			/* device list */
static list_t *l2arc_dev_list;			/* device list pointer */
static kmutex_t l2arc_dev_mtx;			/* device list mutex */
static l2arc_dev_t *l2arc_dev_last;		/* last device used */
static kmutex_t l2arc_buflist_mtx;		/* mutex for all buflists */
static list_t L2ARC_free_on_write;		/* free after write buf list */
static list_t *l2arc_free_on_write;		/* free after write list ptr */
static kmutex_t l2arc_free_on_write_mtx;	/* mutex for list */
static uint64_t l2arc_ndev;			/* number of devices */

typedef struct l2arc_read_callback {
	arc_buf_t	*l2rcb_buf;		/* read buffer */
	spa_t		*l2rcb_spa;		/* spa */
	blkptr_t	l2rcb_bp;		/* original blkptr */
	zbookmark_t	l2rcb_zb;		/* original bookmark */
	int		l2rcb_flags;		/* original flags */
} l2arc_read_callback_t;

typedef struct l2arc_write_callback {
	l2arc_dev_t	*l2wcb_dev;		/* device info */
	arc_buf_hdr_t	*l2wcb_head;		/* head of write buflist */
} l2arc_write_callback_t;

struct l2arc_buf_hdr {
	/* protected by arc_buf_hdr  mutex */
	l2arc_dev_t	*b_dev;			/* L2ARC device */
	daddr_t		b_daddr;		/* disk address, offset byte */
};

typedef struct l2arc_data_free {
	/* protected by l2arc_free_on_write_mtx */
	void		*l2df_data;
	size_t		l2df_size;
	void		(*l2df_func)(void *, size_t);
	list_node_t	l2df_list_node;
} l2arc_data_free_t;

static kmutex_t l2arc_feed_thr_lock;
static kcondvar_t l2arc_feed_thr_cv;
static uint8_t l2arc_thread_exit;

static void l2arc_read_done(zio_t *zio);
static void l2arc_hdr_stat_add(void);
static void l2arc_hdr_stat_remove(void);

static uint64_t
buf_hash(uint64_t spa, const dva_t *dva, uint64_t birth)
{
	uint8_t *vdva = (uint8_t *)dva;
	uint64_t crc = -1ULL;
	int i;

	ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY);

	for (i = 0; i < sizeof (dva_t); i++)
		crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ vdva[i]) & 0xFF];

	crc ^= (spa>>8) ^ birth;

	return (crc);
}

#define	BUF_EMPTY(buf)						\
	((buf)->b_dva.dva_word[0] == 0 &&			\
	(buf)->b_dva.dva_word[1] == 0 &&			\
	(buf)->b_birth == 0)

#define	BUF_EQUAL(spa, dva, birth, buf)				\
	((buf)->b_dva.dva_word[0] == (dva)->dva_word[0]) &&	\
	((buf)->b_dva.dva_word[1] == (dva)->dva_word[1]) &&	\
	((buf)->b_birth == birth) && ((buf)->b_spa == spa)

static arc_buf_hdr_t *
buf_hash_find(uint64_t spa, const dva_t *dva, uint64_t birth, kmutex_t **lockp)
{
	uint64_t idx = BUF_HASH_INDEX(spa, dva, birth);
	kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
	arc_buf_hdr_t *buf;

	mutex_enter(hash_lock);
	for (buf = buf_hash_table.ht_table[idx]; buf != NULL;
	    buf = buf->b_hash_next) {
		if (BUF_EQUAL(spa, dva, birth, buf)) {
			*lockp = hash_lock;
			return (buf);
		}
	}
	mutex_exit(hash_lock);
	*lockp = NULL;
	return (NULL);
}

/*
 * Insert an entry into the hash table.  If there is already an element
 * equal to elem in the hash table, then the already existing element
 * will be returned and the new element will not be inserted.
 * Otherwise returns NULL.
 */
static arc_buf_hdr_t *
buf_hash_insert(arc_buf_hdr_t *buf, kmutex_t **lockp)
{
	uint64_t idx = BUF_HASH_INDEX(buf->b_spa, &buf->b_dva, buf->b_birth);
	kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
	arc_buf_hdr_t *fbuf;
	uint32_t i;

	ASSERT(!HDR_IN_HASH_TABLE(buf));
	*lockp = hash_lock;
	mutex_enter(hash_lock);
	for (fbuf = buf_hash_table.ht_table[idx], i = 0; fbuf != NULL;
	    fbuf = fbuf->b_hash_next, i++) {
		if (BUF_EQUAL(buf->b_spa, &buf->b_dva, buf->b_birth, fbuf))
			return (fbuf);
	}

	buf->b_hash_next = buf_hash_table.ht_table[idx];
	buf_hash_table.ht_table[idx] = buf;
	buf->b_flags |= ARC_IN_HASH_TABLE;

	/* collect some hash table performance data */
	if (i > 0) {
		ARCSTAT_BUMP(arcstat_hash_collisions);
		if (i == 1)
			ARCSTAT_BUMP(arcstat_hash_chains);

		ARCSTAT_MAX(arcstat_hash_chain_max, i);
	}

	ARCSTAT_BUMP(arcstat_hash_elements);
	ARCSTAT_MAXSTAT(arcstat_hash_elements);

	return (NULL);
}

static void
buf_hash_remove(arc_buf_hdr_t *buf)
{
	arc_buf_hdr_t *fbuf, **bufp;
	uint64_t idx = BUF_HASH_INDEX(buf->b_spa, &buf->b_dva, buf->b_birth);

	ASSERT(MUTEX_HELD(BUF_HASH_LOCK(idx)));
	ASSERT(HDR_IN_HASH_TABLE(buf));

	bufp = &buf_hash_table.ht_table[idx];
	while ((fbuf = *bufp) != buf) {
		ASSERT(fbuf != NULL);
		bufp = &fbuf->b_hash_next;
	}
	*bufp = buf->b_hash_next;
	buf->b_hash_next = NULL;
	buf->b_flags &= ~ARC_IN_HASH_TABLE;

	/* collect some hash table performance data */
	ARCSTAT_BUMPDOWN(arcstat_hash_elements);

	if (buf_hash_table.ht_table[idx] &&
	    buf_hash_table.ht_table[idx]->b_hash_next == NULL)
		ARCSTAT_BUMPDOWN(arcstat_hash_chains);
}

/*
 * Global data structures and functions for the buf kmem cache.
 */
static kmem_cache_t *hdr_cache;
static kmem_cache_t *buf_cache;

static void
buf_fini(void)
{
	int i;

	kmem_free(buf_hash_table.ht_table,
	    (buf_hash_table.ht_mask + 1) * sizeof (void *));
	for (i = 0; i < BUF_LOCKS; i++)
		mutex_destroy(&buf_hash_table.ht_locks[i].ht_lock);
	kmem_cache_destroy(hdr_cache);
	kmem_cache_destroy(buf_cache);
}

/*
 * Constructor callback - called when the cache is empty
 * and a new buf is requested.
 */
/* ARGSUSED */
static int
hdr_cons(void *vbuf, void *unused, int kmflag)
{
	arc_buf_hdr_t *buf = vbuf;

	bzero(buf, sizeof (arc_buf_hdr_t));
	refcount_create(&buf->b_refcnt);
	cv_init(&buf->b_cv, NULL, CV_DEFAULT, NULL);
	mutex_init(&buf->b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
	arc_space_consume(sizeof (arc_buf_hdr_t), ARC_SPACE_HDRS);

	return (0);
}

/* ARGSUSED */
static int
buf_cons(void *vbuf, void *unused, int kmflag)
{
	arc_buf_t *buf = vbuf;

	bzero(buf, sizeof (arc_buf_t));
	rw_init(&buf->b_lock, NULL, RW_DEFAULT, NULL);
	arc_space_consume(sizeof (arc_buf_t), ARC_SPACE_HDRS);

	return (0);
}

/*
 * Destructor callback - called when a cached buf is
 * no longer required.
 */
/* ARGSUSED */
static void
hdr_dest(void *vbuf, void *unused)
{
	arc_buf_hdr_t *buf = vbuf;

	refcount_destroy(&buf->b_refcnt);
	cv_destroy(&buf->b_cv);
	mutex_destroy(&buf->b_freeze_lock);
	arc_space_return(sizeof (arc_buf_hdr_t), ARC_SPACE_HDRS);
}

/* ARGSUSED */
static void
buf_dest(void *vbuf, void *unused)
{
	arc_buf_t *buf = vbuf;

	rw_destroy(&buf->b_lock);
	arc_space_return(sizeof (arc_buf_t), ARC_SPACE_HDRS);
}

/*
 * Reclaim callback -- invoked when memory is low.
 */
/* ARGSUSED */
static void
hdr_recl(void *unused)
{
	dprintf("hdr_recl called\n");
	/*
	 * umem calls the reclaim func when we destroy the buf cache,
	 * which is after we do arc_fini().
	 */
	if (!arc_dead)
		cv_signal(&arc_reclaim_thr_cv);
}

static void
buf_init(void)
{
	uint64_t *ct;
	uint64_t hsize = 1ULL << 12;
	int i, j;

	/*
	 * The hash table is big enough to fill all of physical memory
	 * with an average 64K block size.  The table will take up
	 * totalmem*sizeof(void*)/64K (eg. 128KB/GB with 8-byte pointers).
	 */
	while (hsize * 65536 < physmem * PAGESIZE)
		hsize <<= 1;
retry:
	buf_hash_table.ht_mask = hsize - 1;
	buf_hash_table.ht_table =
	    kmem_zalloc(hsize * sizeof (void*), KM_NOSLEEP);
	if (buf_hash_table.ht_table == NULL) {
		ASSERT(hsize > (1ULL << 8));
		hsize >>= 1;
		goto retry;
	}

	hdr_cache = kmem_cache_create("arc_buf_hdr_t", sizeof (arc_buf_hdr_t),
	    0, hdr_cons, hdr_dest, hdr_recl, NULL, NULL, 0);
	buf_cache = kmem_cache_create("arc_buf_t", sizeof (arc_buf_t),
	    0, buf_cons, buf_dest, NULL, NULL, NULL, 0);

	for (i = 0; i < 256; i++)
		for (ct = zfs_crc64_table + i, *ct = i, j = 8; j > 0; j--)
			*ct = (*ct >> 1) ^ (-(*ct & 1) & ZFS_CRC64_POLY);

	for (i = 0; i < BUF_LOCKS; i++) {
		mutex_init(&buf_hash_table.ht_locks[i].ht_lock,
		    NULL, MUTEX_DEFAULT, NULL);
	}
}

#define	ARC_MINTIME	(hz>>4) /* 62 ms */

static void
arc_cksum_verify(arc_buf_t *buf)
{
	zio_cksum_t zc;

	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
		return;

	mutex_enter(&buf->b_hdr->b_freeze_lock);
	if (buf->b_hdr->b_freeze_cksum == NULL ||
	    (buf->b_hdr->b_flags & ARC_IO_ERROR)) {
		mutex_exit(&buf->b_hdr->b_freeze_lock);
		return;
	}
	fletcher_2_native(buf->b_data, buf->b_hdr->b_size, &zc);
	if (!ZIO_CHECKSUM_EQUAL(*buf->b_hdr->b_freeze_cksum, zc))
		panic("buffer modified while frozen!");
	mutex_exit(&buf->b_hdr->b_freeze_lock);
}

static int
arc_cksum_equal(arc_buf_t *buf)
{
	zio_cksum_t zc;
	int equal;

	mutex_enter(&buf->b_hdr->b_freeze_lock);
	fletcher_2_native(buf->b_data, buf->b_hdr->b_size, &zc);
	equal = ZIO_CHECKSUM_EQUAL(*buf->b_hdr->b_freeze_cksum, zc);
	mutex_exit(&buf->b_hdr->b_freeze_lock);

	return (equal);
}

static void
arc_cksum_compute(arc_buf_t *buf, boolean_t force)
{
	if (!force && !(zfs_flags & ZFS_DEBUG_MODIFY))
		return;

	mutex_enter(&buf->b_hdr->b_freeze_lock);
	if (buf->b_hdr->b_freeze_cksum != NULL) {
		mutex_exit(&buf->b_hdr->b_freeze_lock);
		return;
	}
	buf->b_hdr->b_freeze_cksum = kmem_alloc(sizeof (zio_cksum_t), KM_SLEEP);
	fletcher_2_native(buf->b_data, buf->b_hdr->b_size,
	    buf->b_hdr->b_freeze_cksum);
	mutex_exit(&buf->b_hdr->b_freeze_lock);
}

void
arc_buf_thaw(arc_buf_t *buf)
{
	if (zfs_flags & ZFS_DEBUG_MODIFY) {
		if (buf->b_hdr->b_state != arc_anon)
			panic("modifying non-anon buffer!");
		if (buf->b_hdr->b_flags & ARC_IO_IN_PROGRESS)
			panic("modifying buffer while i/o in progress!");
		arc_cksum_verify(buf);
	}

	mutex_enter(&buf->b_hdr->b_freeze_lock);
	if (buf->b_hdr->b_freeze_cksum != NULL) {
		kmem_free(buf->b_hdr->b_freeze_cksum, sizeof (zio_cksum_t));
		buf->b_hdr->b_freeze_cksum = NULL;
	}
	mutex_exit(&buf->b_hdr->b_freeze_lock);
}

void
arc_buf_freeze(arc_buf_t *buf)
{
	if (!(zfs_flags & ZFS_DEBUG_MODIFY))
		return;

	ASSERT(buf->b_hdr->b_freeze_cksum != NULL ||
	    buf->b_hdr->b_state == arc_anon);
	arc_cksum_compute(buf, B_FALSE);
}

static void
add_reference(arc_buf_hdr_t *ab, kmutex_t *hash_lock, void *tag)
{
	ASSERT(MUTEX_HELD(hash_lock));

	if ((refcount_add(&ab->b_refcnt, tag) == 1) &&
	    (ab->b_state != arc_anon)) {
		uint64_t delta = ab->b_size * ab->b_datacnt;
		list_t *list = &ab->b_state->arcs_list[ab->b_type];
		uint64_t *size = &ab->b_state->arcs_lsize[ab->b_type];

		ASSERT(!MUTEX_HELD(&ab->b_state->arcs_mtx));
		mutex_enter(&ab->b_state->arcs_mtx);
		ASSERT(list_link_active(&ab->b_arc_node));
		list_remove(list, ab);
		if (GHOST_STATE(ab->b_state)) {
			ASSERT3U(ab->b_datacnt, ==, 0);
			ASSERT3P(ab->b_buf, ==, NULL);
			delta = ab->b_size;
		}
		ASSERT(delta > 0);
		ASSERT3U(*size, >=, delta);
		atomic_add_64(size, -delta);
		mutex_exit(&ab->b_state->arcs_mtx);
		/* remove the prefetch flag if we get a reference */
		if (ab->b_flags & ARC_PREFETCH)
			ab->b_flags &= ~ARC_PREFETCH;
	}
}

static int
remove_reference(arc_buf_hdr_t *ab, kmutex_t *hash_lock, void *tag)
{
	int cnt;
	arc_state_t *state = ab->b_state;

	ASSERT(state == arc_anon || MUTEX_HELD(hash_lock));
	ASSERT(!GHOST_STATE(state));

	if (((cnt = refcount_remove(&ab->b_refcnt, tag)) == 0) &&
	    (state != arc_anon)) {
		uint64_t *size = &state->arcs_lsize[ab->b_type];

		ASSERT(!MUTEX_HELD(&state->arcs_mtx));
		mutex_enter(&state->arcs_mtx);
		ASSERT(!list_link_active(&ab->b_arc_node));
		list_insert_head(&state->arcs_list[ab->b_type], ab);
		ASSERT(ab->b_datacnt > 0);
		atomic_add_64(size, ab->b_size * ab->b_datacnt);
		mutex_exit(&state->arcs_mtx);
	}
	return (cnt);
}

/*
 * Move the supplied buffer to the indicated state.  The mutex
 * for the buffer must be held by the caller.
 */
static void
arc_change_state(arc_state_t *new_state, arc_buf_hdr_t *ab, kmutex_t *hash_lock)
{
	arc_state_t *old_state = ab->b_state;
	int64_t refcnt = refcount_count(&ab->b_refcnt);
	uint64_t from_delta, to_delta;

	ASSERT(MUTEX_HELD(hash_lock));
	ASSERT(new_state != old_state);
	ASSERT(refcnt == 0 || ab->b_datacnt > 0);
	ASSERT(ab->b_datacnt == 0 || !GHOST_STATE(new_state));

	from_delta = to_delta = ab->b_datacnt * ab->b_size;

	/*
	 * If this buffer is evictable, transfer it from the
	 * old state list to the new state list.
	 */
	if (refcnt == 0) {
		if (old_state != arc_anon) {
			int use_mutex = !MUTEX_HELD(&old_state->arcs_mtx);
			uint64_t *size = &old_state->arcs_lsize[ab->b_type];

			if (use_mutex)
				mutex_enter(&old_state->arcs_mtx);

			ASSERT(list_link_active(&ab->b_arc_node));
			list_remove(&old_state->arcs_list[ab->b_type], ab);

			/*
			 * If prefetching out of the ghost cache,
			 * we will have a non-null datacnt.
			 */
			if (GHOST_STATE(old_state) && ab->b_datacnt == 0) {
				/* ghost elements have a ghost size */
				ASSERT(ab->b_buf == NULL);
				from_delta = ab->b_size;
			}
			ASSERT3U(*size, >=, from_delta);
			atomic_add_64(size, -from_delta);

			if (use_mutex)
				mutex_exit(&old_state->arcs_mtx);
		}
		if (new_state != arc_anon) {
			int use_mutex = !MUTEX_HELD(&new_state->arcs_mtx);
			uint64_t *size = &new_state->arcs_lsize[ab->b_type];

			if (use_mutex)
				mutex_enter(&new_state->arcs_mtx);

			list_insert_head(&new_state->arcs_list[ab->b_type], ab);

			/* ghost elements have a ghost size */
			if (GHOST_STATE(new_state)) {
				ASSERT(ab->b_datacnt == 0);
				ASSERT(ab->b_buf == NULL);
				to_delta = ab->b_size;
			}
			atomic_add_64(size, to_delta);

			if (use_mutex)
				mutex_exit(&new_state->arcs_mtx);
		}
	}

	ASSERT(!BUF_EMPTY(ab));
	if (new_state == arc_anon) {
		buf_hash_remove(ab);
	}

	/* adjust state sizes */
	if (to_delta)
		atomic_add_64(&new_state->arcs_size, to_delta);
	if (from_delta) {
		ASSERT3U(old_state->arcs_size, >=, from_delta);
		atomic_add_64(&old_state->arcs_size, -from_delta);
	}
	ab->b_state = new_state;

	/* adjust l2arc hdr stats */
	if (new_state == arc_l2c_only)
		l2arc_hdr_stat_add();
	else if (old_state == arc_l2c_only)
		l2arc_hdr_stat_remove();
}

void
arc_space_consume(uint64_t space, arc_space_type_t type)
{
	ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);

	switch (type) {
	case ARC_SPACE_DATA:
		ARCSTAT_INCR(arcstat_data_size, space);
		break;
	case ARC_SPACE_OTHER:
		ARCSTAT_INCR(arcstat_other_size, space);
		break;
	case ARC_SPACE_HDRS:
		ARCSTAT_INCR(arcstat_hdr_size, space);
		break;
	case ARC_SPACE_L2HDRS:
		ARCSTAT_INCR(arcstat_l2_hdr_size, space);
		break;
	}

	atomic_add_64(&arc_meta_used, space);
	atomic_add_64(&arc_size, space);
}

void
arc_space_return(uint64_t space, arc_space_type_t type)
{
	ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);

	switch (type) {
	case ARC_SPACE_DATA:
		ARCSTAT_INCR(arcstat_data_size, -space);
		break;
	case ARC_SPACE_OTHER:
		ARCSTAT_INCR(arcstat_other_size, -space);
		break;
	case ARC_SPACE_HDRS:
		ARCSTAT_INCR(arcstat_hdr_size, -space);
		break;
	case ARC_SPACE_L2HDRS:
		ARCSTAT_INCR(arcstat_l2_hdr_size, -space);
		break;
	}

	ASSERT(arc_meta_used >= space);
	if (arc_meta_max < arc_meta_used)
		arc_meta_max = arc_meta_used;
	atomic_add_64(&arc_meta_used, -space);
	ASSERT(arc_size >= space);
	atomic_add_64(&arc_size, -space);
}

void *
arc_data_buf_alloc(uint64_t size)
{
	if (arc_evict_needed(ARC_BUFC_DATA))
		cv_signal(&arc_reclaim_thr_cv);
	atomic_add_64(&arc_size, size);
	return (zio_data_buf_alloc(size));
}

void
arc_data_buf_free(void *buf, uint64_t size)
{
	zio_data_buf_free(buf, size);
	ASSERT(arc_size >= size);
	atomic_add_64(&arc_size, -size);
}

arc_buf_t *
arc_buf_alloc(spa_t *spa, int size, void *tag, arc_buf_contents_t type)
{
	arc_buf_hdr_t *hdr;
	arc_buf_t *buf;

	ASSERT3U(size, >, 0);
	hdr = kmem_cache_alloc(hdr_cache, KM_PUSHPAGE);
	ASSERT(BUF_EMPTY(hdr));
	hdr->b_size = size;
	hdr->b_type = type;
	hdr->b_spa = spa_guid(spa);
	hdr->b_state = arc_anon;
	hdr->b_arc_access = 0;
	buf = kmem_cache_alloc(buf_cache, KM_PUSHPAGE);
	buf->b_hdr = hdr;
	buf->b_data = NULL;
	buf->b_efunc = NULL;
	buf->b_private = NULL;
	buf->b_next = NULL;
	hdr->b_buf = buf;
	arc_get_data_buf(buf);
	hdr->b_datacnt = 1;
	hdr->b_flags = 0;
	ASSERT(refcount_is_zero(&hdr->b_refcnt));
	(void) refcount_add(&hdr->b_refcnt, tag);

	return (buf);
}

static arc_buf_t *
arc_buf_clone(arc_buf_t *from)
{
	arc_buf_t *buf;
	arc_buf_hdr_t *hdr = from->b_hdr;
	uint64_t size = hdr->b_size;

	buf = kmem_cache_alloc(buf_cache, KM_PUSHPAGE);
	buf->b_hdr = hdr;
	buf->b_data = NULL;
	buf->b_efunc = NULL;
	buf->b_private = NULL;
	buf->b_next = hdr->b_buf;
	hdr->b_buf = buf;
	arc_get_data_buf(buf);
	bcopy(from->b_data, buf->b_data, size);
	hdr->b_datacnt += 1;
	return (buf);
}

void
arc_buf_add_ref(arc_buf_t *buf, void* tag)
{
	arc_buf_hdr_t *hdr;
	kmutex_t *hash_lock;

	/*
	 * Check to see if this buffer is evicted.  Callers
	 * must verify b_data != NULL to know if the add_ref
	 * was successful.
	 */
	rw_enter(&buf->b_lock, RW_READER);
	if (buf->b_data == NULL) {
		rw_exit(&buf->b_lock);
		return;
	}
	hdr = buf->b_hdr;
	ASSERT(hdr != NULL);
	hash_lock = HDR_LOCK(hdr);
	mutex_enter(hash_lock);
	rw_exit(&buf->b_lock);

	ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);
	add_reference(hdr, hash_lock, tag);
	DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
	arc_access(hdr, hash_lock);
	mutex_exit(hash_lock);
	ARCSTAT_BUMP(arcstat_hits);
	ARCSTAT_CONDSTAT(!(hdr->b_flags & ARC_PREFETCH),
	    demand, prefetch, hdr->b_type != ARC_BUFC_METADATA,
	    data, metadata, hits);
}

/*
 * Free the arc data buffer.  If it is an l2arc write in progress,
 * the buffer is placed on l2arc_free_on_write to be freed later.
 */
static void
arc_buf_data_free(arc_buf_hdr_t *hdr, void (*free_func)(void *, size_t),
    void *data, size_t size)
{
	if (HDR_L2_WRITING(hdr)) {
		l2arc_data_free_t *df;
		df = kmem_alloc(sizeof (l2arc_data_free_t), KM_SLEEP);
		df->l2df_data = data;
		df->l2df_size = size;
		df->l2df_func = free_func;
		mutex_enter(&l2arc_free_on_write_mtx);
		list_insert_head(l2arc_free_on_write, df);
		mutex_exit(&l2arc_free_on_write_mtx);
		ARCSTAT_BUMP(arcstat_l2_free_on_write);
	} else {
		free_func(data, size);
	}
}

static void
arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t all)
{
	arc_buf_t **bufp;

	/* free up data associated with the buf */
	if (buf->b_data) {
		arc_state_t *state = buf->b_hdr->b_state;
		uint64_t size = buf->b_hdr->b_size;
		arc_buf_contents_t type = buf->b_hdr->b_type;

		arc_cksum_verify(buf);
		if (!recycle) {
			if (type == ARC_BUFC_METADATA) {
				arc_buf_data_free(buf->b_hdr, zio_buf_free,
				    buf->b_data, size);
				arc_space_return(size, ARC_SPACE_DATA);
			} else {
				ASSERT(type == ARC_BUFC_DATA);
				arc_buf_data_free(buf->b_hdr,
				    zio_data_buf_free, buf->b_data, size);
				ARCSTAT_INCR(arcstat_data_size, -size);
				atomic_add_64(&arc_size, -size);
			}
		}
		if (list_link_active(&buf->b_hdr->b_arc_node)) {
			uint64_t *cnt = &state->arcs_lsize[type];

			ASSERT(refcount_is_zero(&buf->b_hdr->b_refcnt));
			ASSERT(state != arc_anon);

			ASSERT3U(*cnt, >=, size);
			atomic_add_64(cnt, -size);
		}
		ASSERT3U(state->arcs_size, >=, size);
		atomic_add_64(&state->arcs_size, -size);
		buf->b_data = NULL;
		ASSERT(buf->b_hdr->b_datacnt > 0);
		buf->b_hdr->b_datacnt -= 1;
	}

	/* only remove the buf if requested */
	if (!all)
		return;

	/* remove the buf from the hdr list */
	for (bufp = &buf->b_hdr->b_buf; *bufp != buf; bufp = &(*bufp)->b_next)
		continue;
	*bufp = buf->b_next;

	ASSERT(buf->b_efunc == NULL);

	/* clean up the buf */
	buf->b_hdr = NULL;
	kmem_cache_free(buf_cache, buf);
}

static void
arc_hdr_destroy(arc_buf_hdr_t *hdr)
{
	ASSERT(refcount_is_zero(&hdr->b_refcnt));
	ASSERT3P(hdr->b_state, ==, arc_anon);
	ASSERT(!HDR_IO_IN_PROGRESS(hdr));
	ASSERT(!(hdr->b_flags & ARC_STORED));

	if (hdr->b_l2hdr != NULL) {
		if (!MUTEX_HELD(&l2arc_buflist_mtx)) {
			/*
			 * To prevent arc_free() and l2arc_evict() from
			 * attempting to free the same buffer at the same time,
			 * a FREE_IN_PROGRESS flag is given to arc_free() to
			 * give it priority.  l2arc_evict() can't destroy this
			 * header while we are waiting on l2arc_buflist_mtx.
			 *
			 * The hdr may be removed from l2ad_buflist before we
			 * grab l2arc_buflist_mtx, so b_l2hdr is rechecked.
			 */
			mutex_enter(&l2arc_buflist_mtx);
			if (hdr->b_l2hdr != NULL) {
				list_remove(hdr->b_l2hdr->b_dev->l2ad_buflist,
				    hdr);
			}
			mutex_exit(&l2arc_buflist_mtx);
		} else {
			list_remove(hdr->b_l2hdr->b_dev->l2ad_buflist, hdr);
		}
		ARCSTAT_INCR(arcstat_l2_size, -hdr->b_size);
		kmem_free(hdr->b_l2hdr, sizeof (l2arc_buf_hdr_t));
		if (hdr->b_state == arc_l2c_only)
			l2arc_hdr_stat_remove();
		hdr->b_l2hdr = NULL;
	}

	if (!BUF_EMPTY(hdr)) {
		ASSERT(!HDR_IN_HASH_TABLE(hdr));
		bzero(&hdr->b_dva, sizeof (dva_t));
		hdr->b_birth = 0;
		hdr->b_cksum0 = 0;
	}
	while (hdr->b_buf) {
		arc_buf_t *buf = hdr->b_buf;

		if (buf->b_efunc) {
			mutex_enter(&arc_eviction_mtx);
			rw_enter(&buf->b_lock, RW_WRITER);
			ASSERT(buf->b_hdr != NULL);
			arc_buf_destroy(hdr->b_buf, FALSE, FALSE);
			hdr->b_buf = buf->b_next;
			buf->b_hdr = &arc_eviction_hdr;
			buf->b_next = arc_eviction_list;
			arc_eviction_list = buf;
			rw_exit(&buf->b_lock);
			mutex_exit(&arc_eviction_mtx);
		} else {
			arc_buf_destroy(hdr->b_buf, FALSE, TRUE);
		}
	}
	if (hdr->b_freeze_cksum != NULL) {
		kmem_free(hdr->b_freeze_cksum, sizeof (zio_cksum_t));
		hdr->b_freeze_cksum = NULL;
	}

	ASSERT(!list_link_active(&hdr->b_arc_node));
	ASSERT3P(hdr->b_hash_next, ==, NULL);
	ASSERT3P(hdr->b_acb, ==, NULL);
	kmem_cache_free(hdr_cache, hdr);
}

void
arc_buf_free(arc_buf_t *buf, void *tag)
{
	arc_buf_hdr_t *hdr = buf->b_hdr;
	int hashed = hdr->b_state != arc_anon;

	ASSERT(buf->b_efunc == NULL);
	ASSERT(buf->b_data != NULL);

	if (hashed) {
		kmutex_t *hash_lock = HDR_LOCK(hdr);

		mutex_enter(hash_lock);
		(void) remove_reference(hdr, hash_lock, tag);
		if (hdr->b_datacnt > 1)
			arc_buf_destroy(buf, FALSE, TRUE);
		else
			hdr->b_flags |= ARC_BUF_AVAILABLE;
		mutex_exit(hash_lock);
	} else if (HDR_IO_IN_PROGRESS(hdr)) {
		int destroy_hdr;
		/*
		 * We are in the middle of an async write.  Don't destroy
		 * this buffer unless the write completes before we finish
		 * decrementing the reference count.
		 */
		mutex_enter(&arc_eviction_mtx);
		(void) remove_reference(hdr, NULL, tag);
		ASSERT(refcount_is_zero(&hdr->b_refcnt));
		destroy_hdr = !HDR_IO_IN_PROGRESS(hdr);
		mutex_exit(&arc_eviction_mtx);
		if (destroy_hdr)
			arc_hdr_destroy(hdr);
	} else {
		if (remove_reference(hdr, NULL, tag) > 0) {
			ASSERT(HDR_IO_ERROR(hdr));
			arc_buf_destroy(buf, FALSE, TRUE);
		} else {
			arc_hdr_destroy(hdr);
		}
	}
}

int
arc_buf_remove_ref(arc_buf_t *buf, void* tag)
{
	arc_buf_hdr_t *hdr = buf->b_hdr;
	kmutex_t *hash_lock = HDR_LOCK(hdr);
	int no_callback = (buf->b_efunc == NULL);

	if (hdr->b_state == arc_anon) {
		arc_buf_free(buf, tag);
		return (no_callback);
	}

	mutex_enter(hash_lock);
	ASSERT(hdr->b_state != arc_anon);
	ASSERT(buf->b_data != NULL);

	(void) remove_reference(hdr, hash_lock, tag);
	if (hdr->b_datacnt > 1) {
		if (no_callback)
			arc_buf_destroy(buf, FALSE, TRUE);
	} else if (no_callback) {
		ASSERT(hdr->b_buf == buf && buf->b_next == NULL);
		hdr->b_flags |= ARC_BUF_AVAILABLE;
	}
	ASSERT(no_callback || hdr->b_datacnt > 1 ||
	    refcount_is_zero(&hdr->b_refcnt));
	mutex_exit(hash_lock);
	return (no_callback);
}

int
arc_buf_size(arc_buf_t *buf)
{
	return (buf->b_hdr->b_size);
}

/*
 * Evict buffers from list until we've removed the specified number of
 * bytes.  Move the removed buffers to the appropriate evict state.
 * If the recycle flag is set, then attempt to "recycle" a buffer:
 * - look for a buffer to evict that is `bytes' long.
 * - return the data block from this buffer rather than freeing it.
 * This flag is used by callers that are trying to make space for a
 * new buffer in a full arc cache.
 *
 * This function makes a "best effort".  It skips over any buffers
 * it can't get a hash_lock on, and so may not catch all candidates.
 * It may also return without evicting as much space as requested.
 */
static void *
arc_evict(arc_state_t *state, uint64_t spa, int64_t bytes, boolean_t recycle,
    arc_buf_contents_t type)
{
	arc_state_t *evicted_state;
	uint64_t bytes_evicted = 0, skipped = 0, missed = 0;
	arc_buf_hdr_t *ab, *ab_prev = NULL;
	list_t *list = &state->arcs_list[type];
	kmutex_t *hash_lock;
	boolean_t have_lock;
	void *stolen = NULL;

	ASSERT(state == arc_mru || state == arc_mfu);

	evicted_state = (state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;

	mutex_enter(&state->arcs_mtx);
	mutex_enter(&evicted_state->arcs_mtx);

	for (ab = list_tail(list); ab; ab = ab_prev) {
		ab_prev = list_prev(list, ab);
		/* prefetch buffers have a minimum lifespan */
		if (HDR_IO_IN_PROGRESS(ab) ||
		    (spa && ab->b_spa != spa) ||
		    (ab->b_flags & (ARC_PREFETCH|ARC_INDIRECT) &&
		    lbolt - ab->b_arc_access < arc_min_prefetch_lifespan)) {
			skipped++;
			continue;
		}
		/* "lookahead" for better eviction candidate */
		if (recycle && ab->b_size != bytes &&
		    ab_prev && ab_prev->b_size == bytes)
			continue;
		hash_lock = HDR_LOCK(ab);
		have_lock = MUTEX_HELD(hash_lock);
		if (have_lock || mutex_tryenter(hash_lock)) {
			ASSERT3U(refcount_count(&ab->b_refcnt), ==, 0);
			ASSERT(ab->b_datacnt > 0);
			while (ab->b_buf) {
				arc_buf_t *buf = ab->b_buf;
				if (!rw_tryenter(&buf->b_lock, RW_WRITER)) {
					missed += 1;
					break;
				}
				if (buf->b_data) {
					bytes_evicted += ab->b_size;
					if (recycle && ab->b_type == type &&
					    ab->b_size == bytes &&
					    !HDR_L2_WRITING(ab)) {
						stolen = buf->b_data;
						recycle = FALSE;
					}
				}
				if (buf->b_efunc) {
					mutex_enter(&arc_eviction_mtx);
					arc_buf_destroy(buf,
					    buf->b_data == stolen, FALSE);
					ab->b_buf = buf->b_next;
					buf->b_hdr = &arc_eviction_hdr;
					buf->b_next = arc_eviction_list;
					arc_eviction_list = buf;
					mutex_exit(&arc_eviction_mtx);
					rw_exit(&buf->b_lock);
				} else {
					rw_exit(&buf->b_lock);
					arc_buf_destroy(buf,
					    buf->b_data == stolen, TRUE);
				}
			}
			if (ab->b_datacnt == 0) {
				arc_change_state(evicted_state, ab, hash_lock);
				ASSERT(HDR_IN_HASH_TABLE(ab));
				ab->b_flags |= ARC_IN_HASH_TABLE;
				ab->b_flags &= ~ARC_BUF_AVAILABLE;
				DTRACE_PROBE1(arc__evict, arc_buf_hdr_t *, ab);
			}
			if (!have_lock)
				mutex_exit(hash_lock);
			if (bytes >= 0 && bytes_evicted >= bytes)
				break;
		} else {
			missed += 1;
		}
	}

	mutex_exit(&evicted_state->arcs_mtx);
	mutex_exit(&state->arcs_mtx);

	if (bytes_evicted < bytes)
		dprintf("only evicted %lld bytes from %x",
		    (longlong_t)bytes_evicted, state);

	if (skipped)
		ARCSTAT_INCR(arcstat_evict_skip, skipped);

	if (missed)
		ARCSTAT_INCR(arcstat_mutex_miss, missed);

	/*
	 * We have just evicted some date into the ghost state, make
	 * sure we also adjust the ghost state size if necessary.
	 */
	if (arc_no_grow &&
	    arc_mru_ghost->arcs_size + arc_mfu_ghost->arcs_size > arc_c) {
		int64_t mru_over = arc_anon->arcs_size + arc_mru->arcs_size +
		    arc_mru_ghost->arcs_size - arc_c;

		if (mru_over > 0 && arc_mru_ghost->arcs_lsize[type] > 0) {
			int64_t todelete =
			    MIN(arc_mru_ghost->arcs_lsize[type], mru_over);
			arc_evict_ghost(arc_mru_ghost, NULL, todelete);
		} else if (arc_mfu_ghost->arcs_lsize[type] > 0) {
			int64_t todelete = MIN(arc_mfu_ghost->arcs_lsize[type],
			    arc_mru_ghost->arcs_size +
			    arc_mfu_ghost->arcs_size - arc_c);
			arc_evict_ghost(arc_mfu_ghost, NULL, todelete);
		}
	}

	return (stolen);
}

/*
 * Remove buffers from list until we've removed the specified number of
 * bytes.  Destroy the buffers that are removed.
 */
static void
arc_evict_ghost(arc_state_t *state, uint64_t spa, int64_t bytes)
{
	arc_buf_hdr_t *ab, *ab_prev;
	list_t *list = &state->arcs_list[ARC_BUFC_DATA];
	kmutex_t *hash_lock;
	uint64_t bytes_deleted = 0;
	uint64_t bufs_skipped = 0;

	ASSERT(GHOST_STATE(state));
top:
	mutex_enter(&state->arcs_mtx);
	for (ab = list_tail(list); ab; ab = ab_prev) {
		ab_prev = list_prev(list, ab);
		if (spa && ab->b_spa != spa)
			continue;
		hash_lock = HDR_LOCK(ab);
		if (mutex_tryenter(hash_lock)) {
			ASSERT(!HDR_IO_IN_PROGRESS(ab));
			ASSERT(ab->b_buf == NULL);
			ARCSTAT_BUMP(arcstat_deleted);
			bytes_deleted += ab->b_size;

			if (ab->b_l2hdr != NULL) {
				/*
				 * This buffer is cached on the 2nd Level ARC;
				 * don't destroy the header.
				 */
				arc_change_state(arc_l2c_only, ab, hash_lock);
				mutex_exit(hash_lock);
			} else {
				arc_change_state(arc_anon, ab, hash_lock);
				mutex_exit(hash_lock);
				arc_hdr_destroy(ab);
			}

			DTRACE_PROBE1(arc__delete, arc_buf_hdr_t *, ab);
			if (bytes >= 0 && bytes_deleted >= bytes)
				break;
		} else {
			if (bytes < 0) {
				mutex_exit(&state->arcs_mtx);
				mutex_enter(hash_lock);
				mutex_exit(hash_lock);
				goto top;
			}
			bufs_skipped += 1;
		}
	}
	mutex_exit(&state->arcs_mtx);

	if (list == &state->arcs_list[ARC_BUFC_DATA] &&
	    (bytes < 0 || bytes_deleted < bytes)) {
		list = &state->arcs_list[ARC_BUFC_METADATA];
		goto top;
	}

	if (bufs_skipped) {
		ARCSTAT_INCR(arcstat_mutex_miss, bufs_skipped);
		ASSERT(bytes >= 0);
	}

	if (bytes_deleted < bytes)
		dprintf("only deleted %lld bytes from %p",
		    (longlong_t)bytes_deleted, state);
}

static void
arc_adjust(void)
{
	int64_t adjustment, delta;

	/*
	 * Adjust MRU size
	 */

	adjustment = MIN(arc_size - arc_c,
	    arc_anon->arcs_size + arc_mru->arcs_size + arc_meta_used - arc_p);

	if (adjustment > 0 && arc_mru->arcs_lsize[ARC_BUFC_DATA] > 0) {
		delta = MIN(arc_mru->arcs_lsize[ARC_BUFC_DATA], adjustment);
		(void) arc_evict(arc_mru, NULL, delta, FALSE, ARC_BUFC_DATA);
		adjustment -= delta;
	}

	if (adjustment > 0 && arc_mru->arcs_lsize[ARC_BUFC_METADATA] > 0) {
		delta = MIN(arc_mru->arcs_lsize[ARC_BUFC_METADATA], adjustment);
		(void) arc_evict(arc_mru, NULL, delta, FALSE,
		    ARC_BUFC_METADATA);
	}

	/*
	 * Adjust MFU size
	 */

	adjustment = arc_size - arc_c;

	if (adjustment > 0 && arc_mfu->arcs_lsize[ARC_BUFC_DATA] > 0) {
		delta = MIN(adjustment, arc_mfu->arcs_lsize[ARC_BUFC_DATA]);
		(void) arc_evict(arc_mfu, NULL, delta, FALSE, ARC_BUFC_DATA);
		adjustment -= delta;
	}

	if (adjustment > 0 && arc_mfu->arcs_lsize[ARC_BUFC_METADATA] > 0) {
		int64_t delta = MIN(adjustment,
		    arc_mfu->arcs_lsize[ARC_BUFC_METADATA]);
		(void) arc_evict(arc_mfu, NULL, delta, FALSE,
		    ARC_BUFC_METADATA);
	}

	/*
	 * Adjust ghost lists
	 */

	adjustment = arc_mru->arcs_size + arc_mru_ghost->arcs_size - arc_c;

	if (adjustment > 0 && arc_mru_ghost->arcs_size > 0) {
		delta = MIN(arc_mru_ghost->arcs_size, adjustment);
		arc_evict_ghost(arc_mru_ghost, NULL, delta);
	}

	adjustment =
	    arc_mru_ghost->arcs_size + arc_mfu_ghost->arcs_size - arc_c;

	if (adjustment > 0 && arc_mfu_ghost->arcs_size > 0) {
		delta = MIN(arc_mfu_ghost->arcs_size, adjustment);
		arc_evict_ghost(arc_mfu_ghost, NULL, delta);
	}
}

static void
arc_do_user_evicts(void)
{
	mutex_enter(&arc_eviction_mtx);
	while (arc_eviction_list != NULL) {
		arc_buf_t *buf = arc_eviction_list;
		arc_eviction_list = buf->b_next;
		rw_enter(&buf->b_lock, RW_WRITER);
		buf->b_hdr = NULL;
		rw_exit(&buf->b_lock);
		mutex_exit(&arc_eviction_mtx);

		if (buf->b_efunc != NULL)
			VERIFY(buf->b_efunc(buf) == 0);

		buf->b_efunc = NULL;
		buf->b_private = NULL;
		kmem_cache_free(buf_cache, buf);
		mutex_enter(&arc_eviction_mtx);
	}
	mutex_exit(&arc_eviction_mtx);
}

/*
 * Flush all *evictable* data from the cache for the given spa.
 * NOTE: this will not touch "active" (i.e. referenced) data.
 */
void
arc_flush(spa_t *spa)
{
	uint64_t guid = 0;

	if (spa)
		guid = spa_guid(spa);

	while (list_head(&arc_mru->arcs_list[ARC_BUFC_DATA])) {
		(void) arc_evict(arc_mru, guid, -1, FALSE, ARC_BUFC_DATA);
		if (spa)
			break;
	}
	while (list_head(&arc_mru->arcs_list[ARC_BUFC_METADATA])) {
		(void) arc_evict(arc_mru, guid, -1, FALSE, ARC_BUFC_METADATA);
		if (spa)
			break;
	}
	while (list_head(&arc_mfu->arcs_list[ARC_BUFC_DATA])) {
		(void) arc_evict(arc_mfu, guid, -1, FALSE, ARC_BUFC_DATA);
		if (spa)
			break;
	}
	while (list_head(&arc_mfu->arcs_list[ARC_BUFC_METADATA])) {
		(void) arc_evict(arc_mfu, guid, -1, FALSE, ARC_BUFC_METADATA);
		if (spa)
			break;
	}

	arc_evict_ghost(arc_mru_ghost, guid, -1);
	arc_evict_ghost(arc_mfu_ghost, guid, -1);

	mutex_enter(&arc_reclaim_thr_lock);
	arc_do_user_evicts();
	mutex_exit(&arc_reclaim_thr_lock);
	ASSERT(spa || arc_eviction_list == NULL);
}

void
arc_shrink(void)
{
	if (arc_c > arc_c_min) {
		uint64_t to_free;

#ifdef _KERNEL
		to_free = MAX(arc_c >> arc_shrink_shift, ptob(needfree));
#else
		to_free = arc_c >> arc_shrink_shift;
#endif
		if (arc_c > arc_c_min + to_free)
			atomic_add_64(&arc_c, -to_free);
		else
			arc_c = arc_c_min;

		atomic_add_64(&arc_p, -(arc_p >> arc_shrink_shift));
		if (arc_c > arc_size)
			arc_c = MAX(arc_size, arc_c_min);
		if (arc_p > arc_c)
			arc_p = (arc_c >> 1);
		ASSERT(arc_c >= arc_c_min);
		ASSERT((int64_t)arc_p >= 0);
	}

	if (arc_size > arc_c)
		arc_adjust();
}

static int
arc_reclaim_needed(void)
{
	uint64_t extra;

#ifdef _KERNEL

	if (needfree)
		return (1);

	/*
	 * take 'desfree' extra pages, so we reclaim sooner, rather than later
	 */
	extra = desfree;

	/*
	 * check that we're out of range of the pageout scanner.  It starts to
	 * schedule paging if freemem is less than lotsfree and needfree.
	 * lotsfree is the high-water mark for pageout, and needfree is the
	 * number of needed free pages.  We add extra pages here to make sure
	 * the scanner doesn't start up while we're freeing memory.
	 */
	if (freemem < lotsfree + needfree + extra)
		return (1);

	/*
	 * check to make sure that swapfs has enough space so that anon
	 * reservations can still succeed. anon_resvmem() checks that the
	 * availrmem is greater than swapfs_minfree, and the number of reserved
	 * swap pages.  We also add a bit of extra here just to prevent
	 * circumstances from getting really dire.
	 */
	if (availrmem < swapfs_minfree + swapfs_reserve + extra)
		return (1);

#if defined(__i386)
	/*
	 * If we're on an i386 platform, it's possible that we'll exhaust the
	 * kernel heap space before we ever run out of available physical
	 * memory.  Most checks of the size of the heap_area compare against
	 * tune.t_minarmem, which is the minimum available real memory that we
	 * can have in the system.  However, this is generally fixed at 25 pages
	 * which is so low that it's useless.  In this comparison, we seek to
	 * calculate the total heap-size, and reclaim if more than 3/4ths of the
	 * heap is allocated.  (Or, in the calculation, if less than 1/4th is
	 * free)
	 */
	if (btop(vmem_size(heap_arena, VMEM_FREE)) <
	    (btop(vmem_size(heap_arena, VMEM_FREE | VMEM_ALLOC)) >> 2))
		return (1);
#endif

#else
	if (spa_get_random(100) == 0)
		return (1);
#endif
	return (0);
}

static void
arc_kmem_reap_now(arc_reclaim_strategy_t strat)
{
	size_t			i;
	kmem_cache_t		*prev_cache = NULL;
	kmem_cache_t		*prev_data_cache = NULL;
	extern kmem_cache_t	*zio_buf_cache[];
	extern kmem_cache_t	*zio_data_buf_cache[];

#ifdef _KERNEL
	if (arc_meta_used >= arc_meta_limit) {
		/*
		 * We are exceeding our meta-data cache limit.
		 * Purge some DNLC entries to release holds on meta-data.
		 */
		dnlc_reduce_cache((void *)(uintptr_t)arc_reduce_dnlc_percent);
	}
#if defined(__i386)
	/*
	 * Reclaim unused memory from all kmem caches.
	 */
	kmem_reap();
#endif
#endif

	/*
	 * An aggressive reclamation will shrink the cache size as well as
	 * reap free buffers from the arc kmem caches.
	 */
	if (strat == ARC_RECLAIM_AGGR)
		arc_shrink();

	for (i = 0; i < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; i++) {
		if (zio_buf_cache[i] != prev_cache) {
			prev_cache = zio_buf_cache[i];
			kmem_cache_reap_now(zio_buf_cache[i]);
		}
		if (zio_data_buf_cache[i] != prev_data_cache) {
			prev_data_cache = zio_data_buf_cache[i];
			kmem_cache_reap_now(zio_data_buf_cache[i]);
		}
	}
	kmem_cache_reap_now(buf_cache);
	kmem_cache_reap_now(hdr_cache);
}

static void
arc_reclaim_thread(void)
{
	clock_t			growtime = 0;
	arc_reclaim_strategy_t	last_reclaim = ARC_RECLAIM_CONS;
	callb_cpr_t		cpr;

	CALLB_CPR_INIT(&cpr, &arc_reclaim_thr_lock, callb_generic_cpr, FTAG);

	mutex_enter(&arc_reclaim_thr_lock);
	while (arc_thread_exit == 0) {
		if (arc_reclaim_needed()) {

			if (arc_no_grow) {
				if (last_reclaim == ARC_RECLAIM_CONS) {
					last_reclaim = ARC_RECLAIM_AGGR;
				} else {
					last_reclaim = ARC_RECLAIM_CONS;
				}
			} else {
				arc_no_grow = TRUE;
				last_reclaim = ARC_RECLAIM_AGGR;
				membar_producer();
			}

			/* reset the growth delay for every reclaim */
			growtime = lbolt + (arc_grow_retry * hz);

			arc_kmem_reap_now(last_reclaim);
			arc_warm = B_TRUE;

		} else if (arc_no_grow && lbolt >= growtime) {
			arc_no_grow = FALSE;
		}

		if (2 * arc_c < arc_size +
		    arc_mru_ghost->arcs_size + arc_mfu_ghost->arcs_size)
			arc_adjust();

		if (arc_eviction_list != NULL)
			arc_do_user_evicts();

		/* block until needed, or one second, whichever is shorter */
		CALLB_CPR_SAFE_BEGIN(&cpr);
		(void) cv_timedwait(&arc_reclaim_thr_cv,
		    &arc_reclaim_thr_lock, (lbolt + hz));
		CALLB_CPR_SAFE_END(&cpr, &arc_reclaim_thr_lock);
	}

	arc_thread_exit = 0;
	cv_broadcast(&arc_reclaim_thr_cv);
	CALLB_CPR_EXIT(&cpr);		/* drops arc_reclaim_thr_lock */
	thread_exit();
}

/*
 * Adapt arc info given the number of bytes we are trying to add and
 * the state that we are comming from.  This function is only called
 * when we are adding new content to the cache.
 */
static void
arc_adapt(int bytes, arc_state_t *state)
{
	int mult;
	uint64_t arc_p_min = (arc_c >> arc_p_min_shift);

	if (state == arc_l2c_only)
		return;

	ASSERT(bytes > 0);
	/*
	 * Adapt the target size of the MRU list:
	 *	- if we just hit in the MRU ghost list, then increase
	 *	  the target size of the MRU list.
	 *	- if we just hit in the MFU ghost list, then increase
	 *	  the target size of the MFU list by decreasing the
	 *	  target size of the MRU list.
	 */
	if (state == arc_mru_ghost) {
		mult = ((arc_mru_ghost->arcs_size >= arc_mfu_ghost->arcs_size) ?
		    1 : (arc_mfu_ghost->arcs_size/arc_mru_ghost->arcs_size));

		arc_p = MIN(arc_c - arc_p_min, arc_p + bytes * mult);
	} else if (state == arc_mfu_ghost) {
		uint64_t delta;

		mult = ((arc_mfu_ghost->arcs_size >= arc_mru_ghost->arcs_size) ?
		    1 : (arc_mru_ghost->arcs_size/arc_mfu_ghost->arcs_size));

		delta = MIN(bytes * mult, arc_p);
		arc_p = MAX(arc_p_min, arc_p - delta);
	}
	ASSERT((int64_t)arc_p >= 0);

	if (arc_reclaim_needed()) {
		cv_signal(&arc_reclaim_thr_cv);
		return;
	}

	if (arc_no_grow)
		return;

	if (arc_c >= arc_c_max)
		return;

	/*
	 * If we're within (2 * maxblocksize) bytes of the target
	 * cache size, increment the target cache size
	 */
	if (arc_size > arc_c - (2ULL << SPA_MAXBLOCKSHIFT)) {
		atomic_add_64(&arc_c, (int64_t)bytes);
		if (arc_c > arc_c_max)
			arc_c = arc_c_max;
		else if (state == arc_anon)
			atomic_add_64(&arc_p, (int64_t)bytes);
		if (arc_p > arc_c)
			arc_p = arc_c;
	}
	ASSERT((int64_t)arc_p >= 0);
}

/*
 * Check if the cache has reached its limits and eviction is required
 * prior to insert.
 */
static int
arc_evict_needed(arc_buf_contents_t type)
{
	if (type == ARC_BUFC_METADATA && arc_meta_used >= arc_meta_limit)
		return (1);

#ifdef _KERNEL
	/*
	 * If zio data pages are being allocated out of a separate heap segment,
	 * then enforce that the size of available vmem for this area remains
	 * above about 1/32nd free.
	 */
	if (type == ARC_BUFC_DATA && zio_arena != NULL &&
	    vmem_size(zio_arena, VMEM_FREE) <
	    (vmem_size(zio_arena, VMEM_ALLOC) >> 5))
		return (1);
#endif

	if (arc_reclaim_needed())
		return (1);

	return (arc_size > arc_c);
}

/*
 * The buffer, supplied as the first argument, needs a data block.
 * So, if we are at cache max, determine which cache should be victimized.
 * We have the following cases:
 *
 * 1. Insert for MRU, p > sizeof(arc_anon + arc_mru) ->
 * In this situation if we're out of space, but the resident size of the MFU is
 * under the limit, victimize the MFU cache to satisfy this insertion request.
 *
 * 2. Insert for MRU, p <= sizeof(arc_anon + arc_mru) ->
 * Here, we've used up all of the available space for the MRU, so we need to
 * evict from our own cache instead.  Evict from the set of resident MRU
 * entries.
 *
 * 3. Insert for MFU (c - p) > sizeof(arc_mfu) ->
 * c minus p represents the MFU space in the cache, since p is the size of the
 * cache that is dedicated to the MRU.  In this situation there's still space on
 * the MFU side, so the MRU side needs to be victimized.
 *
 * 4. Insert for MFU (c - p) < sizeof(arc_mfu) ->
 * MFU's resident set is consuming more space than it has been allotted.  In
 * this situation, we must victimize our own cache, the MFU, for this insertion.
 */
static void
arc_get_data_buf(arc_buf_t *buf)
{
	arc_state_t		*state = buf->b_hdr->b_state;
	uint64_t		size = buf->b_hdr->b_size;
	arc_buf_contents_t	type = buf->b_hdr->b_type;

	arc_adapt(size, state);

	/*
	 * We have not yet reached cache maximum size,
	 * just allocate a new buffer.
	 */
	if (!arc_evict_needed(type)) {
		if (type == ARC_BUFC_METADATA) {
			buf->b_data = zio_buf_alloc(size);
			arc_space_consume(size, ARC_SPACE_DATA);
		} else {
			ASSERT(type == ARC_BUFC_DATA);
			buf->b_data = zio_data_buf_alloc(size);
			ARCSTAT_INCR(arcstat_data_size, size);
			atomic_add_64(&arc_size, size);
		}
		goto out;
	}

	/*
	 * If we are prefetching from the mfu ghost list, this buffer
	 * will end up on the mru list; so steal space from there.
	 */
	if (state == arc_mfu_ghost)
		state = buf->b_hdr->b_flags & ARC_PREFETCH ? arc_mru : arc_mfu;
	else if (state == arc_mru_ghost)
		state = arc_mru;

	if (state == arc_mru || state == arc_anon) {
		uint64_t mru_used = arc_anon->arcs_size + arc_mru->arcs_size;
		state = (arc_mfu->arcs_lsize[type] >= size &&
		    arc_p > mru_used) ? arc_mfu : arc_mru;
	} else {
		/* MFU cases */
		uint64_t mfu_space = arc_c - arc_p;
		state =  (arc_mru->arcs_lsize[type] >= size &&
		    mfu_space > arc_mfu->arcs_size) ? arc_mru : arc_mfu;
	}
	if ((buf->b_data = arc_evict(state, NULL, size, TRUE, type)) == NULL) {
		if (type == ARC_BUFC_METADATA) {
			buf->b_data = zio_buf_alloc(size);
			arc_space_consume(size, ARC_SPACE_DATA);
		} else {
			ASSERT(type == ARC_BUFC_DATA);
			buf->b_data = zio_data_buf_alloc(size);
			ARCSTAT_INCR(arcstat_data_size, size);
			atomic_add_64(&arc_size, size);
		}
		ARCSTAT_BUMP(arcstat_recycle_miss);
	}
	ASSERT(buf->b_data != NULL);
out:
	/*
	 * Update the state size.  Note that ghost states have a
	 * "ghost size" and so don't need to be updated.
	 */
	if (!GHOST_STATE(buf->b_hdr->b_state)) {
		arc_buf_hdr_t *hdr = buf->b_hdr;

		atomic_add_64(&hdr->b_state->arcs_size, size);
		if (list_link_active(&hdr->b_arc_node)) {
			ASSERT(refcount_is_zero(&hdr->b_refcnt));
			atomic_add_64(&hdr->b_state->arcs_lsize[type], size);
		}
		/*
		 * If we are growing the cache, and we are adding anonymous
		 * data, and we have outgrown arc_p, update arc_p
		 */
		if (arc_size < arc_c && hdr->b_state == arc_anon &&
		    arc_anon->arcs_size + arc_mru->arcs_size > arc_p)
			arc_p = MIN(arc_c, arc_p + size);
	}
}

/*
 * This routine is called whenever a buffer is accessed.
 * NOTE: the hash lock is dropped in this function.
 */
static void
arc_access(arc_buf_hdr_t *buf, kmutex_t *hash_lock)
{
	ASSERT(MUTEX_HELD(hash_lock));

	if (buf->b_state == arc_anon) {
		/*
		 * This buffer is not in the cache, and does not
		 * appear in our "ghost" list.  Add the new buffer
		 * to the MRU state.
		 */

		ASSERT(buf->b_arc_access == 0);
		buf->b_arc_access = lbolt;
		DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, buf);
		arc_change_state(arc_mru, buf, hash_lock);

	} else if (buf->b_state == arc_mru) {
		/*
		 * If this buffer is here because of a prefetch, then either:
		 * - clear the flag if this is a "referencing" read
		 *   (any subsequent access will bump this into the MFU state).
		 * or
		 * - move the buffer to the head of the list if this is
		 *   another prefetch (to make it less likely to be evicted).
		 */
		if ((buf->b_flags & ARC_PREFETCH) != 0) {
			if (refcount_count(&buf->b_refcnt) == 0) {
				ASSERT(list_link_active(&buf->b_arc_node));
			} else {
				buf->b_flags &= ~ARC_PREFETCH;
				ARCSTAT_BUMP(arcstat_mru_hits);
			}
			buf->b_arc_access = lbolt;
			return;
		}

		/*
		 * This buffer has been "accessed" only once so far,
		 * but it is still in the cache. Move it to the MFU
		 * state.
		 */
		if (lbolt > buf->b_arc_access + ARC_MINTIME) {
			/*
			 * More than 125ms have passed since we
			 * instantiated this buffer.  Move it to the
			 * most frequently used state.
			 */
			buf->b_arc_access = lbolt;
			DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
			arc_change_state(arc_mfu, buf, hash_lock);
		}
		ARCSTAT_BUMP(arcstat_mru_hits);
	} else if (buf->b_state == arc_mru_ghost) {
		arc_state_t	*new_state;
		/*
		 * This buffer has been "accessed" recently, but
		 * was evicted from the cache.  Move it to the
		 * MFU state.
		 */

		if (buf->b_flags & ARC_PREFETCH) {
			new_state = arc_mru;
			if (refcount_count(&buf->b_refcnt) > 0)
				buf->b_flags &= ~ARC_PREFETCH;
			DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, buf);
		} else {
			new_state = arc_mfu;
			DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
		}

		buf->b_arc_access = lbolt;
		arc_change_state(new_state, buf, hash_lock);

		ARCSTAT_BUMP(arcstat_mru_ghost_hits);
	} else if (buf->b_state == arc_mfu) {
		/*
		 * This buffer has been accessed more than once and is
		 * still in the cache.  Keep it in the MFU state.
		 *
		 * NOTE: an add_reference() that occurred when we did
		 * the arc_read() will have kicked this off the list.
		 * If it was a prefetch, we will explicitly move it to
		 * the head of the list now.
		 */
		if ((buf->b_flags & ARC_PREFETCH) != 0) {
			ASSERT(refcount_count(&buf->b_refcnt) == 0);
			ASSERT(list_link_active(&buf->b_arc_node));
		}
		ARCSTAT_BUMP(arcstat_mfu_hits);
		buf->b_arc_access = lbolt;
	} else if (buf->b_state == arc_mfu_ghost) {
		arc_state_t	*new_state = arc_mfu;
		/*
		 * This buffer has been accessed more than once but has
		 * been evicted from the cache.  Move it back to the
		 * MFU state.
		 */

		if (buf->b_flags & ARC_PREFETCH) {
			/*
			 * This is a prefetch access...
			 * move this block back to the MRU state.
			 */
			ASSERT3U(refcount_count(&buf->b_refcnt), ==, 0);
			new_state = arc_mru;
		}

		buf->b_arc_access = lbolt;
		DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
		arc_change_state(new_state, buf, hash_lock);

		ARCSTAT_BUMP(arcstat_mfu_ghost_hits);
	} else if (buf->b_state == arc_l2c_only) {
		/*
		 * This buffer is on the 2nd Level ARC.
		 */

		buf->b_arc_access = lbolt;
		DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, buf);
		arc_change_state(arc_mfu, buf, hash_lock);
	} else {
		ASSERT(!"invalid arc state");
	}
}

/* a generic arc_done_func_t which you can use */
/* ARGSUSED */
void
arc_bcopy_func(zio_t *zio, arc_buf_t *buf, void *arg)
{
	bcopy(buf->b_data, arg, buf->b_hdr->b_size);
	VERIFY(arc_buf_remove_ref(buf, arg) == 1);
}

/* a generic arc_done_func_t */
void
arc_getbuf_func(zio_t *zio, arc_buf_t *buf, void *arg)
{
	arc_buf_t **bufp = arg;
	if (zio && zio->io_error) {
		VERIFY(arc_buf_remove_ref(buf, arg) == 1);
		*bufp = NULL;
	} else {
		*bufp = buf;
	}
}

static void
arc_read_done(zio_t *zio)
{
	arc_buf_hdr_t	*hdr, *found;
	arc_buf_t	*buf;
	arc_buf_t	*abuf;	/* buffer we're assigning to callback */
	kmutex_t	*hash_lock;
	arc_callback_t	*callback_list, *acb;
	int		freeable = FALSE;

	buf = zio->io_private;
	hdr = buf->b_hdr;

	/*
	 * The hdr was inserted into hash-table and removed from lists
	 * prior to starting I/O.  We should find this header, since
	 * it's in the hash table, and it should be legit since it's
	 * not possible to evict it during the I/O.  The only possible
	 * reason for it not to be found is if we were freed during the
	 * read.
	 */
	found = buf_hash_find(hdr->b_spa, &hdr->b_dva, hdr->b_birth,
	    &hash_lock);

	ASSERT((found == NULL && HDR_FREED_IN_READ(hdr) && hash_lock == NULL) ||
	    (found == hdr && DVA_EQUAL(&hdr->b_dva, BP_IDENTITY(zio->io_bp))) ||
	    (found == hdr && HDR_L2_READING(hdr)));

	hdr->b_flags &= ~ARC_L2_EVICTED;
	if (l2arc_noprefetch && (hdr->b_flags & ARC_PREFETCH))
		hdr->b_flags &= ~ARC_L2CACHE;

	/* byteswap if necessary */
	callback_list = hdr->b_acb;
	ASSERT(callback_list != NULL);
	if (BP_SHOULD_BYTESWAP(zio->io_bp)) {
		arc_byteswap_func_t *func = BP_GET_LEVEL(zio->io_bp) > 0 ?
		    byteswap_uint64_array :
		    dmu_ot[BP_GET_TYPE(zio->io_bp)].ot_byteswap;
		func(buf->b_data, hdr->b_size);
	}

	arc_cksum_compute(buf, B_FALSE);

	/* create copies of the data buffer for the callers */
	abuf = buf;
	for (acb = callback_list; acb; acb = acb->acb_next) {
		if (acb->acb_done) {
			if (abuf == NULL)
				abuf = arc_buf_clone(buf);
			acb->acb_buf = abuf;
			abuf = NULL;
		}
	}
	hdr->b_acb = NULL;
	hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
	ASSERT(!HDR_BUF_AVAILABLE(hdr));
	if (abuf == buf)
		hdr->b_flags |= ARC_BUF_AVAILABLE;

	ASSERT(refcount_is_zero(&hdr->b_refcnt) || callback_list != NULL);

	if (zio->io_error != 0) {
		hdr->b_flags |= ARC_IO_ERROR;
		if (hdr->b_state != arc_anon)
			arc_change_state(arc_anon, hdr, hash_lock);
		if (HDR_IN_HASH_TABLE(hdr))
			buf_hash_remove(hdr);
		freeable = refcount_is_zero(&hdr->b_refcnt);
	}

	/*
	 * Broadcast before we drop the hash_lock to avoid the possibility
	 * that the hdr (and hence the cv) might be freed before we get to
	 * the cv_broadcast().
	 */
	cv_broadcast(&hdr->b_cv);

	if (hash_lock) {
		/*
		 * Only call arc_access on anonymous buffers.  This is because
		 * if we've issued an I/O for an evicted buffer, we've already
		 * called arc_access (to prevent any simultaneous readers from
		 * getting confused).
		 */
		if (zio->io_error == 0 && hdr->b_state == arc_anon)
			arc_access(hdr, hash_lock);
		mutex_exit(hash_lock);
	} else {
		/*
		 * This block was freed while we waited for the read to
		 * complete.  It has been removed from the hash table and
		 * moved to the anonymous state (so that it won't show up
		 * in the cache).
		 */
		ASSERT3P(hdr->b_state, ==, arc_anon);
		freeable = refcount_is_zero(&hdr->b_refcnt);
	}

	/* execute each callback and free its structure */
	while ((acb = callback_list) != NULL) {
		if (acb->acb_done)
			acb->acb_done(zio, acb->acb_buf, acb->acb_private);

		if (acb->acb_zio_dummy != NULL) {
			acb->acb_zio_dummy->io_error = zio->io_error;
			zio_nowait(acb->acb_zio_dummy);
		}

		callback_list = acb->acb_next;
		kmem_free(acb, sizeof (arc_callback_t));
	}

	if (freeable)
		arc_hdr_destroy(hdr);
}

/*
 * "Read" the block block at the specified DVA (in bp) via the
 * cache.  If the block is found in the cache, invoke the provided
 * callback immediately and return.  Note that the `zio' parameter
 * in the callback will be NULL in this case, since no IO was
 * required.  If the block is not in the cache pass the read request
 * on to the spa with a substitute callback function, so that the
 * requested block will be added to the cache.
 *
 * If a read request arrives for a block that has a read in-progress,
 * either wait for the in-progress read to complete (and return the
 * results); or, if this is a read with a "done" func, add a record
 * to the read to invoke the "done" func when the read completes,
 * and return; or just return.
 *
 * arc_read_done() will invoke all the requested "done" functions
 * for readers of this block.
 *
 * Normal callers should use arc_read and pass the arc buffer and offset
 * for the bp.  But if you know you don't need locking, you can use
 * arc_read_bp.
 */
int
arc_read(zio_t *pio, spa_t *spa, blkptr_t *bp, arc_buf_t *pbuf,
    arc_done_func_t *done, void *private, int priority, int zio_flags,
    uint32_t *arc_flags, const zbookmark_t *zb)
{
	int err;
	arc_buf_hdr_t *hdr = pbuf->b_hdr;

	ASSERT(!refcount_is_zero(&pbuf->b_hdr->b_refcnt));
	ASSERT3U((char *)bp - (char *)pbuf->b_data, <, pbuf->b_hdr->b_size);
	rw_enter(&pbuf->b_lock, RW_READER);

	err = arc_read_nolock(pio, spa, bp, done, private, priority,
	    zio_flags, arc_flags, zb);

	ASSERT3P(hdr, ==, pbuf->b_hdr);
	rw_exit(&pbuf->b_lock);
	return (err);
}

int
arc_read_nolock(zio_t *pio, spa_t *spa, blkptr_t *bp,
    arc_done_func_t *done, void *private, int priority, int zio_flags,
    uint32_t *arc_flags, const zbookmark_t *zb)
{
	arc_buf_hdr_t *hdr;
	arc_buf_t *buf;
	kmutex_t *hash_lock;
	zio_t *rzio;
	uint64_t guid = spa_guid(spa);

top:
	hdr = buf_hash_find(guid, BP_IDENTITY(bp), bp->blk_birth, &hash_lock);
	if (hdr && hdr->b_datacnt > 0) {

		*arc_flags |= ARC_CACHED;

		if (HDR_IO_IN_PROGRESS(hdr)) {

			if (*arc_flags & ARC_WAIT) {
				cv_wait(&hdr->b_cv, hash_lock);
				mutex_exit(hash_lock);
				goto top;
			}
			ASSERT(*arc_flags & ARC_NOWAIT);

			if (done) {
				arc_callback_t	*acb = NULL;

				acb = kmem_zalloc(sizeof (arc_callback_t),
				    KM_SLEEP);
				acb->acb_done = done;
				acb->acb_private = private;
				if (pio != NULL)
					acb->acb_zio_dummy = zio_null(pio,
					    spa, NULL, NULL, NULL, zio_flags);

				ASSERT(acb->acb_done != NULL);
				acb->acb_next = hdr->b_acb;
				hdr->b_acb = acb;
				add_reference(hdr, hash_lock, private);
				mutex_exit(hash_lock);
				return (0);
			}
			mutex_exit(hash_lock);
			return (0);
		}

		ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);

		if (done) {
			add_reference(hdr, hash_lock, private);
			/*
			 * If this block is already in use, create a new
			 * copy of the data so that we will be guaranteed
			 * that arc_release() will always succeed.
			 */
			buf = hdr->b_buf;
			ASSERT(buf);
			ASSERT(buf->b_data);
			if (HDR_BUF_AVAILABLE(hdr)) {
				ASSERT(buf->b_efunc == NULL);
				hdr->b_flags &= ~ARC_BUF_AVAILABLE;
			} else {
				buf = arc_buf_clone(buf);
			}
		} else if (*arc_flags & ARC_PREFETCH &&
		    refcount_count(&hdr->b_refcnt) == 0) {
			hdr->b_flags |= ARC_PREFETCH;
		}
		DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
		arc_access(hdr, hash_lock);
		if (*arc_flags & ARC_L2CACHE)
			hdr->b_flags |= ARC_L2CACHE;
		mutex_exit(hash_lock);
		ARCSTAT_BUMP(arcstat_hits);
		ARCSTAT_CONDSTAT(!(hdr->b_flags & ARC_PREFETCH),
		    demand, prefetch, hdr->b_type != ARC_BUFC_METADATA,
		    data, metadata, hits);

		if (done)
			done(NULL, buf, private);
	} else {
		uint64_t size = BP_GET_LSIZE(bp);
		arc_callback_t	*acb;
		vdev_t *vd = NULL;
		daddr_t addr;
		boolean_t devw = B_FALSE;

		if (hdr == NULL) {
			/* this block is not in the cache */
			arc_buf_hdr_t	*exists;
			arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
			buf = arc_buf_alloc(spa, size, private, type);
			hdr = buf->b_hdr;
			hdr->b_dva = *BP_IDENTITY(bp);
			hdr->b_birth = bp->blk_birth;
			hdr->b_cksum0 = bp->blk_cksum.zc_word[0];
			exists = buf_hash_insert(hdr, &hash_lock);
			if (exists) {
				/* somebody beat us to the hash insert */
				mutex_exit(hash_lock);
				bzero(&hdr->b_dva, sizeof (dva_t));
				hdr->b_birth = 0;
				hdr->b_cksum0 = 0;
				(void) arc_buf_remove_ref(buf, private);
				goto top; /* restart the IO request */
			}
			/* if this is a prefetch, we don't have a reference */
			if (*arc_flags & ARC_PREFETCH) {
				(void) remove_reference(hdr, hash_lock,
				    private);
				hdr->b_flags |= ARC_PREFETCH;
			}
			if (*arc_flags & ARC_L2CACHE)
				hdr->b_flags |= ARC_L2CACHE;
			if (BP_GET_LEVEL(bp) > 0)
				hdr->b_flags |= ARC_INDIRECT;
		} else {
			/* this block is in the ghost cache */
			ASSERT(GHOST_STATE(hdr->b_state));
			ASSERT(!HDR_IO_IN_PROGRESS(hdr));
			ASSERT3U(refcount_count(&hdr->b_refcnt), ==, 0);
			ASSERT(hdr->b_buf == NULL);

			/* if this is a prefetch, we don't have a reference */
			if (*arc_flags & ARC_PREFETCH)
				hdr->b_flags |= ARC_PREFETCH;
			else
				add_reference(hdr, hash_lock, private);
			if (*arc_flags & ARC_L2CACHE)
				hdr->b_flags |= ARC_L2CACHE;
			buf = kmem_cache_alloc(buf_cache, KM_PUSHPAGE);
			buf->b_hdr = hdr;
			buf->b_data = NULL;
			buf->b_efunc = NULL;
			buf->b_private = NULL;
			buf->b_next = NULL;
			hdr->b_buf = buf;
			arc_get_data_buf(buf);
			ASSERT(hdr->b_datacnt == 0);
			hdr->b_datacnt = 1;

		}

		acb = kmem_zalloc(sizeof (arc_callback_t), KM_SLEEP);
		acb->acb_done = done;
		acb->acb_private = private;

		ASSERT(hdr->b_acb == NULL);
		hdr->b_acb = acb;
		hdr->b_flags |= ARC_IO_IN_PROGRESS;

		/*
		 * If the buffer has been evicted, migrate it to a present state
		 * before issuing the I/O.  Once we drop the hash-table lock,
		 * the header will be marked as I/O in progress and have an
		 * attached buffer.  At this point, anybody who finds this
		 * buffer ought to notice that it's legit but has a pending I/O.
		 */

		if (GHOST_STATE(hdr->b_state))
			arc_access(hdr, hash_lock);

		if (HDR_L2CACHE(hdr) && hdr->b_l2hdr != NULL &&
		    (vd = hdr->b_l2hdr->b_dev->l2ad_vdev) != NULL) {
			devw = hdr->b_l2hdr->b_dev->l2ad_writing;
			addr = hdr->b_l2hdr->b_daddr;
			/*
			 * Lock out device removal.
			 */
			if (vdev_is_dead(vd) ||
			    !spa_config_tryenter(spa, SCL_L2ARC, vd, RW_READER))
				vd = NULL;
		}

		mutex_exit(hash_lock);

		ASSERT3U(hdr->b_size, ==, size);
		DTRACE_PROBE3(arc__miss, blkptr_t *, bp, uint64_t, size,
		    zbookmark_t *, zb);
		ARCSTAT_BUMP(arcstat_misses);
		ARCSTAT_CONDSTAT(!(hdr->b_flags & ARC_PREFETCH),
		    demand, prefetch, hdr->b_type != ARC_BUFC_METADATA,
		    data, metadata, misses);

		if (vd != NULL && l2arc_ndev != 0 && !(l2arc_norw && devw)) {
			/*
			 * Read from the L2ARC if the following are true:
			 * 1. The L2ARC vdev was previously cached.
			 * 2. This buffer still has L2ARC metadata.
			 * 3. This buffer isn't currently writing to the L2ARC.
			 * 4. The L2ARC entry wasn't evicted, which may
			 *    also have invalidated the vdev.
			 * 5. This isn't prefetch and l2arc_noprefetch is set.
			 */
			if (hdr->b_l2hdr != NULL &&
			    !HDR_L2_WRITING(hdr) && !HDR_L2_EVICTED(hdr) &&
			    !(l2arc_noprefetch && HDR_PREFETCH(hdr))) {
				l2arc_read_callback_t *cb;

				DTRACE_PROBE1(l2arc__hit, arc_buf_hdr_t *, hdr);
				ARCSTAT_BUMP(arcstat_l2_hits);

				cb = kmem_zalloc(sizeof (l2arc_read_callback_t),
				    KM_SLEEP);
				cb->l2rcb_buf = buf;
				cb->l2rcb_spa = spa;
				cb->l2rcb_bp = *bp;
				cb->l2rcb_zb = *zb;
				cb->l2rcb_flags = zio_flags;

				/*
				 * l2arc read.  The SCL_L2ARC lock will be
				 * released by l2arc_read_done().
				 */
				rzio = zio_read_phys(pio, vd, addr, size,
				    buf->b_data, ZIO_CHECKSUM_OFF,
				    l2arc_read_done, cb, priority, zio_flags |
				    ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
				    ZIO_FLAG_DONT_PROPAGATE |
				    ZIO_FLAG_DONT_RETRY, B_FALSE);
				DTRACE_PROBE2(l2arc__read, vdev_t *, vd,
				    zio_t *, rzio);
				ARCSTAT_INCR(arcstat_l2_read_bytes, size);

				if (*arc_flags & ARC_NOWAIT) {
					zio_nowait(rzio);
					return (0);
				}

				ASSERT(*arc_flags & ARC_WAIT);
				if (zio_wait(rzio) == 0)
					return (0);

				/* l2arc read error; goto zio_read() */
			} else {
				DTRACE_PROBE1(l2arc__miss,
				    arc_buf_hdr_t *, hdr);
				ARCSTAT_BUMP(arcstat_l2_misses);
				if (HDR_L2_WRITING(hdr))
					ARCSTAT_BUMP(arcstat_l2_rw_clash);
				spa_config_exit(spa, SCL_L2ARC, vd);
			}
		} else {
			if (vd != NULL)
				spa_config_exit(spa, SCL_L2ARC, vd);
			if (l2arc_ndev != 0) {
				DTRACE_PROBE1(l2arc__miss,
				    arc_buf_hdr_t *, hdr);
				ARCSTAT_BUMP(arcstat_l2_misses);
			}
		}

		rzio = zio_read(pio, spa, bp, buf->b_data, size,
		    arc_read_done, buf, priority, zio_flags, zb);

		if (*arc_flags & ARC_WAIT)
			return (zio_wait(rzio));

		ASSERT(*arc_flags & ARC_NOWAIT);
		zio_nowait(rzio);
	}
	return (0);
}

/*
 * arc_read() variant to support pool traversal.  If the block is already
 * in the ARC, make a copy of it; otherwise, the caller will do the I/O.
 * The idea is that we don't want pool traversal filling up memory, but
 * if the ARC already has the data anyway, we shouldn't pay for the I/O.
 */
int
arc_tryread(spa_t *spa, blkptr_t *bp, void *data)
{
	arc_buf_hdr_t *hdr;
	kmutex_t *hash_mtx;
	uint64_t guid = spa_guid(spa);
	int rc = 0;

	hdr = buf_hash_find(guid, BP_IDENTITY(bp), bp->blk_birth, &hash_mtx);

	if (hdr && hdr->b_datacnt > 0 && !HDR_IO_IN_PROGRESS(hdr)) {
		arc_buf_t *buf = hdr->b_buf;

		ASSERT(buf);
		while (buf->b_data == NULL) {
			buf = buf->b_next;
			ASSERT(buf);
		}
		bcopy(buf->b_data, data, hdr->b_size);
	} else {
		rc = ENOENT;
	}

	if (hash_mtx)
		mutex_exit(hash_mtx);

	return (rc);
}

void
arc_set_callback(arc_buf_t *buf, arc_evict_func_t *func, void *private)
{
	ASSERT(buf->b_hdr != NULL);
	ASSERT(buf->b_hdr->b_state != arc_anon);
	ASSERT(!refcount_is_zero(&buf->b_hdr->b_refcnt) || func == NULL);
	buf->b_efunc = func;
	buf->b_private = private;
}

/*
 * This is used by the DMU to let the ARC know that a buffer is
 * being evicted, so the ARC should clean up.  If this arc buf
 * is not yet in the evicted state, it will be put there.
 */
int
arc_buf_evict(arc_buf_t *buf)
{
	arc_buf_hdr_t *hdr;
	kmutex_t *hash_lock;
	arc_buf_t **bufp;

	rw_enter(&buf->b_lock, RW_WRITER);
	hdr = buf->b_hdr;
	if (hdr == NULL) {
		/*
		 * We are in arc_do_user_evicts().
		 */
		ASSERT(buf->b_data == NULL);
		rw_exit(&buf->b_lock);
		return (0);
	} else if (buf->b_data == NULL) {
		arc_buf_t copy = *buf; /* structure assignment */
		/*
		 * We are on the eviction list; process this buffer now
		 * but let arc_do_user_evicts() do the reaping.
		 */
		buf->b_efunc = NULL;
		rw_exit(&buf->b_lock);
		VERIFY(copy.b_efunc(&copy) == 0);
		return (1);
	}
	hash_lock = HDR_LOCK(hdr);
	mutex_enter(hash_lock);

	ASSERT(buf->b_hdr == hdr);
	ASSERT3U(refcount_count(&hdr->b_refcnt), <, hdr->b_datacnt);
	ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);

	/*
	 * Pull this buffer off of the hdr
	 */
	bufp = &hdr->b_buf;
	while (*bufp != buf)
		bufp = &(*bufp)->b_next;
	*bufp = buf->b_next;

	ASSERT(buf->b_data != NULL);
	arc_buf_destroy(buf, FALSE, FALSE);

	if (hdr->b_datacnt == 0) {
		arc_state_t *old_state = hdr->b_state;
		arc_state_t *evicted_state;

		ASSERT(refcount_is_zero(&hdr->b_refcnt));

		evicted_state =
		    (old_state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;

		mutex_enter(&old_state->arcs_mtx);
		mutex_enter(&evicted_state->arcs_mtx);

		arc_change_state(evicted_state, hdr, hash_lock);
		ASSERT(HDR_IN_HASH_TABLE(hdr));
		hdr->b_flags |= ARC_IN_HASH_TABLE;
		hdr->b_flags &= ~ARC_BUF_AVAILABLE;

		mutex_exit(&evicted_state->arcs_mtx);
		mutex_exit(&old_state->arcs_mtx);
	}
	mutex_exit(hash_lock);
	rw_exit(&buf->b_lock);

	VERIFY(buf->b_efunc(buf) == 0);
	buf->b_efunc = NULL;
	buf->b_private = NULL;
	buf->b_hdr = NULL;
	kmem_cache_free(buf_cache, buf);
	return (1);
}

/*
 * Release this buffer from the cache.  This must be done
 * after a read and prior to modifying the buffer contents.
 * If the buffer has more than one reference, we must make
 * a new hdr for the buffer.
 */
void
arc_release(arc_buf_t *buf, void *tag)
{
	arc_buf_hdr_t *hdr;
	kmutex_t *hash_lock;
	l2arc_buf_hdr_t *l2hdr;
	uint64_t buf_size;

	rw_enter(&buf->b_lock, RW_WRITER);
	hdr = buf->b_hdr;

	/* this buffer is not on any list */
	ASSERT(refcount_count(&hdr->b_refcnt) > 0);
	ASSERT(!(hdr->b_flags & ARC_STORED));

	if (hdr->b_state == arc_anon) {
		/* this buffer is already released */
		ASSERT3U(refcount_count(&hdr->b_refcnt), ==, 1);
		ASSERT(BUF_EMPTY(hdr));
		ASSERT(buf->b_efunc == NULL);
		arc_buf_thaw(buf);
		rw_exit(&buf->b_lock);
		return;
	}

	hash_lock = HDR_LOCK(hdr);
	mutex_enter(hash_lock);

	l2hdr = hdr->b_l2hdr;
	if (l2hdr) {
		mutex_enter(&l2arc_buflist_mtx);
		hdr->b_l2hdr = NULL;
		buf_size = hdr->b_size;
	}

	/*
	 * Do we have more than one buf?
	 */
	if (hdr->b_datacnt > 1) {
		arc_buf_hdr_t *nhdr;
		arc_buf_t **bufp;
		uint64_t blksz = hdr->b_size;
		uint64_t spa = hdr->b_spa;
		arc_buf_contents_t type = hdr->b_type;
		uint32_t flags = hdr->b_flags;

		ASSERT(hdr->b_buf != buf || buf->b_next != NULL);
		/*
		 * Pull the data off of this buf and attach it to
		 * a new anonymous buf.
		 */
		(void) remove_reference(hdr, hash_lock, tag);
		bufp = &hdr->b_buf;
		while (*bufp != buf)
			bufp = &(*bufp)->b_next;
		*bufp = (*bufp)->b_next;
		buf->b_next = NULL;

		ASSERT3U(hdr->b_state->arcs_size, >=, hdr->b_size);
		atomic_add_64(&hdr->b_state->arcs_size, -hdr->b_size);
		if (refcount_is_zero(&hdr->b_refcnt)) {
			uint64_t *size = &hdr->b_state->arcs_lsize[hdr->b_type];
			ASSERT3U(*size, >=, hdr->b_size);
			atomic_add_64(size, -hdr->b_size);
		}
		hdr->b_datacnt -= 1;
		arc_cksum_verify(buf);

		mutex_exit(hash_lock);

		nhdr = kmem_cache_alloc(hdr_cache, KM_PUSHPAGE);
		nhdr->b_size = blksz;
		nhdr->b_spa = spa;
		nhdr->b_type = type;
		nhdr->b_buf = buf;
		nhdr->b_state = arc_anon;
		nhdr->b_arc_access = 0;
		nhdr->b_flags = flags & ARC_L2_WRITING;
		nhdr->b_l2hdr = NULL;
		nhdr->b_datacnt = 1;
		nhdr->b_freeze_cksum = NULL;
		(void) refcount_add(&nhdr->b_refcnt, tag);
		buf->b_hdr = nhdr;
		rw_exit(&buf->b_lock);
		atomic_add_64(&arc_anon->arcs_size, blksz);
	} else {
		rw_exit(&buf->b_lock);
		ASSERT(refcount_count(&hdr->b_refcnt) == 1);
		ASSERT(!list_link_active(&hdr->b_arc_node));
		ASSERT(!HDR_IO_IN_PROGRESS(hdr));
		arc_change_state(arc_anon, hdr, hash_lock);
		hdr->b_arc_access = 0;
		mutex_exit(hash_lock);

		bzero(&hdr->b_dva, sizeof (dva_t));
		hdr->b_birth = 0;
		hdr->b_cksum0 = 0;
		arc_buf_thaw(buf);
	}
	buf->b_efunc = NULL;
	buf->b_private = NULL;

	if (l2hdr) {
		list_remove(l2hdr->b_dev->l2ad_buflist, hdr);
		kmem_free(l2hdr, sizeof (l2arc_buf_hdr_t));
		ARCSTAT_INCR(arcstat_l2_size, -buf_size);
		mutex_exit(&l2arc_buflist_mtx);
	}
}

int
arc_released(arc_buf_t *buf)
{
	int released;

	rw_enter(&buf->b_lock, RW_READER);
	released = (buf->b_data != NULL && buf->b_hdr->b_state == arc_anon);
	rw_exit(&buf->b_lock);
	return (released);
}

int
arc_has_callback(arc_buf_t *buf)
{
	int callback;

	rw_enter(&buf->b_lock, RW_READER);
	callback = (buf->b_efunc != NULL);
	rw_exit(&buf->b_lock);
	return (callback);
}

#ifdef ZFS_DEBUG
int
arc_referenced(arc_buf_t *buf)
{
	int referenced;

	rw_enter(&buf->b_lock, RW_READER);
	referenced = (refcount_count(&buf->b_hdr->b_refcnt));
	rw_exit(&buf->b_lock);
	return (referenced);
}
#endif

static void
arc_write_ready(zio_t *zio)
{
	arc_write_callback_t *callback = zio->io_private;
	arc_buf_t *buf = callback->awcb_buf;
	arc_buf_hdr_t *hdr = buf->b_hdr;

	ASSERT(!refcount_is_zero(&buf->b_hdr->b_refcnt));
	callback->awcb_ready(zio, buf, callback->awcb_private);

	/*
	 * If the IO is already in progress, then this is a re-write
	 * attempt, so we need to thaw and re-compute the cksum.
	 * It is the responsibility of the callback to handle the
	 * accounting for any re-write attempt.
	 */
	if (HDR_IO_IN_PROGRESS(hdr)) {
		mutex_enter(&hdr->b_freeze_lock);
		if (hdr->b_freeze_cksum != NULL) {
			kmem_free(hdr->b_freeze_cksum, sizeof (zio_cksum_t));
			hdr->b_freeze_cksum = NULL;
		}
		mutex_exit(&hdr->b_freeze_lock);
	}
	arc_cksum_compute(buf, B_FALSE);
	hdr->b_flags |= ARC_IO_IN_PROGRESS;
}

static void
arc_write_done(zio_t *zio)
{
	arc_write_callback_t *callback = zio->io_private;
	arc_buf_t *buf = callback->awcb_buf;
	arc_buf_hdr_t *hdr = buf->b_hdr;

	hdr->b_acb = NULL;

	hdr->b_dva = *BP_IDENTITY(zio->io_bp);
	hdr->b_birth = zio->io_bp->blk_birth;
	hdr->b_cksum0 = zio->io_bp->blk_cksum.zc_word[0];
	/*
	 * If the block to be written was all-zero, we may have
	 * compressed it away.  In this case no write was performed
	 * so there will be no dva/birth-date/checksum.  The buffer
	 * must therefor remain anonymous (and uncached).
	 */
	if (!BUF_EMPTY(hdr)) {
		arc_buf_hdr_t *exists;
		kmutex_t *hash_lock;

		arc_cksum_verify(buf);

		exists = buf_hash_insert(hdr, &hash_lock);
		if (exists) {
			/*
			 * This can only happen if we overwrite for
			 * sync-to-convergence, because we remove
			 * buffers from the hash table when we arc_free().
			 */
			ASSERT(zio->io_flags & ZIO_FLAG_IO_REWRITE);
			ASSERT(DVA_EQUAL(BP_IDENTITY(&zio->io_bp_orig),
			    BP_IDENTITY(zio->io_bp)));
			ASSERT3U(zio->io_bp_orig.blk_birth, ==,
			    zio->io_bp->blk_birth);

			ASSERT(refcount_is_zero(&exists->b_refcnt));
			arc_change_state(arc_anon, exists, hash_lock);
			mutex_exit(hash_lock);
			arc_hdr_destroy(exists);
			exists = buf_hash_insert(hdr, &hash_lock);
			ASSERT3P(exists, ==, NULL);
		}
		hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
		/* if it's not anon, we are doing a scrub */
		if (hdr->b_state == arc_anon)
			arc_access(hdr, hash_lock);
		mutex_exit(hash_lock);
	} else if (callback->awcb_done == NULL) {
		int destroy_hdr;
		/*
		 * This is an anonymous buffer with no user callback,
		 * destroy it if there are no active references.
		 */
		mutex_enter(&arc_eviction_mtx);
		destroy_hdr = refcount_is_zero(&hdr->b_refcnt);
		hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
		mutex_exit(&arc_eviction_mtx);
		if (destroy_hdr)
			arc_hdr_destroy(hdr);
	} else {
		hdr->b_flags &= ~ARC_IO_IN_PROGRESS;
	}
	hdr->b_flags &= ~ARC_STORED;

	if (callback->awcb_done) {
		ASSERT(!refcount_is_zero(&hdr->b_refcnt));
		callback->awcb_done(zio, buf, callback->awcb_private);
	}

	kmem_free(callback, sizeof (arc_write_callback_t));
}

void
write_policy(spa_t *spa, const writeprops_t *wp, zio_prop_t *zp)
{
	boolean_t ismd = (wp->wp_level > 0 || dmu_ot[wp->wp_type].ot_metadata);

	/* Determine checksum setting */
	if (ismd) {
		/*
		 * Metadata always gets checksummed.  If the data
		 * checksum is multi-bit correctable, and it's not a
		 * ZBT-style checksum, then it's suitable for metadata
		 * as well.  Otherwise, the metadata checksum defaults
		 * to fletcher4.
		 */
		if (zio_checksum_table[wp->wp_oschecksum].ci_correctable &&
		    !zio_checksum_table[wp->wp_oschecksum].ci_zbt)
			zp->zp_checksum = wp->wp_oschecksum;
		else
			zp->zp_checksum = ZIO_CHECKSUM_FLETCHER_4;
	} else {
		zp->zp_checksum = zio_checksum_select(wp->wp_dnchecksum,
		    wp->wp_oschecksum);
	}

	/* Determine compression setting */
	if (ismd) {
		/*
		 * XXX -- we should design a compression algorithm
		 * that specializes in arrays of bps.
		 */
		zp->zp_compress = zfs_mdcomp_disable ? ZIO_COMPRESS_EMPTY :
		    ZIO_COMPRESS_LZJB;
	} else {
		zp->zp_compress = zio_compress_select(wp->wp_dncompress,
		    wp->wp_oscompress);
	}

	zp->zp_type = wp->wp_type;
	zp->zp_level = wp->wp_level;
	zp->zp_ndvas = MIN(wp->wp_copies + ismd, spa_max_replication(spa));
}

zio_t *
arc_write(zio_t *pio, spa_t *spa, const writeprops_t *wp,
    boolean_t l2arc, uint64_t txg, blkptr_t *bp, arc_buf_t *buf,
    arc_done_func_t *ready, arc_done_func_t *done, void *private, int priority,
    int zio_flags, const zbookmark_t *zb)
{
	arc_buf_hdr_t *hdr = buf->b_hdr;
	arc_write_callback_t *callback;
	zio_t *zio;
	zio_prop_t zp;

	ASSERT(ready != NULL);
	ASSERT(!HDR_IO_ERROR(hdr));
	ASSERT((hdr->b_flags & ARC_IO_IN_PROGRESS) == 0);
	ASSERT(hdr->b_acb == 0);
	if (l2arc)
		hdr->b_flags |= ARC_L2CACHE;
	callback = kmem_zalloc(sizeof (arc_write_callback_t), KM_SLEEP);
	callback->awcb_ready = ready;
	callback->awcb_done = done;
	callback->awcb_private = private;
	callback->awcb_buf = buf;

	write_policy(spa, wp, &zp);
	zio = zio_write(pio, spa, txg, bp, buf->b_data, hdr->b_size, &zp,
	    arc_write_ready, arc_write_done, callback, priority, zio_flags, zb);

	return (zio);
}

int
arc_free(zio_t *pio, spa_t *spa, uint64_t txg, blkptr_t *bp,
    zio_done_func_t *done, void *private, uint32_t arc_flags)
{
	arc_buf_hdr_t *ab;
	kmutex_t *hash_lock;
	zio_t	*zio;
	uint64_t guid = spa_guid(spa);

	/*
	 * If this buffer is in the cache, release it, so it
	 * can be re-used.
	 */
	ab = buf_hash_find(guid, BP_IDENTITY(bp), bp->blk_birth, &hash_lock);
	if (ab != NULL) {
		/*
		 * The checksum of blocks to free is not always
		 * preserved (eg. on the deadlist).  However, if it is
		 * nonzero, it should match what we have in the cache.
		 */
		ASSERT(bp->blk_cksum.zc_word[0] == 0 ||
		    bp->blk_cksum.zc_word[0] == ab->b_cksum0 ||
		    bp->blk_fill == BLK_FILL_ALREADY_FREED);

		if (ab->b_state != arc_anon)
			arc_change_state(arc_anon, ab, hash_lock);
		if (HDR_IO_IN_PROGRESS(ab)) {
			/*
			 * This should only happen when we prefetch.
			 */
			ASSERT(ab->b_flags & ARC_PREFETCH);
			ASSERT3U(ab->b_datacnt, ==, 1);
			ab->b_flags |= ARC_FREED_IN_READ;
			if (HDR_IN_HASH_TABLE(ab))
				buf_hash_remove(ab);
			ab->b_arc_access = 0;
			bzero(&ab->b_dva, sizeof (dva_t));
			ab->b_birth = 0;
			ab->b_cksum0 = 0;
			ab->b_buf->b_efunc = NULL;
			ab->b_buf->b_private = NULL;
			mutex_exit(hash_lock);
		} else if (refcount_is_zero(&ab->b_refcnt)) {
			ab->b_flags |= ARC_FREE_IN_PROGRESS;
			mutex_exit(hash_lock);
			arc_hdr_destroy(ab);
			ARCSTAT_BUMP(arcstat_deleted);
		} else {
			/*
			 * We still have an active reference on this
			 * buffer.  This can happen, e.g., from
			 * dbuf_unoverride().
			 */
			ASSERT(!HDR_IN_HASH_TABLE(ab));
			ab->b_arc_access = 0;
			bzero(&ab->b_dva, sizeof (dva_t));
			ab->b_birth = 0;
			ab->b_cksum0 = 0;
			ab->b_buf->b_efunc = NULL;
			ab->b_buf->b_private = NULL;
			mutex_exit(hash_lock);
		}
	}

	zio = zio_free(pio, spa, txg, bp, done, private, ZIO_FLAG_MUSTSUCCEED);

	if (arc_flags & ARC_WAIT)
		return (zio_wait(zio));

	ASSERT(arc_flags & ARC_NOWAIT);
	zio_nowait(zio);

	return (0);
}

static int
arc_memory_throttle(uint64_t reserve, uint64_t txg)
{
#ifdef _KERNEL
	uint64_t inflight_data = arc_anon->arcs_size;
	uint64_t available_memory = ptob(freemem);
	static uint64_t page_load = 0;
	static uint64_t last_txg = 0;

#if defined(__i386)
	available_memory =
	    MIN(available_memory, vmem_size(heap_arena, VMEM_FREE));
#endif
	if (available_memory >= zfs_write_limit_max)
		return (0);

	if (txg > last_txg) {
		last_txg = txg;
		page_load = 0;
	}
	/*
	 * If we are in pageout, we know that memory is already tight,
	 * the arc is already going to be evicting, so we just want to
	 * continue to let page writes occur as quickly as possible.
	 */
	if (curproc == proc_pageout) {
		if (page_load > MAX(ptob(minfree), available_memory) / 4)
			return (ERESTART);
		/* Note: reserve is inflated, so we deflate */
		page_load += reserve / 8;
		return (0);
	} else if (page_load > 0 && arc_reclaim_needed()) {
		/* memory is low, delay before restarting */
		ARCSTAT_INCR(arcstat_memory_throttle_count, 1);
		return (EAGAIN);
	}
	page_load = 0;

	if (arc_size > arc_c_min) {
		uint64_t evictable_memory =
		    arc_mru->arcs_lsize[ARC_BUFC_DATA] +
		    arc_mru->arcs_lsize[ARC_BUFC_METADATA] +
		    arc_mfu->arcs_lsize[ARC_BUFC_DATA] +
		    arc_mfu->arcs_lsize[ARC_BUFC_METADATA];
		available_memory += MIN(evictable_memory, arc_size - arc_c_min);
	}

	if (inflight_data > available_memory / 4) {
		ARCSTAT_INCR(arcstat_memory_throttle_count, 1);
		return (ERESTART);
	}
#endif
	return (0);
}

void
arc_tempreserve_clear(uint64_t reserve)
{
	atomic_add_64(&arc_tempreserve, -reserve);
	ASSERT((int64_t)arc_tempreserve >= 0);
}

int
arc_tempreserve_space(uint64_t reserve, uint64_t txg)
{
	int error;

#ifdef ZFS_DEBUG
	/*
	 * Once in a while, fail for no reason.  Everything should cope.
	 */
	if (spa_get_random(10000) == 0) {
		dprintf("forcing random failure\n");
		return (ERESTART);
	}
#endif
	if (reserve > arc_c/4 && !arc_no_grow)
		arc_c = MIN(arc_c_max, reserve * 4);
	if (reserve > arc_c)
		return (ENOMEM);

	/*
	 * Writes will, almost always, require additional memory allocations
	 * in order to compress/encrypt/etc the data.  We therefor need to
	 * make sure that there is sufficient available memory for this.
	 */
	if (error = arc_memory_throttle(reserve, txg))
		return (error);

	/*
	 * Throttle writes when the amount of dirty data in the cache
	 * gets too large.  We try to keep the cache less than half full
	 * of dirty blocks so that our sync times don't grow too large.
	 * Note: if two requests come in concurrently, we might let them
	 * both succeed, when one of them should fail.  Not a huge deal.
	 */
	if (reserve + arc_tempreserve + arc_anon->arcs_size > arc_c / 2 &&
	    arc_anon->arcs_size > arc_c / 4) {
		dprintf("failing, arc_tempreserve=%lluK anon_meta=%lluK "
		    "anon_data=%lluK tempreserve=%lluK arc_c=%lluK\n",
		    arc_tempreserve>>10,
		    arc_anon->arcs_lsize[ARC_BUFC_METADATA]>>10,
		    arc_anon->arcs_lsize[ARC_BUFC_DATA]>>10,
		    reserve>>10, arc_c>>10);
		return (ERESTART);
	}
	atomic_add_64(&arc_tempreserve, reserve);
	return (0);
}

void
arc_init(void)
{
	mutex_init(&arc_reclaim_thr_lock, NULL, MUTEX_DEFAULT, NULL);
	cv_init(&arc_reclaim_thr_cv, NULL, CV_DEFAULT, NULL);

	/* Convert seconds to clock ticks */
	arc_min_prefetch_lifespan = 1 * hz;

	/* Start out with 1/8 of all memory */
	arc_c = physmem * PAGESIZE / 8;

#ifdef _KERNEL
	/*
	 * On architectures where the physical memory can be larger
	 * than the addressable space (intel in 32-bit mode), we may
	 * need to limit the cache to 1/8 of VM size.
	 */
	arc_c = MIN(arc_c, vmem_size(heap_arena, VMEM_ALLOC | VMEM_FREE) / 8);
#endif

	/* set min cache to 1/32 of all memory, or 64MB, whichever is more */
	arc_c_min = MAX(arc_c / 4, 64<<20);
	/* set max to 3/4 of all memory, or all but 1GB, whichever is more */
	if (arc_c * 8 >= 1<<30)
		arc_c_max = (arc_c * 8) - (1<<30);
	else
		arc_c_max = arc_c_min;
	arc_c_max = MAX(arc_c * 6, arc_c_max);

	/*
	 * Allow the tunables to override our calculations if they are
	 * reasonable (ie. over 64MB)
	 */
	if (zfs_arc_max > 64<<20 && zfs_arc_max < physmem * PAGESIZE)
		arc_c_max = zfs_arc_max;
	if (zfs_arc_min > 64<<20 && zfs_arc_min <= arc_c_max)
		arc_c_min = zfs_arc_min;

	arc_c = arc_c_max;
	arc_p = (arc_c >> 1);

	/* limit meta-data to 1/4 of the arc capacity */
	arc_meta_limit = arc_c_max / 4;

	/* Allow the tunable to override if it is reasonable */
	if (zfs_arc_meta_limit > 0 && zfs_arc_meta_limit <= arc_c_max)
		arc_meta_limit = zfs_arc_meta_limit;

	if (arc_c_min < arc_meta_limit / 2 && zfs_arc_min == 0)
		arc_c_min = arc_meta_limit / 2;

	if (zfs_arc_grow_retry > 0)
		arc_grow_retry = zfs_arc_grow_retry;

	if (zfs_arc_shrink_shift > 0)
		arc_shrink_shift = zfs_arc_shrink_shift;

	if (zfs_arc_p_min_shift > 0)
		arc_p_min_shift = zfs_arc_p_min_shift;

	/* if kmem_flags are set, lets try to use less memory */
	if (kmem_debugging())
		arc_c = arc_c / 2;
	if (arc_c < arc_c_min)
		arc_c = arc_c_min;

	arc_anon = &ARC_anon;
	arc_mru = &ARC_mru;
	arc_mru_ghost = &ARC_mru_ghost;
	arc_mfu = &ARC_mfu;
	arc_mfu_ghost = &ARC_mfu_ghost;
	arc_l2c_only = &ARC_l2c_only;
	arc_size = 0;

	mutex_init(&arc_anon->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&arc_mru->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&arc_mru_ghost->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&arc_mfu->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&arc_mfu_ghost->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&arc_l2c_only->arcs_mtx, NULL, MUTEX_DEFAULT, NULL);

	list_create(&arc_mru->arcs_list[ARC_BUFC_METADATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mru->arcs_list[ARC_BUFC_DATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mru_ghost->arcs_list[ARC_BUFC_METADATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mru_ghost->arcs_list[ARC_BUFC_DATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mfu->arcs_list[ARC_BUFC_METADATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mfu->arcs_list[ARC_BUFC_DATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_mfu_ghost->arcs_list[ARC_BUFC_DATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_l2c_only->arcs_list[ARC_BUFC_METADATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));
	list_create(&arc_l2c_only->arcs_list[ARC_BUFC_DATA],
	    sizeof (arc_buf_hdr_t), offsetof(arc_buf_hdr_t, b_arc_node));

	buf_init();

	arc_thread_exit = 0;
	arc_eviction_list = NULL;
	mutex_init(&arc_eviction_mtx, NULL, MUTEX_DEFAULT, NULL);
	bzero(&arc_eviction_hdr, sizeof (arc_buf_hdr_t));

	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);

	if (arc_ksp != NULL) {
		arc_ksp->ks_data = &arc_stats;
		kstat_install(arc_ksp);
	}

	(void) thread_create(NULL, 0, arc_reclaim_thread, NULL, 0, &p0,
	    TS_RUN, minclsyspri);

	arc_dead = FALSE;
	arc_warm = B_FALSE;

	if (zfs_write_limit_max == 0)
		zfs_write_limit_max = ptob(physmem) >> zfs_write_limit_shift;
	else
		zfs_write_limit_shift = 0;
	mutex_init(&zfs_write_limit_lock, NULL, MUTEX_DEFAULT, NULL);
}

void
arc_fini(void)
{
	mutex_enter(&arc_reclaim_thr_lock);
	arc_thread_exit = 1;
	while (arc_thread_exit != 0)
		cv_wait(&arc_reclaim_thr_cv, &arc_reclaim_thr_lock);
	mutex_exit(&arc_reclaim_thr_lock);

	arc_flush(NULL);

	arc_dead = TRUE;

	if (arc_ksp != NULL) {
		kstat_delete(arc_ksp);
		arc_ksp = NULL;
	}

	mutex_destroy(&arc_eviction_mtx);
	mutex_destroy(&arc_reclaim_thr_lock);
	cv_destroy(&arc_reclaim_thr_cv);

	list_destroy(&arc_mru->arcs_list[ARC_BUFC_METADATA]);
	list_destroy(&arc_mru_ghost->arcs_list[ARC_BUFC_METADATA]);
	list_destroy(&arc_mfu->arcs_list[ARC_BUFC_METADATA]);
	list_destroy(&arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA]);
	list_destroy(&arc_mru->arcs_list[ARC_BUFC_DATA]);
	list_destroy(&arc_mru_ghost->arcs_list[ARC_BUFC_DATA]);
	list_destroy(&arc_mfu->arcs_list[ARC_BUFC_DATA]);
	list_destroy(&arc_mfu_ghost->arcs_list[ARC_BUFC_DATA]);

	mutex_destroy(&arc_anon->arcs_mtx);
	mutex_destroy(&arc_mru->arcs_mtx);
	mutex_destroy(&arc_mru_ghost->arcs_mtx);
	mutex_destroy(&arc_mfu->arcs_mtx);
	mutex_destroy(&arc_mfu_ghost->arcs_mtx);
	mutex_destroy(&arc_l2c_only->arcs_mtx);

	mutex_destroy(&zfs_write_limit_lock);

	buf_fini();
}

/*
 * Level 2 ARC
 *
 * The level 2 ARC (L2ARC) is a cache layer in-between main memory and disk.
 * It uses dedicated storage devices to hold cached data, which are populated
 * using large infrequent writes.  The main role of this cache is to boost
 * the performance of random read workloads.  The intended L2ARC devices
 * include short-stroked disks, solid state disks, and other media with
 * substantially faster read latency than disk.
 *
 *                 +-----------------------+
 *                 |         ARC           |
 *                 +-----------------------+
 *                    |         ^     ^
 *                    |         |     |
 *      l2arc_feed_thread()    arc_read()
 *                    |         |     |
 *                    |  l2arc read   |
 *                    V         |     |
 *               +---------------+    |
 *               |     L2ARC     |    |
 *               +---------------+    |
 *                   |    ^           |
 *          l2arc_write() |           |
 *                   |    |           |
 *                   V    |           |
 *                 +-------+      +-------+
 *                 | vdev  |      | vdev  |
 *                 | cache |      | cache |
 *                 +-------+      +-------+
 *                 +=========+     .-----.
 *                 :  L2ARC  :    |-_____-|
 *                 : devices :    | Disks |
 *                 +=========+    `-_____-'
 *
 * Read requests are satisfied from the following sources, in order:
 *
 *	1) ARC
 *	2) vdev cache of L2ARC devices
 *	3) L2ARC devices
 *	4) vdev cache of disks
 *	5) disks
 *
 * Some L2ARC device types exhibit extremely slow write performance.
 * To accommodate for this there are some significant differences between
 * the L2ARC and traditional cache design:
 *
 * 1. There is no eviction path from the ARC to the L2ARC.  Evictions from
 * the ARC behave as usual, freeing buffers and placing headers on ghost
 * lists.  The ARC does not send buffers to the L2ARC during eviction as
 * this would add inflated write latencies for all ARC memory pressure.
 *
 * 2. The L2ARC attempts to cache data from the ARC before it is evicted.
 * It does this by periodically scanning buffers from the eviction-end of
 * the MFU and MRU ARC lists, copying them to the L2ARC devices if they are
 * not already there.  It scans until a headroom of buffers is satisfied,
 * which itself is a buffer for ARC eviction.  The thread that does this is
 * l2arc_feed_thread(), illustrated below; example sizes are included to
 * provide a better sense of ratio than this diagram:
 *
 *	       head -->                        tail
 *	        +---------------------+----------+
 *	ARC_mfu |:::::#:::::::::::::::|o#o###o###|-->.   # already on L2ARC
 *	        +---------------------+----------+   |   o L2ARC eligible
 *	ARC_mru |:#:::::::::::::::::::|#o#ooo####|-->|   : ARC buffer
 *	        +---------------------+----------+   |
 *	             15.9 Gbytes      ^ 32 Mbytes    |
 *	                           headroom          |
 *	                                      l2arc_feed_thread()
 *	                                             |
 *	                 l2arc write hand <--[oooo]--'
 *	                         |           8 Mbyte
 *	                         |          write max
 *	                         V
 *		  +==============================+
 *	L2ARC dev |####|#|###|###|    |####| ... |
 *	          +==============================+
 *	                     32 Gbytes
 *
 * 3. If an ARC buffer is copied to the L2ARC but then hit instead of
 * evicted, then the L2ARC has cached a buffer much sooner than it probably
 * needed to, potentially wasting L2ARC device bandwidth and storage.  It is
 * safe to say that this is an uncommon case, since buffers at the end of
 * the ARC lists have moved there due to inactivity.
 *
 * 4. If the ARC evicts faster than the L2ARC can maintain a headroom,
 * then the L2ARC simply misses copying some buffers.  This serves as a
 * pressure valve to prevent heavy read workloads from both stalling the ARC
 * with waits and clogging the L2ARC with writes.  This also helps prevent
 * the potential for the L2ARC to churn if it attempts to cache content too
 * quickly, such as during backups of the entire pool.
 *
 * 5. After system boot and before the ARC has filled main memory, there are
 * no evictions from the ARC and so the tails of the ARC_mfu and ARC_mru
 * lists can remain mostly static.  Instead of searching from tail of these
 * lists as pictured, the l2arc_feed_thread() will search from the list heads
 * for eligible buffers, greatly increasing its chance of finding them.
 *
 * The L2ARC device write speed is also boosted during this time so that
 * the L2ARC warms up faster.  Since there have been no ARC evictions yet,
 * there are no L2ARC reads, and no fear of degrading read performance
 * through increased writes.
 *
 * 6. Writes to the L2ARC devices are grouped and sent in-sequence, so that
 * the vdev queue can aggregate them into larger and fewer writes.  Each
 * device is written to in a rotor fashion, sweeping writes through
 * available space then repeating.
 *
 * 7. The L2ARC does not store dirty content.  It never needs to flush
 * write buffers back to disk based storage.
 *
 * 8. If an ARC buffer is written (and dirtied) which also exists in the
 * L2ARC, the now stale L2ARC buffer is immediately dropped.
 *
 * The performance of the L2ARC can be tweaked by a number of tunables, which
 * may be necessary for different workloads:
 *
 *	l2arc_write_max		max write bytes per interval
 *	l2arc_write_boost	extra write bytes during device warmup
 *	l2arc_noprefetch	skip caching prefetched buffers
 *	l2arc_headroom		number of max device writes to precache
 *	l2arc_feed_secs		seconds between L2ARC writing
 *
 * Tunables may be removed or added as future performance improvements are
 * integrated, and also may become zpool properties.
 *
 * There are three key functions that control how the L2ARC warms up:
 *
 *	l2arc_write_eligible()	check if a buffer is eligible to cache
 *	l2arc_write_size()	calculate how much to write
 *	l2arc_write_interval()	calculate sleep delay between writes
 *
 * These three functions determine what to write, how much, and how quickly
 * to send writes.
 */

static boolean_t
l2arc_write_eligible(uint64_t spa_guid, arc_buf_hdr_t *ab)
{
	/*
	 * A buffer is *not* eligible for the L2ARC if it:
	 * 1. belongs to a different spa.
	 * 2. has no attached buffer.
	 * 3. is already cached on the L2ARC.
	 * 4. has an I/O in progress (it may be an incomplete read).
	 * 5. is flagged not eligible (zfs property).
	 */
	if (ab->b_spa != spa_guid || ab->b_buf == NULL || ab->b_l2hdr != NULL ||
	    HDR_IO_IN_PROGRESS(ab) || !HDR_L2CACHE(ab))
		return (B_FALSE);

	return (B_TRUE);
}

static uint64_t
l2arc_write_size(l2arc_dev_t *dev)
{
	uint64_t size;

	size = dev->l2ad_write;

	if (arc_warm == B_FALSE)
		size += dev->l2ad_boost;

	return (size);

}

static clock_t
l2arc_write_interval(clock_t began, uint64_t wanted, uint64_t wrote)
{
	clock_t interval, next;

	/*
	 * If the ARC lists are busy, increase our write rate; if the
	 * lists are stale, idle back.  This is achieved by checking
	 * how much we previously wrote - if it was more than half of
	 * what we wanted, schedule the next write much sooner.
	 */
	if (l2arc_feed_again && wrote > (wanted / 2))
		interval = (hz * l2arc_feed_min_ms) / 1000;
	else
		interval = hz * l2arc_feed_secs;

	next = MAX(lbolt, MIN(lbolt + interval, began + interval));

	return (next);
}

static void
l2arc_hdr_stat_add(void)
{
	ARCSTAT_INCR(arcstat_l2_hdr_size, HDR_SIZE + L2HDR_SIZE);
	ARCSTAT_INCR(arcstat_hdr_size, -HDR_SIZE);
}

static void
l2arc_hdr_stat_remove(void)
{
	ARCSTAT_INCR(arcstat_l2_hdr_size, -(HDR_SIZE + L2HDR_SIZE));
	ARCSTAT_INCR(arcstat_hdr_size, HDR_SIZE);
}

/*
 * Cycle through L2ARC devices.  This is how L2ARC load balances.
 * If a device is returned, this also returns holding the spa config lock.
 */
static l2arc_dev_t *
l2arc_dev_get_next(void)
{
	l2arc_dev_t *first, *next = NULL;

	/*
	 * Lock out the removal of spas (spa_namespace_lock), then removal
	 * of cache devices (l2arc_dev_mtx).  Once a device has been selected,
	 * both locks will be dropped and a spa config lock held instead.
	 */
	mutex_enter(&spa_namespace_lock);
	mutex_enter(&l2arc_dev_mtx);

	/* if there are no vdevs, there is nothing to do */
	if (l2arc_ndev == 0)
		goto out;

	first = NULL;
	next = l2arc_dev_last;
	do {
		/* loop around the list looking for a non-faulted vdev */
		if (next == NULL) {
			next = list_head(l2arc_dev_list);
		} else {
			next = list_next(l2arc_dev_list, next);
			if (next == NULL)
				next = list_head(l2arc_dev_list);
		}

		/* if we have come back to the start, bail out */
		if (first == NULL)
			first = next;
		else if (next == first)
			break;

	} while (vdev_is_dead(next->l2ad_vdev));

	/* if we were unable to find any usable vdevs, return NULL */
	if (vdev_is_dead(next->l2ad_vdev))
		next = NULL;

	l2arc_dev_last = next;

out:
	mutex_exit(&l2arc_dev_mtx);

	/*
	 * Grab the config lock to prevent the 'next' device from being
	 * removed while we are writing to it.
	 */
	if (next != NULL)
		spa_config_enter(next->l2ad_spa, SCL_L2ARC, next, RW_READER);
	mutex_exit(&spa_namespace_lock);

	return (next);
}

/*
 * Free buffers that were tagged for destruction.
 */
static void
l2arc_do_free_on_write()
{
	list_t *buflist;
	l2arc_data_free_t *df, *df_prev;

	mutex_enter(&l2arc_free_on_write_mtx);
	buflist = l2arc_free_on_write;

	for (df = list_tail(buflist); df; df = df_prev) {
		df_prev = list_prev(buflist, df);
		ASSERT(df->l2df_data != NULL);
		ASSERT(df->l2df_func != NULL);
		df->l2df_func(df->l2df_data, df->l2df_size);
		list_remove(buflist, df);
		kmem_free(df, sizeof (l2arc_data_free_t));
	}

	mutex_exit(&l2arc_free_on_write_mtx);
}

/*
 * A write to a cache device has completed.  Update all headers to allow
 * reads from these buffers to begin.
 */
static void
l2arc_write_done(zio_t *zio)
{
	l2arc_write_callback_t *cb;
	l2arc_dev_t *dev;
	list_t *buflist;
	arc_buf_hdr_t *head, *ab, *ab_prev;
	l2arc_buf_hdr_t *abl2;
	kmutex_t *hash_lock;

	cb = zio->io_private;
	ASSERT(cb != NULL);
	dev = cb->l2wcb_dev;
	ASSERT(dev != NULL);
	head = cb->l2wcb_head;
	ASSERT(head != NULL);
	buflist = dev->l2ad_buflist;
	ASSERT(buflist != NULL);
	DTRACE_PROBE2(l2arc__iodone, zio_t *, zio,
	    l2arc_write_callback_t *, cb);

	if (zio->io_error != 0)
		ARCSTAT_BUMP(arcstat_l2_writes_error);

	mutex_enter(&l2arc_buflist_mtx);

	/*
	 * All writes completed, or an error was hit.
	 */
	for (ab = list_prev(buflist, head); ab; ab = ab_prev) {
		ab_prev = list_prev(buflist, ab);

		hash_lock = HDR_LOCK(ab);
		if (!mutex_tryenter(hash_lock)) {
			/*
			 * This buffer misses out.  It may be in a stage
			 * of eviction.  Its ARC_L2_WRITING flag will be
			 * left set, denying reads to this buffer.
			 */
			ARCSTAT_BUMP(arcstat_l2_writes_hdr_miss);
			continue;
		}

		if (zio->io_error != 0) {
			/*
			 * Error - drop L2ARC entry.
			 */
			list_remove(buflist, ab);
			abl2 = ab->b_l2hdr;
			ab->b_l2hdr = NULL;
			kmem_free(abl2, sizeof (l2arc_buf_hdr_t));
			ARCSTAT_INCR(arcstat_l2_size, -ab->b_size);
		}

		/*
		 * Allow ARC to begin reads to this L2ARC entry.
		 */
		ab->b_flags &= ~ARC_L2_WRITING;

		mutex_exit(hash_lock);
	}

	atomic_inc_64(&l2arc_writes_done);
	list_remove(buflist, head);
	kmem_cache_free(hdr_cache, head);
	mutex_exit(&l2arc_buflist_mtx);

	l2arc_do_free_on_write();

	kmem_free(cb, sizeof (l2arc_write_callback_t));
}

/*
 * A read to a cache device completed.  Validate buffer contents before
 * handing over to the regular ARC routines.
 */
static void
l2arc_read_done(zio_t *zio)
{
	l2arc_read_callback_t *cb;
	arc_buf_hdr_t *hdr;
	arc_buf_t *buf;
	kmutex_t *hash_lock;
	int equal;

	ASSERT(zio->io_vd != NULL);
	ASSERT(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE);

	spa_config_exit(zio->io_spa, SCL_L2ARC, zio->io_vd);

	cb = zio->io_private;
	ASSERT(cb != NULL);
	buf = cb->l2rcb_buf;
	ASSERT(buf != NULL);
	hdr = buf->b_hdr;
	ASSERT(hdr != NULL);

	hash_lock = HDR_LOCK(hdr);
	mutex_enter(hash_lock);

	/*
	 * Check this survived the L2ARC journey.
	 */
	equal = arc_cksum_equal(buf);
	if (equal && zio->io_error == 0 && !HDR_L2_EVICTED(hdr)) {
		mutex_exit(hash_lock);
		zio->io_private = buf;
		zio->io_bp_copy = cb->l2rcb_bp;	/* XXX fix in L2ARC 2.0	*/
		zio->io_bp = &zio->io_bp_copy;	/* XXX fix in L2ARC 2.0	*/
		arc_read_done(zio);
	} else {
		mutex_exit(hash_lock);
		/*
		 * Buffer didn't survive caching.  Increment stats and
		 * reissue to the original storage device.
		 */
		if (zio->io_error != 0) {
			ARCSTAT_BUMP(arcstat_l2_io_error);
		} else {
			zio->io_error = EIO;
		}
		if (!equal)
			ARCSTAT_BUMP(arcstat_l2_cksum_bad);

		/*
		 * If there's no waiter, issue an async i/o to the primary
		 * storage now.  If there *is* a waiter, the caller must
		 * issue the i/o in a context where it's OK to block.
		 */
		if (zio->io_waiter == NULL) {
			zio_t *pio = zio_unique_parent(zio);

			ASSERT(!pio || pio->io_child_type == ZIO_CHILD_LOGICAL);

			zio_nowait(zio_read(pio, cb->l2rcb_spa, &cb->l2rcb_bp,
			    buf->b_data, zio->io_size, arc_read_done, buf,
			    zio->io_priority, cb->l2rcb_flags, &cb->l2rcb_zb));
		}
	}

	kmem_free(cb, sizeof (l2arc_read_callback_t));
}

/*
 * This is the list priority from which the L2ARC will search for pages to
 * cache.  This is used within loops (0..3) to cycle through lists in the
 * desired order.  This order can have a significant effect on cache
 * performance.
 *
 * Currently the metadata lists are hit first, MFU then MRU, followed by
 * the data lists.  This function returns a locked list, and also returns
 * the lock pointer.
 */
static list_t *
l2arc_list_locked(int list_num, kmutex_t **lock)
{
	list_t *list;

	ASSERT(list_num >= 0 && list_num <= 3);

	switch (list_num) {
	case 0:
		list = &arc_mfu->arcs_list[ARC_BUFC_METADATA];
		*lock = &arc_mfu->arcs_mtx;
		break;
	case 1:
		list = &arc_mru->arcs_list[ARC_BUFC_METADATA];
		*lock = &arc_mru->arcs_mtx;
		break;
	case 2:
		list = &arc_mfu->arcs_list[ARC_BUFC_DATA];
		*lock = &arc_mfu->arcs_mtx;
		break;
	case 3:
		list = &arc_mru->arcs_list[ARC_BUFC_DATA];
		*lock = &arc_mru->arcs_mtx;
		break;
	}

	ASSERT(!(MUTEX_HELD(*lock)));
	mutex_enter(*lock);
	return (list);
}

/*
 * Evict buffers from the device write hand to the distance specified in
 * bytes.  This distance may span populated buffers, it may span nothing.
 * This is clearing a region on the L2ARC device ready for writing.
 * If the 'all' boolean is set, every buffer is evicted.
 */
static void
l2arc_evict(l2arc_dev_t *dev, uint64_t distance, boolean_t all)
{
	list_t *buflist;
	l2arc_buf_hdr_t *abl2;
	arc_buf_hdr_t *ab, *ab_prev;
	kmutex_t *hash_lock;
	uint64_t taddr;

	buflist = dev->l2ad_buflist;

	if (buflist == NULL)
		return;

	if (!all && dev->l2ad_first) {
		/*
		 * This is the first sweep through the device.  There is
		 * nothing to evict.
		 */
		return;
	}

	if (dev->l2ad_hand >= (dev->l2ad_end - (2 * distance))) {
		/*
		 * When nearing the end of the device, evict to the end
		 * before the device write hand jumps to the start.
		 */
		taddr = dev->l2ad_end;
	} else {
		taddr = dev->l2ad_hand + distance;
	}
	DTRACE_PROBE4(l2arc__evict, l2arc_dev_t *, dev, list_t *, buflist,
	    uint64_t, taddr, boolean_t, all);

top:
	mutex_enter(&l2arc_buflist_mtx);
	for (ab = list_tail(buflist); ab; ab = ab_prev) {
		ab_prev = list_prev(buflist, ab);

		hash_lock = HDR_LOCK(ab);
		if (!mutex_tryenter(hash_lock)) {
			/*
			 * Missed the hash lock.  Retry.
			 */
			ARCSTAT_BUMP(arcstat_l2_evict_lock_retry);
			mutex_exit(&l2arc_buflist_mtx);
			mutex_enter(hash_lock);
			mutex_exit(hash_lock);
			goto top;
		}

		if (HDR_L2_WRITE_HEAD(ab)) {
			/*
			 * We hit a write head node.  Leave it for
			 * l2arc_write_done().
			 */
			list_remove(buflist, ab);
			mutex_exit(hash_lock);
			continue;
		}

		if (!all && ab->b_l2hdr != NULL &&
		    (ab->b_l2hdr->b_daddr > taddr ||
		    ab->b_l2hdr->b_daddr < dev->l2ad_hand)) {
			/*
			 * We've evicted to the target address,
			 * or the end of the device.
			 */
			mutex_exit(hash_lock);
			break;
		}

		if (HDR_FREE_IN_PROGRESS(ab)) {
			/*
			 * Already on the path to destruction.
			 */
			mutex_exit(hash_lock);
			continue;
		}

		if (ab->b_state == arc_l2c_only) {
			ASSERT(!HDR_L2_READING(ab));
			/*
			 * This doesn't exist in the ARC.  Destroy.
			 * arc_hdr_destroy() will call list_remove()
			 * and decrement arcstat_l2_size.
			 */
			arc_change_state(arc_anon, ab, hash_lock);
			arc_hdr_destroy(ab);
		} else {
			/*
			 * Invalidate issued or about to be issued
			 * reads, since we may be about to write
			 * over this location.
			 */
			if (HDR_L2_READING(ab)) {
				ARCSTAT_BUMP(arcstat_l2_evict_reading);
				ab->b_flags |= ARC_L2_EVICTED;
			}

			/*
			 * Tell ARC this no longer exists in L2ARC.
			 */
			if (ab->b_l2hdr != NULL) {
				abl2 = ab->b_l2hdr;
				ab->b_l2hdr = NULL;
				kmem_free(abl2, sizeof (l2arc_buf_hdr_t));
				ARCSTAT_INCR(arcstat_l2_size, -ab->b_size);
			}
			list_remove(buflist, ab);

			/*
			 * This may have been leftover after a
			 * failed write.
			 */
			ab->b_flags &= ~ARC_L2_WRITING;
		}
		mutex_exit(hash_lock);
	}
	mutex_exit(&l2arc_buflist_mtx);

	spa_l2cache_space_update(dev->l2ad_vdev, 0, -(taddr - dev->l2ad_evict));
	dev->l2ad_evict = taddr;
}

/*
 * Find and write ARC buffers to the L2ARC device.
 *
 * An ARC_L2_WRITING flag is set so that the L2ARC buffers are not valid
 * for reading until they have completed writing.
 */
static uint64_t
l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
{
	arc_buf_hdr_t *ab, *ab_prev, *head;
	l2arc_buf_hdr_t *hdrl2;
	list_t *list;
	uint64_t passed_sz, write_sz, buf_sz, headroom;
	void *buf_data;
	kmutex_t *hash_lock, *list_lock;
	boolean_t have_lock, full;
	l2arc_write_callback_t *cb;
	zio_t *pio, *wzio;
	uint64_t guid = spa_guid(spa);

	ASSERT(dev->l2ad_vdev != NULL);

	pio = NULL;
	write_sz = 0;
	full = B_FALSE;
	head = kmem_cache_alloc(hdr_cache, KM_PUSHPAGE);
	head->b_flags |= ARC_L2_WRITE_HEAD;

	/*
	 * Copy buffers for L2ARC writing.
	 */
	mutex_enter(&l2arc_buflist_mtx);
	for (int try = 0; try <= 3; try++) {
		list = l2arc_list_locked(try, &list_lock);
		passed_sz = 0;

		/*
		 * L2ARC fast warmup.
		 *
		 * Until the ARC is warm and starts to evict, read from the
		 * head of the ARC lists rather than the tail.
		 */
		headroom = target_sz * l2arc_headroom;
		if (arc_warm == B_FALSE)
			ab = list_head(list);
		else
			ab = list_tail(list);

		for (; ab; ab = ab_prev) {
			if (arc_warm == B_FALSE)
				ab_prev = list_next(list, ab);
			else
				ab_prev = list_prev(list, ab);

			hash_lock = HDR_LOCK(ab);
			have_lock = MUTEX_HELD(hash_lock);
			if (!have_lock && !mutex_tryenter(hash_lock)) {
				/*
				 * Skip this buffer rather than waiting.
				 */
				continue;
			}

			passed_sz += ab->b_size;
			if (passed_sz > headroom) {
				/*
				 * Searched too far.
				 */
				mutex_exit(hash_lock);
				break;
			}

			if (!l2arc_write_eligible(guid, ab)) {
				mutex_exit(hash_lock);
				continue;
			}

			if ((write_sz + ab->b_size) > target_sz) {
				full = B_TRUE;
				mutex_exit(hash_lock);
				break;
			}

			if (pio == NULL) {
				/*
				 * Insert a dummy header on the buflist so
				 * l2arc_write_done() can find where the
				 * write buffers begin without searching.
				 */
				list_insert_head(dev->l2ad_buflist, head);

				cb = kmem_alloc(
				    sizeof (l2arc_write_callback_t), KM_SLEEP);
				cb->l2wcb_dev = dev;
				cb->l2wcb_head = head;
				pio = zio_root(spa, l2arc_write_done, cb,
				    ZIO_FLAG_CANFAIL);
			}

			/*
			 * Create and add a new L2ARC header.
			 */
			hdrl2 = kmem_zalloc(sizeof (l2arc_buf_hdr_t), KM_SLEEP);
			hdrl2->b_dev = dev;
			hdrl2->b_daddr = dev->l2ad_hand;

			ab->b_flags |= ARC_L2_WRITING;
			ab->b_l2hdr = hdrl2;
			list_insert_head(dev->l2ad_buflist, ab);
			buf_data = ab->b_buf->b_data;
			buf_sz = ab->b_size;

			/*
			 * Compute and store the buffer cksum before
			 * writing.  On debug the cksum is verified first.
			 */
			arc_cksum_verify(ab->b_buf);
			arc_cksum_compute(ab->b_buf, B_TRUE);

			mutex_exit(hash_lock);

			wzio = zio_write_phys(pio, dev->l2ad_vdev,
			    dev->l2ad_hand, buf_sz, buf_data, ZIO_CHECKSUM_OFF,
			    NULL, NULL, ZIO_PRIORITY_ASYNC_WRITE,
			    ZIO_FLAG_CANFAIL, B_FALSE);

			DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev,
			    zio_t *, wzio);
			(void) zio_nowait(wzio);

			/*
			 * Keep the clock hand suitably device-aligned.
			 */
			buf_sz = vdev_psize_to_asize(dev->l2ad_vdev, buf_sz);

			write_sz += buf_sz;
			dev->l2ad_hand += buf_sz;
		}

		mutex_exit(list_lock);

		if (full == B_TRUE)
			break;
	}
	mutex_exit(&l2arc_buflist_mtx);

	if (pio == NULL) {
		ASSERT3U(write_sz, ==, 0);
		kmem_cache_free(hdr_cache, head);
		return (0);
	}

	ASSERT3U(write_sz, <=, target_sz);
	ARCSTAT_BUMP(arcstat_l2_writes_sent);
	ARCSTAT_INCR(arcstat_l2_write_bytes, write_sz);
	ARCSTAT_INCR(arcstat_l2_size, write_sz);
	spa_l2cache_space_update(dev->l2ad_vdev, 0, write_sz);

	/*
	 * Bump device hand to the device start if it is approaching the end.
	 * l2arc_evict() will already have evicted ahead for this case.
	 */
	if (dev->l2ad_hand >= (dev->l2ad_end - target_sz)) {
		spa_l2cache_space_update(dev->l2ad_vdev, 0,
		    dev->l2ad_end - dev->l2ad_hand);
		dev->l2ad_hand = dev->l2ad_start;
		dev->l2ad_evict = dev->l2ad_start;
		dev->l2ad_first = B_FALSE;
	}

	dev->l2ad_writing = B_TRUE;
	(void) zio_wait(pio);
	dev->l2ad_writing = B_FALSE;

	return (write_sz);
}

/*
 * This thread feeds the L2ARC at regular intervals.  This is the beating
 * heart of the L2ARC.
 */
static void
l2arc_feed_thread(void)
{
	callb_cpr_t cpr;
	l2arc_dev_t *dev;
	spa_t *spa;
	uint64_t size, wrote;
	clock_t begin, next = lbolt;

	CALLB_CPR_INIT(&cpr, &l2arc_feed_thr_lock, callb_generic_cpr, FTAG);

	mutex_enter(&l2arc_feed_thr_lock);

	while (l2arc_thread_exit == 0) {
		CALLB_CPR_SAFE_BEGIN(&cpr);
		(void) cv_timedwait(&l2arc_feed_thr_cv, &l2arc_feed_thr_lock,
		    next);
		CALLB_CPR_SAFE_END(&cpr, &l2arc_feed_thr_lock);
		next = lbolt + hz;

		/*
		 * Quick check for L2ARC devices.
		 */
		mutex_enter(&l2arc_dev_mtx);
		if (l2arc_ndev == 0) {
			mutex_exit(&l2arc_dev_mtx);
			continue;
		}
		mutex_exit(&l2arc_dev_mtx);
		begin = lbolt;

		/*
		 * This selects the next l2arc device to write to, and in
		 * doing so the next spa to feed from: dev->l2ad_spa.   This
		 * will return NULL if there are now no l2arc devices or if
		 * they are all faulted.
		 *
		 * If a device is returned, its spa's config lock is also
		 * held to prevent device removal.  l2arc_dev_get_next()
		 * will grab and release l2arc_dev_mtx.
		 */
		if ((dev = l2arc_dev_get_next()) == NULL)
			continue;

		spa = dev->l2ad_spa;
		ASSERT(spa != NULL);

		/*
		 * Avoid contributing to memory pressure.
		 */
		if (arc_reclaim_needed()) {
			ARCSTAT_BUMP(arcstat_l2_abort_lowmem);
			spa_config_exit(spa, SCL_L2ARC, dev);
			continue;
		}

		ARCSTAT_BUMP(arcstat_l2_feeds);

		size = l2arc_write_size(dev);

		/*
		 * Evict L2ARC buffers that will be overwritten.
		 */
		l2arc_evict(dev, size, B_FALSE);

		/*
		 * Write ARC buffers.
		 */
		wrote = l2arc_write_buffers(spa, dev, size);

		/*
		 * Calculate interval between writes.
		 */
		next = l2arc_write_interval(begin, size, wrote);
		spa_config_exit(spa, SCL_L2ARC, dev);
	}

	l2arc_thread_exit = 0;
	cv_broadcast(&l2arc_feed_thr_cv);
	CALLB_CPR_EXIT(&cpr);		/* drops l2arc_feed_thr_lock */
	thread_exit();
}

boolean_t
l2arc_vdev_present(vdev_t *vd)
{
	l2arc_dev_t *dev;

	mutex_enter(&l2arc_dev_mtx);
	for (dev = list_head(l2arc_dev_list); dev != NULL;
	    dev = list_next(l2arc_dev_list, dev)) {
		if (dev->l2ad_vdev == vd)
			break;
	}
	mutex_exit(&l2arc_dev_mtx);

	return (dev != NULL);
}

/*
 * Add a vdev for use by the L2ARC.  By this point the spa has already
 * validated the vdev and opened it.
 */
void
l2arc_add_vdev(spa_t *spa, vdev_t *vd, uint64_t start, uint64_t end)
{
	l2arc_dev_t *adddev;

	ASSERT(!l2arc_vdev_present(vd));

	/*
	 * Create a new l2arc device entry.
	 */
	adddev = kmem_zalloc(sizeof (l2arc_dev_t), KM_SLEEP);
	adddev->l2ad_spa = spa;
	adddev->l2ad_vdev = vd;
	adddev->l2ad_write = l2arc_write_max;
	adddev->l2ad_boost = l2arc_write_boost;
	adddev->l2ad_start = start;
	adddev->l2ad_end = end;
	adddev->l2ad_hand = adddev->l2ad_start;
	adddev->l2ad_evict = adddev->l2ad_start;
	adddev->l2ad_first = B_TRUE;
	adddev->l2ad_writing = B_FALSE;
	ASSERT3U(adddev->l2ad_write, >, 0);

	/*
	 * This is a list of all ARC buffers that are still valid on the
	 * device.
	 */
	adddev->l2ad_buflist = kmem_zalloc(sizeof (list_t), KM_SLEEP);
	list_create(adddev->l2ad_buflist, sizeof (arc_buf_hdr_t),
	    offsetof(arc_buf_hdr_t, b_l2node));

	spa_l2cache_space_update(vd, adddev->l2ad_end - adddev->l2ad_hand, 0);

	/*
	 * Add device to global list
	 */
	mutex_enter(&l2arc_dev_mtx);
	list_insert_head(l2arc_dev_list, adddev);
	atomic_inc_64(&l2arc_ndev);
	mutex_exit(&l2arc_dev_mtx);
}

/*
 * Remove a vdev from the L2ARC.
 */
void
l2arc_remove_vdev(vdev_t *vd)
{
	l2arc_dev_t *dev, *nextdev, *remdev = NULL;

	/*
	 * Find the device by vdev
	 */
	mutex_enter(&l2arc_dev_mtx);
	for (dev = list_head(l2arc_dev_list); dev; dev = nextdev) {
		nextdev = list_next(l2arc_dev_list, dev);
		if (vd == dev->l2ad_vdev) {
			remdev = dev;
			break;
		}
	}
	ASSERT(remdev != NULL);

	/*
	 * Remove device from global list
	 */
	list_remove(l2arc_dev_list, remdev);
	l2arc_dev_last = NULL;		/* may have been invalidated */
	atomic_dec_64(&l2arc_ndev);
	mutex_exit(&l2arc_dev_mtx);

	/*
	 * Clear all buflists and ARC references.  L2ARC device flush.
	 */
	l2arc_evict(remdev, 0, B_TRUE);
	list_destroy(remdev->l2ad_buflist);
	kmem_free(remdev->l2ad_buflist, sizeof (list_t));
	kmem_free(remdev, sizeof (l2arc_dev_t));
}

void
l2arc_init(void)
{
	l2arc_thread_exit = 0;
	l2arc_ndev = 0;
	l2arc_writes_sent = 0;
	l2arc_writes_done = 0;

	mutex_init(&l2arc_feed_thr_lock, NULL, MUTEX_DEFAULT, NULL);
	cv_init(&l2arc_feed_thr_cv, NULL, CV_DEFAULT, NULL);
	mutex_init(&l2arc_dev_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&l2arc_buflist_mtx, NULL, MUTEX_DEFAULT, NULL);
	mutex_init(&l2arc_free_on_write_mtx, NULL, MUTEX_DEFAULT, NULL);

	l2arc_dev_list = &L2ARC_dev_list;
	l2arc_free_on_write = &L2ARC_free_on_write;
	list_create(l2arc_dev_list, sizeof (l2arc_dev_t),
	    offsetof(l2arc_dev_t, l2ad_node));
	list_create(l2arc_free_on_write, sizeof (l2arc_data_free_t),
	    offsetof(l2arc_data_free_t, l2df_list_node));
}

void
l2arc_fini(void)
{
	/*
	 * This is called from dmu_fini(), which is called from spa_fini();
	 * Because of this, we can assume that all l2arc devices have
	 * already been removed when the pools themselves were removed.
	 */

	l2arc_do_free_on_write();

	mutex_destroy(&l2arc_feed_thr_lock);
	cv_destroy(&l2arc_feed_thr_cv);
	mutex_destroy(&l2arc_dev_mtx);
	mutex_destroy(&l2arc_buflist_mtx);
	mutex_destroy(&l2arc_free_on_write_mtx);

	list_destroy(l2arc_dev_list);
	list_destroy(l2arc_free_on_write);
}

void
l2arc_start(void)
{
	if (!(spa_mode_global & FWRITE))
		return;

	(void) thread_create(NULL, 0, l2arc_feed_thread, NULL, 0, &p0,
	    TS_RUN, minclsyspri);
}

void
l2arc_stop(void)
{
	if (!(spa_mode_global & FWRITE))
		return;

	mutex_enter(&l2arc_feed_thr_lock);
	cv_signal(&l2arc_feed_thr_cv);	/* kick thread out of startup */
	l2arc_thread_exit = 1;
	while (l2arc_thread_exit != 0)
		cv_wait(&l2arc_feed_thr_cv, &l2arc_feed_thr_lock);
	mutex_exit(&l2arc_feed_thr_lock);
}