1 /* 2 * CDDL HEADER START 3 * 4 * The contents of this file are subject to the terms of the 5 * Common Development and Distribution License (the "License"). 6 * You may not use this file except in compliance with the License. 7 * 8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE 9 * or http://www.opensolaris.org/os/licensing. 10 * See the License for the specific language governing permissions 11 * and limitations under the License. 12 * 13 * When distributing Covered Code, include this CDDL HEADER in each 14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE. 15 * If applicable, add the following below this CDDL HEADER, with the 16 * fields enclosed by brackets "[]" replaced with your own identifying 17 * information: Portions Copyright [yyyy] [name of copyright owner] 18 * 19 * CDDL HEADER END 20 */ 21 /* 22 * Copyright 2009 Sun Microsystems, Inc. All rights reserved. 23 * Use is subject to license terms. 24 */ 25 26 #include <sys/zfs_context.h> 27 #include <sys/spa.h> 28 #include <sys/vdev_impl.h> 29 #include <sys/zio.h> 30 #include <sys/kstat.h> 31 32 /* 33 * Virtual device read-ahead caching. 34 * 35 * This file implements a simple LRU read-ahead cache. When the DMU reads 36 * a given block, it will often want other, nearby blocks soon thereafter. 37 * We take advantage of this by reading a larger disk region and caching 38 * the result. In the best case, this can turn 128 back-to-back 512-byte 39 * reads into a single 64k read followed by 127 cache hits; this reduces 40 * latency dramatically. In the worst case, it can turn an isolated 512-byte 41 * read into a 64k read, which doesn't affect latency all that much but is 42 * terribly wasteful of bandwidth. A more intelligent version of the cache 43 * could keep track of access patterns and not do read-ahead unless it sees 44 * at least two temporally close I/Os to the same region. Currently, only 45 * metadata I/O is inflated. A futher enhancement could take advantage of 46 * more semantic information about the I/O. And it could use something 47 * faster than an AVL tree; that was chosen solely for convenience. 48 * 49 * There are five cache operations: allocate, fill, read, write, evict. 50 * 51 * (1) Allocate. This reserves a cache entry for the specified region. 52 * We separate the allocate and fill operations so that multiple threads 53 * don't generate I/O for the same cache miss. 54 * 55 * (2) Fill. When the I/O for a cache miss completes, the fill routine 56 * places the data in the previously allocated cache entry. 57 * 58 * (3) Read. Read data from the cache. 59 * 60 * (4) Write. Update cache contents after write completion. 61 * 62 * (5) Evict. When allocating a new entry, we evict the oldest (LRU) entry 63 * if the total cache size exceeds zfs_vdev_cache_size. 64 */ 65 66 /* 67 * These tunables are for performance analysis. 68 */ 69 /* 70 * All i/os smaller than zfs_vdev_cache_max will be turned into 71 * 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software 72 * track buffer). At most zfs_vdev_cache_size bytes will be kept in each 73 * vdev's vdev_cache. 74 * 75 * TODO: Note that with the current ZFS code, it turns out that the 76 * vdev cache is not helpful, and in some cases actually harmful. It 77 * is better if we disable this. Once some time has passed, we should 78 * actually remove this to simplify the code. For now we just disable 79 * it by setting the zfs_vdev_cache_size to zero. Note that Solaris 11 80 * has made these same changes. 81 */ 82 int zfs_vdev_cache_max = 1<<14; /* 16KB */ 83 int zfs_vdev_cache_size = 0; 84 int zfs_vdev_cache_bshift = 16; 85 86 #define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */ 87 88 kstat_t *vdc_ksp = NULL; 89 90 typedef struct vdc_stats { 91 kstat_named_t vdc_stat_delegations; 92 kstat_named_t vdc_stat_hits; 93 kstat_named_t vdc_stat_misses; 94 } vdc_stats_t; 95 96 static vdc_stats_t vdc_stats = { 97 { "delegations", KSTAT_DATA_UINT64 }, 98 { "hits", KSTAT_DATA_UINT64 }, 99 { "misses", KSTAT_DATA_UINT64 } 100 }; 101 102 #define VDCSTAT_BUMP(stat) atomic_add_64(&vdc_stats.stat.value.ui64, 1); 103 104 static int 105 vdev_cache_offset_compare(const void *a1, const void *a2) 106 { 107 const vdev_cache_entry_t *ve1 = a1; 108 const vdev_cache_entry_t *ve2 = a2; 109 110 if (ve1->ve_offset < ve2->ve_offset) 111 return (-1); 112 if (ve1->ve_offset > ve2->ve_offset) 113 return (1); 114 return (0); 115 } 116 117 static int 118 vdev_cache_lastused_compare(const void *a1, const void *a2) 119 { 120 const vdev_cache_entry_t *ve1 = a1; 121 const vdev_cache_entry_t *ve2 = a2; 122 123 if (ve1->ve_lastused < ve2->ve_lastused) 124 return (-1); 125 if (ve1->ve_lastused > ve2->ve_lastused) 126 return (1); 127 128 /* 129 * Among equally old entries, sort by offset to ensure uniqueness. 130 */ 131 return (vdev_cache_offset_compare(a1, a2)); 132 } 133 134 /* 135 * Evict the specified entry from the cache. 136 */ 137 static void 138 vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve) 139 { 140 ASSERT(MUTEX_HELD(&vc->vc_lock)); 141 ASSERT(ve->ve_fill_io == NULL); 142 ASSERT(ve->ve_data != NULL); 143 144 avl_remove(&vc->vc_lastused_tree, ve); 145 avl_remove(&vc->vc_offset_tree, ve); 146 zio_buf_free(ve->ve_data, VCBS); 147 kmem_free(ve, sizeof (vdev_cache_entry_t)); 148 } 149 150 /* 151 * Allocate an entry in the cache. At the point we don't have the data, 152 * we're just creating a placeholder so that multiple threads don't all 153 * go off and read the same blocks. 154 */ 155 static vdev_cache_entry_t * 156 vdev_cache_allocate(zio_t *zio) 157 { 158 vdev_cache_t *vc = &zio->io_vd->vdev_cache; 159 uint64_t offset = P2ALIGN(zio->io_offset, VCBS); 160 vdev_cache_entry_t *ve; 161 162 ASSERT(MUTEX_HELD(&vc->vc_lock)); 163 164 if (zfs_vdev_cache_size == 0) 165 return (NULL); 166 167 /* 168 * If adding a new entry would exceed the cache size, 169 * evict the oldest entry (LRU). 170 */ 171 if ((avl_numnodes(&vc->vc_lastused_tree) << zfs_vdev_cache_bshift) > 172 zfs_vdev_cache_size) { 173 ve = avl_first(&vc->vc_lastused_tree); 174 if (ve->ve_fill_io != NULL) 175 return (NULL); 176 ASSERT(ve->ve_hits != 0); 177 vdev_cache_evict(vc, ve); 178 } 179 180 ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP); 181 ve->ve_offset = offset; 182 ve->ve_lastused = ddi_get_lbolt(); 183 ve->ve_data = zio_buf_alloc(VCBS); 184 185 avl_add(&vc->vc_offset_tree, ve); 186 avl_add(&vc->vc_lastused_tree, ve); 187 188 return (ve); 189 } 190 191 static void 192 vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio) 193 { 194 uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS); 195 196 ASSERT(MUTEX_HELD(&vc->vc_lock)); 197 ASSERT(ve->ve_fill_io == NULL); 198 199 if (ve->ve_lastused != ddi_get_lbolt()) { 200 avl_remove(&vc->vc_lastused_tree, ve); 201 ve->ve_lastused = ddi_get_lbolt(); 202 avl_add(&vc->vc_lastused_tree, ve); 203 } 204 205 ve->ve_hits++; 206 bcopy(ve->ve_data + cache_phase, zio->io_data, zio->io_size); 207 } 208 209 /* 210 * Fill a previously allocated cache entry with data. 211 */ 212 static void 213 vdev_cache_fill(zio_t *fio) 214 { 215 vdev_t *vd = fio->io_vd; 216 vdev_cache_t *vc = &vd->vdev_cache; 217 vdev_cache_entry_t *ve = fio->io_private; 218 zio_t *pio; 219 220 ASSERT(fio->io_size == VCBS); 221 222 /* 223 * Add data to the cache. 224 */ 225 mutex_enter(&vc->vc_lock); 226 227 ASSERT(ve->ve_fill_io == fio); 228 ASSERT(ve->ve_offset == fio->io_offset); 229 ASSERT(ve->ve_data == fio->io_data); 230 231 ve->ve_fill_io = NULL; 232 233 /* 234 * Even if this cache line was invalidated by a missed write update, 235 * any reads that were queued up before the missed update are still 236 * valid, so we can satisfy them from this line before we evict it. 237 */ 238 while ((pio = zio_walk_parents(fio)) != NULL) 239 vdev_cache_hit(vc, ve, pio); 240 241 if (fio->io_error || ve->ve_missed_update) 242 vdev_cache_evict(vc, ve); 243 244 mutex_exit(&vc->vc_lock); 245 } 246 247 /* 248 * Read data from the cache. Returns 0 on cache hit, errno on a miss. 249 */ 250 int 251 vdev_cache_read(zio_t *zio) 252 { 253 vdev_cache_t *vc = &zio->io_vd->vdev_cache; 254 vdev_cache_entry_t *ve, ve_search; 255 uint64_t cache_offset = P2ALIGN(zio->io_offset, VCBS); 256 uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS); 257 zio_t *fio; 258 259 ASSERT(zio->io_type == ZIO_TYPE_READ); 260 261 if (zio->io_flags & ZIO_FLAG_DONT_CACHE) 262 return (EINVAL); 263 264 if (zio->io_size > zfs_vdev_cache_max) 265 return (EOVERFLOW); 266 267 /* 268 * If the I/O straddles two or more cache blocks, don't cache it. 269 */ 270 if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS)) 271 return (EXDEV); 272 273 ASSERT(cache_phase + zio->io_size <= VCBS); 274 275 mutex_enter(&vc->vc_lock); 276 277 ve_search.ve_offset = cache_offset; 278 ve = avl_find(&vc->vc_offset_tree, &ve_search, NULL); 279 280 if (ve != NULL) { 281 if (ve->ve_missed_update) { 282 mutex_exit(&vc->vc_lock); 283 return (ESTALE); 284 } 285 286 if ((fio = ve->ve_fill_io) != NULL) { 287 zio_vdev_io_bypass(zio); 288 zio_add_child(zio, fio); 289 mutex_exit(&vc->vc_lock); 290 VDCSTAT_BUMP(vdc_stat_delegations); 291 return (0); 292 } 293 294 vdev_cache_hit(vc, ve, zio); 295 zio_vdev_io_bypass(zio); 296 297 mutex_exit(&vc->vc_lock); 298 VDCSTAT_BUMP(vdc_stat_hits); 299 return (0); 300 } 301 302 ve = vdev_cache_allocate(zio); 303 304 if (ve == NULL) { 305 mutex_exit(&vc->vc_lock); 306 return (ENOMEM); 307 } 308 309 fio = zio_vdev_delegated_io(zio->io_vd, cache_offset, 310 ve->ve_data, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_CACHE_FILL, 311 ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve); 312 313 ve->ve_fill_io = fio; 314 zio_vdev_io_bypass(zio); 315 zio_add_child(zio, fio); 316 317 mutex_exit(&vc->vc_lock); 318 zio_nowait(fio); 319 VDCSTAT_BUMP(vdc_stat_misses); 320 321 return (0); 322 } 323 324 /* 325 * Update cache contents upon write completion. 326 */ 327 void 328 vdev_cache_write(zio_t *zio) 329 { 330 vdev_cache_t *vc = &zio->io_vd->vdev_cache; 331 vdev_cache_entry_t *ve, ve_search; 332 uint64_t io_start = zio->io_offset; 333 uint64_t io_end = io_start + zio->io_size; 334 uint64_t min_offset = P2ALIGN(io_start, VCBS); 335 uint64_t max_offset = P2ROUNDUP(io_end, VCBS); 336 avl_index_t where; 337 338 ASSERT(zio->io_type == ZIO_TYPE_WRITE); 339 340 mutex_enter(&vc->vc_lock); 341 342 ve_search.ve_offset = min_offset; 343 ve = avl_find(&vc->vc_offset_tree, &ve_search, &where); 344 345 if (ve == NULL) 346 ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER); 347 348 while (ve != NULL && ve->ve_offset < max_offset) { 349 uint64_t start = MAX(ve->ve_offset, io_start); 350 uint64_t end = MIN(ve->ve_offset + VCBS, io_end); 351 352 if (ve->ve_fill_io != NULL) { 353 ve->ve_missed_update = 1; 354 } else { 355 bcopy((char *)zio->io_data + start - io_start, 356 ve->ve_data + start - ve->ve_offset, end - start); 357 } 358 ve = AVL_NEXT(&vc->vc_offset_tree, ve); 359 } 360 mutex_exit(&vc->vc_lock); 361 } 362 363 void 364 vdev_cache_purge(vdev_t *vd) 365 { 366 vdev_cache_t *vc = &vd->vdev_cache; 367 vdev_cache_entry_t *ve; 368 369 mutex_enter(&vc->vc_lock); 370 while ((ve = avl_first(&vc->vc_offset_tree)) != NULL) 371 vdev_cache_evict(vc, ve); 372 mutex_exit(&vc->vc_lock); 373 } 374 375 void 376 vdev_cache_init(vdev_t *vd) 377 { 378 vdev_cache_t *vc = &vd->vdev_cache; 379 380 mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL); 381 382 avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare, 383 sizeof (vdev_cache_entry_t), 384 offsetof(struct vdev_cache_entry, ve_offset_node)); 385 386 avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare, 387 sizeof (vdev_cache_entry_t), 388 offsetof(struct vdev_cache_entry, ve_lastused_node)); 389 } 390 391 void 392 vdev_cache_fini(vdev_t *vd) 393 { 394 vdev_cache_t *vc = &vd->vdev_cache; 395 396 vdev_cache_purge(vd); 397 398 avl_destroy(&vc->vc_offset_tree); 399 avl_destroy(&vc->vc_lastused_tree); 400 401 mutex_destroy(&vc->vc_lock); 402 } 403 404 void 405 vdev_cache_stat_init(void) 406 { 407 vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc", 408 KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t), 409 KSTAT_FLAG_VIRTUAL); 410 if (vdc_ksp != NULL) { 411 vdc_ksp->ks_data = &vdc_stats; 412 kstat_install(vdc_ksp); 413 } 414 } 415 416 void 417 vdev_cache_stat_fini(void) 418 { 419 if (vdc_ksp != NULL) { 420 kstat_delete(vdc_ksp); 421 vdc_ksp = NULL; 422 } 423 } 424