xref: /freebsd/share/man/man5/fs.5 (revision b3e7694832e81d7a904a10f525f8797b753bf0d3)
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28.\"     @(#)fs.5	8.2 (Berkeley) 4/19/94
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31.Dd January 16, 2017
32.Dt FS 5
33.Os
34.Sh NAME
35.Nm fs ,
36.Nm inode
37.Nd format of file system volume
38.Sh SYNOPSIS
39.In sys/param.h
40.In ufs/ffs/fs.h
41.Pp
42.In sys/types.h
43.In sys/lock.h
44.In sys/extattr.h
45.In sys/acl.h
46.In ufs/ufs/quota.h
47.In ufs/ufs/dinode.h
48.In ufs/ufs/extattr.h
49.Sh DESCRIPTION
50The files
51.In fs.h
52and
53.In inode.h
54declare several structures, defined variables and macros
55which are used to create and manage the underlying format of
56file system objects on random access devices (disks).
57.Pp
58The block size and number of blocks which
59comprise a file system are parameters of the file system.
60Sectors beginning at
61.Dv BBLOCK
62and continuing for
63.Dv BBSIZE
64are used
65for a disklabel and for some hardware primary
66and secondary bootstrapping programs.
67.Pp
68The actual file system begins at sector
69.Dv SBLOCK
70with the
71.Em super-block
72that is of size
73.Dv SBLOCKSIZE .
74The following structure describes the super-block and is
75from the file
76.In ufs/ffs/fs.h :
77.Bd -literal
78/*
79 * Super block for an FFS filesystem.
80 */
81struct fs {
82	int32_t	 fs_firstfield;	   /* historic filesystem linked list, */
83	int32_t	 fs_unused_1;      /*     used for incore super blocks */
84	int32_t	 fs_sblkno;        /* offset of super-block in filesys */
85	int32_t	 fs_cblkno;        /* offset of cyl-block in filesys */
86	int32_t	 fs_iblkno;        /* offset of inode-blocks in filesys */
87	int32_t	 fs_dblkno;        /* offset of first data after cg */
88	int32_t	 fs_old_cgoffset;  /* cylinder group offset in cylinder */
89	int32_t	 fs_old_cgmask;    /* used to calc mod fs_ntrak */
90	int32_t  fs_old_time;      /* last time written */
91	int32_t	 fs_old_size;      /* number of blocks in fs */
92	int32_t	 fs_old_dsize;     /* number of data blocks in fs */
93	int32_t	 fs_ncg;           /* number of cylinder groups */
94	int32_t	 fs_bsize;         /* size of basic blocks in fs */
95	int32_t	 fs_fsize;         /* size of frag blocks in fs */
96	int32_t	 fs_frag;          /* number of frags in a block in fs */
97/* these are configuration parameters */
98	int32_t	 fs_minfree;       /* minimum percentage of free blocks */
99	int32_t	 fs_old_rotdelay;  /* num of ms for optimal next block */
100	int32_t	 fs_old_rps;       /* disk revolutions per second */
101/* these fields can be computed from the others */
102	int32_t	 fs_bmask;         /* ``blkoff'' calc of blk offsets */
103	int32_t	 fs_fmask;         /* ``fragoff'' calc of frag offsets */
104	int32_t	 fs_bshift;        /* ``lblkno'' calc of logical blkno */
105	int32_t	 fs_fshift;        /* ``numfrags'' calc number of frags */
106/* these are configuration parameters */
107	int32_t	 fs_maxcontig;     /* max number of contiguous blks */
108	int32_t	 fs_maxbpg;        /* max number of blks per cyl group */
109/* these fields can be computed from the others */
110	int32_t	 fs_fragshift;     /* block to frag shift */
111	int32_t	 fs_fsbtodb;       /* fsbtodb and dbtofsb shift constant */
112	int32_t	 fs_sbsize;        /* actual size of super block */
113	int32_t	 fs_spare1[2];     /* old fs_csmask */
114	                           /* old fs_csshift */
115	int32_t	 fs_nindir;        /* value of NINDIR */
116	int32_t	 fs_inopb;         /* value of INOPB */
117	int32_t	 fs_old_nspf;      /* value of NSPF */
118/* yet another configuration parameter */
119	int32_t	 fs_optim;         /* optimization preference, see below */
120	int32_t	 fs_old_npsect;    /* # sectors/track including spares */
121	int32_t	 fs_old_interleave; /* hardware sector interleave */
122	int32_t	 fs_old_trackskew; /* sector 0 skew, per track */
123	int32_t	 fs_id[2];         /* unique filesystem id */
124/* sizes determined by number of cylinder groups and their sizes */
125	int32_t	 fs_old_csaddr;	   /* blk addr of cyl grp summary area */
126	int32_t	 fs_cssize;        /* size of cyl grp summary area */
127	int32_t	 fs_cgsize;        /* cylinder group size */
128	int32_t	 fs_spare2;        /* old fs_ntrak */
129	int32_t	 fs_old_nsect;     /* sectors per track */
130	int32_t  fs_old_spc;       /* sectors per cylinder */
131	int32_t	 fs_old_ncyl;      /* cylinders in filesystem */
132	int32_t	 fs_old_cpg;       /* cylinders per group */
133	int32_t	 fs_ipg;           /* inodes per group */
134	int32_t	 fs_fpg;           /* blocks per group * fs_frag */
135/* this data must be re-computed after crashes */
136	struct	csum fs_old_cstotal; /* cylinder summary information */
137/* these fields are cleared at mount time */
138	int8_t   fs_fmod;          /* super block modified flag */
139	int8_t   fs_clean;         /* filesystem is clean flag */
140	int8_t 	 fs_ronly;         /* mounted read-only flag */
141	int8_t   fs_old_flags;     /* old FS_ flags */
142	u_char	 fs_fsmnt[MAXMNTLEN]; /* name mounted on */
143	u_char	 fs_volname[MAXVOLLEN]; /* volume name */
144	uint64_t fs_swuid;         /* system-wide uid */
145	int32_t  fs_pad;           /* due to alignment of fs_swuid */
146/* these fields retain the current block allocation info */
147	int32_t	 fs_cgrotor;       /* last cg searched */
148	void 	*fs_ocsp[NOCSPTRS]; /* padding; was list of fs_cs buffers */
149	uint8_t *fs_contigdirs;    /* # of contiguously allocated dirs */
150	struct	csum *fs_csp;      /* cg summary info buffer for fs_cs */
151	int32_t	*fs_maxcluster;    /* max cluster in each cyl group */
152	u_int	*fs_active;        /* used by snapshots to track fs */
153	int32_t	 fs_old_cpc;       /* cyl per cycle in postbl */
154	int32_t	 fs_maxbsize;      /* maximum blocking factor permitted */
155	int64_t	 fs_unrefs;        /* number of unreferenced inodes */
156	int64_t	 fs_sparecon64[16]; /* old rotation block list head */
157	int64_t	 fs_sblockloc;     /* byte offset of standard superblock */
158	struct	csum_total fs_cstotal;  /* cylinder summary information */
159	ufs_time_t fs_time;        /* last time written */
160	int64_t	 fs_size;          /* number of blocks in fs */
161	int64_t	 fs_dsize;         /* number of data blocks in fs */
162	ufs2_daddr_t fs_csaddr;    /* blk addr of cyl grp summary area */
163	int64_t	 fs_pendingblocks; /* blocks in process of being freed */
164	int32_t	 fs_pendinginodes; /* inodes in process of being freed */
165	int32_t	 fs_snapinum[FSMAXSNAP]; /* list of snapshot inode numbers */
166	int32_t	 fs_avgfilesize;   /* expected average file size */
167	int32_t	 fs_avgfpdir;      /* expected # of files per directory */
168	int32_t	 fs_save_cgsize;   /* save real cg size to use fs_bsize */
169	int32_t	 fs_sparecon32[26]; /* reserved for future constants */
170	int32_t  fs_flags;         /* see FS_ flags below */
171	int32_t	 fs_contigsumsize; /* size of cluster summary array */
172	int32_t	 fs_maxsymlinklen; /* max length of an internal symlink */
173	int32_t	 fs_old_inodefmt;  /* format of on-disk inodes */
174	uint64_t fs_maxfilesize;   /* maximum representable file size */
175	int64_t	 fs_qbmask;        /* ~fs_bmask for use with 64-bit size */
176	int64_t	 fs_qfmask;        /* ~fs_fmask for use with 64-bit size */
177	int32_t	 fs_state;         /* validate fs_clean field */
178	int32_t	 fs_old_postblformat; /* format of positional layout tables */
179	int32_t	 fs_old_nrpos;     /* number of rotational positions */
180	int32_t	 fs_spare5[2];     /* old fs_postbloff */
181	                           /* old fs_rotbloff */
182	int32_t	 fs_magic;         /* magic number */
183};
184
185/*
186 * Filesystem identification
187 */
188#define	FS_UFS1_MAGIC	0x011954    /* UFS1 fast filesystem magic number */
189#define	FS_UFS2_MAGIC	0x19540119  /* UFS2 fast filesystem magic number */
190#define	FS_OKAY		0x7c269d38  /* superblock checksum */
191#define FS_42INODEFMT	-1      /* 4.2BSD inode format */
192#define FS_44INODEFMT	2       /* 4.4BSD inode format */
193
194/*
195 * Preference for optimization.
196 */
197#define FS_OPTTIME	0	/* minimize allocation time */
198#define FS_OPTSPACE	1	/* minimize disk fragmentation */
199.Ed
200.Pp
201Each disk drive contains some number of file systems.
202A file system consists of a number of cylinder groups.
203Each cylinder group has inodes and data.
204.Pp
205A file system is described by its super-block, which in turn
206describes the cylinder groups.
207The super-block is critical
208data and is replicated in each cylinder group to protect against
209catastrophic loss.
210This is done at file system creation
211time and the critical
212super-block data does not change, so the copies need not be
213referenced further unless disaster strikes.
214.Pp
215Addresses stored in inodes are capable of addressing fragments
216of `blocks'.
217File system blocks of at most size
218.Dv MAXBSIZE
219can
220be optionally broken into 2, 4, or 8 pieces, each of which is
221addressable; these pieces may be
222.Dv DEV_BSIZE ,
223or some multiple of
224a
225.Dv DEV_BSIZE
226unit.
227.Pp
228Large files consist of exclusively large data blocks.
229To avoid
230undue wasted disk space, the last data block of a small file is
231allocated as only as many fragments of a large block as are
232necessary.
233The file system format retains only a single pointer
234to such a fragment, which is a piece of a single large block that
235has been divided.
236The size of such a fragment is determinable from
237information in the inode, using the
238.Fn blksize fs ip lbn
239macro.
240.Pp
241The file system records space availability at the fragment level;
242to determine block availability, aligned fragments are examined.
243.Pp
244The root inode is the root of the file system.
245Inode 0 cannot be used for normal purposes and
246historically bad blocks were linked to inode 1,
247thus the root inode is 2 (inode 1 is no longer used for
248this purpose, however numerous dump tapes make this
249assumption, so we are stuck with it).
250.Pp
251The
252.Fa fs_minfree
253element gives the minimum acceptable percentage of file system
254blocks that may be free.
255If the freelist drops below this level
256only the super-user may continue to allocate blocks.
257The
258.Fa fs_minfree
259element
260may be set to 0 if no reserve of free blocks is deemed necessary,
261however severe performance degradations will be observed if the
262file system is run at greater than 90% full; thus the default
263value of
264.Fa fs_minfree
265is 8%.
266.Pp
267Empirically the best trade-off between block fragmentation and
268overall disk utilization at a loading of 90% comes with a
269fragmentation of 8, thus the default fragment size is an eighth
270of the block size.
271.Pp
272The element
273.Fa fs_optim
274specifies whether the file system should try to minimize the time spent
275allocating blocks, or if it should attempt to minimize the space
276fragmentation on the disk.
277If the value of fs_minfree (see above) is less than 8%,
278then the file system defaults to optimizing for space to avoid
279running out of full sized blocks.
280If the value of minfree is greater than or equal to 8%,
281fragmentation is unlikely to be problematical, and
282the file system defaults to optimizing for time.
283.Pp
284.Em Cylinder group related limits :
285Each cylinder keeps track of the availability of blocks at different
286rotational positions, so that sequential blocks can be laid out
287with minimum rotational latency.
288With the default of 8 distinguished
289rotational positions, the resolution of the
290summary information is 2ms for a typical 3600 rpm drive.
291.Pp
292The element
293.Fa fs_old_rotdelay
294gives the minimum number of milliseconds to initiate
295another disk transfer on the same cylinder.
296It is used in determining the rotationally optimal
297layout for disk blocks within a file;
298the default value for
299.Fa fs_old_rotdelay
300is 2ms.
301.Pp
302Each file system has a statically allocated number of inodes.
303An inode is allocated for each
304.Dv NBPI
305bytes of disk space.
306The inode allocation strategy is extremely conservative.
307.Pp
308.Dv MINBSIZE
309is the smallest allowable block size.
310With a
311.Dv MINBSIZE
312of 4096
313it is possible to create files of size
3142^32 with only two levels of indirection.
315.Dv MINBSIZE
316must be big enough to hold a cylinder group block,
317thus changes to
318.Pq Fa struct cg
319must keep its size within
320.Dv MINBSIZE .
321Note that super-blocks are never more than size
322.Dv SBLOCKSIZE .
323.Pp
324The path name on which the file system is mounted is maintained in
325.Fa fs_fsmnt .
326.Dv MAXMNTLEN
327defines the amount of space allocated in
328the super-block for this name.
329The limit on the amount of summary information per file system
330is defined by
331.Dv MAXCSBUFS .
332For a 4096 byte block size, it is currently parameterized for a
333maximum of two million cylinders.
334.Pp
335Per cylinder group information is summarized in blocks allocated
336from the first cylinder group's data blocks.
337These blocks are read in from
338.Fa fs_csaddr
339(size
340.Fa fs_cssize )
341in addition to the super-block.
342.Pp
343.Sy N.B. :
344.Fn sizeof "struct csum"
345must be a power of two in order for
346the
347.Fn fs_cs
348macro to work.
349.Pp
350The
351.Em "Super-block for a file system" :
352The size of the rotational layout tables
353is limited by the fact that the super-block is of size
354.Dv SBLOCKSIZE .
355The size of these tables is
356.Em inversely
357proportional to the block
358size of the file system.
359The size of the tables is
360increased when sector sizes are not powers of two,
361as this increases the number of cylinders
362included before the rotational pattern repeats
363.Pq Fa fs_cpc .
364The size of the rotational layout
365tables is derived from the number of bytes remaining in
366.Pq Fa struct fs .
367.Pp
368The number of blocks of data per cylinder group
369is limited because cylinder groups are at most one block.
370The inode and free block tables
371must fit into a single block after deducting space for
372the cylinder group structure
373.Pq Fa struct cg .
374.Pp
375The
376.Em Inode :
377The inode is the focus of all file activity in the
378.Ux
379file system.
380There is a unique inode allocated
381for each active file,
382each current directory, each mounted-on file,
383text file, and the root.
384An inode is `named' by its device/i-number pair.
385For further information, see the include file
386.In ufs/ufs/inode.h .
387.Pp
388The format of an external attribute is defined by the extattr structure:
389.Bd -literal
390struct extattr {
391	uint32_t ea_length;	    /* length of this attribute */
392	uint8_t	ea_namespace;	    /* name space of this attribute */
393	uint8_t	ea_contentpadlen;   /* bytes of padding at end of attribute */
394	uint8_t	ea_namelength;	    /* length of attribute name */
395	char	ea_name[1];	    /* attribute name (NOT nul-terminated) */
396	/* padding, if any, to align attribute content to 8 byte boundary */
397	/* extended attribute content follows */
398};
399.Ed
400.Pp
401Several macros are defined to manipulate these structures.
402Each macro takes a pointer to an extattr structure.
403.Bl -tag -width ".Dv EXTATTR_CONTENT_SIZE(eap)"
404.It Dv EXTATTR_NEXT(eap)
405Returns a pointer to the next extended attribute following
406.Fa eap .
407.It Dv EXTATTR_CONTENT(eap)
408Returns a pointer to the extended attribute content referenced by
409.Fa eap .
410.It Dv EXTATTR_CONTENT_SIZE(eap)
411Returns the size of the extended attribute content referenced by
412.Fa eap .
413.El
414.Pp
415The following code identifies an ACL:
416.Bd -literal
417	if (eap->ea_namespace == EXTATTR_NAMESPACE_SYSTEM &&
418            eap->ea_namelength == sizeof(POSIX1E_ACL_ACCESS_EXTATTR_NAME) - 1 &&
419	    strncmp(eap->ea_name, POSIX1E_ACL_ACCESS_EXTATTR_NAME,
420             sizeof(POSIX1E_ACL_ACCESS_EXTATTR_NAME) - 1) == 0) {
421		aclp = EXTATTR_CONTENT(eap);
422		acllen = EXTATTR_CONTENT_SIZE(eap);
423		...
424	}
425.Ed
426.Sh HISTORY
427A super-block structure named filsys appeared in
428.At v6 .
429The file system described in this manual appeared
430in
431.Bx 4.2 .
432