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32 .\" @(#)fs.5 8.2 (Berkeley) 4/19/94
33 .\" $FreeBSD$
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35 .Dd October 31, 2006
36 .Dt FS 5
37 .Os
38 .Sh NAME
39 .Nm fs ,
40 .Nm inode
41 .Nd format of file system volume
42 .Sh SYNOPSIS
43 .In sys/param.h
44 .In ufs/ffs/fs.h
45 .Pp
46 .In sys/types.h
47 .In sys/lock.h
48 .In sys/extattr.h
49 .In sys/acl.h
50 .In ufs/ufs/quota.h
51 .In ufs/ufs/dinode.h
52 .In ufs/ufs/extattr.h
53 .Sh DESCRIPTION
54 The files
55 .In fs.h
56 and
57 .In inode.h
58 declare several structures, defined variables and macros
59 which are used to create and manage the underlying format of
60 file system objects on random access devices (disks).
61 .Pp
62 The block size and number of blocks which
63 comprise a file system are parameters of the file system.
64 Sectors beginning at
65 .Dv BBLOCK
66 and continuing for
67 .Dv BBSIZE
68 are used
69 for a disklabel and for some hardware primary
70 and secondary bootstrapping programs.
71 .Pp
72 The actual file system begins at sector
73 .Dv SBLOCK
74 with the
75 .Em super-block
76 that is of size
77 .Dv SBLOCKSIZE .
78 The following structure describes the super-block and is
79 from the file
80 .In ufs/ffs/fs.h :
81 .Bd -literal
82 /*
83 * Super block for an FFS filesystem.
84 */
85 struct fs {
86 int32_t fs_firstfield; /* historic filesystem linked list, */
87 int32_t fs_unused_1; /* used for incore super blocks */
88 int32_t fs_sblkno; /* offset of super-block in filesys */
89 int32_t fs_cblkno; /* offset of cyl-block in filesys */
90 int32_t fs_iblkno; /* offset of inode-blocks in filesys */
91 int32_t fs_dblkno; /* offset of first data after cg */
92 int32_t fs_old_cgoffset; /* cylinder group offset in cylinder */
93 int32_t fs_old_cgmask; /* used to calc mod fs_ntrak */
94 int32_t fs_old_time; /* last time written */
95 int32_t fs_old_size; /* number of blocks in fs */
96 int32_t fs_old_dsize; /* number of data blocks in fs */
97 int32_t fs_ncg; /* number of cylinder groups */
98 int32_t fs_bsize; /* size of basic blocks in fs */
99 int32_t fs_fsize; /* size of frag blocks in fs */
100 int32_t fs_frag; /* number of frags in a block in fs */
101 /* these are configuration parameters */
102 int32_t fs_minfree; /* minimum percentage of free blocks */
103 int32_t fs_old_rotdelay; /* num of ms for optimal next block */
104 int32_t fs_old_rps; /* disk revolutions per second */
105 /* these fields can be computed from the others */
106 int32_t fs_bmask; /* ``blkoff'' calc of blk offsets */
107 int32_t fs_fmask; /* ``fragoff'' calc of frag offsets */
108 int32_t fs_bshift; /* ``lblkno'' calc of logical blkno */
109 int32_t fs_fshift; /* ``numfrags'' calc number of frags */
110 /* these are configuration parameters */
111 int32_t fs_maxcontig; /* max number of contiguous blks */
112 int32_t fs_maxbpg; /* max number of blks per cyl group */
113 /* these fields can be computed from the others */
114 int32_t fs_fragshift; /* block to frag shift */
115 int32_t fs_fsbtodb; /* fsbtodb and dbtofsb shift constant */
116 int32_t fs_sbsize; /* actual size of super block */
117 int32_t fs_spare1[2]; /* old fs_csmask */
118 /* old fs_csshift */
119 int32_t fs_nindir; /* value of NINDIR */
120 int32_t fs_inopb; /* value of INOPB */
121 int32_t fs_old_nspf; /* value of NSPF */
122 /* yet another configuration parameter */
123 int32_t fs_optim; /* optimization preference, see below */
124 int32_t fs_old_npsect; /* # sectors/track including spares */
125 int32_t fs_old_interleave; /* hardware sector interleave */
126 int32_t fs_old_trackskew; /* sector 0 skew, per track */
127 int32_t fs_id[2]; /* unique filesystem id */
128 /* sizes determined by number of cylinder groups and their sizes */
129 int32_t fs_old_csaddr; /* blk addr of cyl grp summary area */
130 int32_t fs_cssize; /* size of cyl grp summary area */
131 int32_t fs_cgsize; /* cylinder group size */
132 int32_t fs_spare2; /* old fs_ntrak */
133 int32_t fs_old_nsect; /* sectors per track */
134 int32_t fs_old_spc; /* sectors per cylinder */
135 int32_t fs_old_ncyl; /* cylinders in filesystem */
136 int32_t fs_old_cpg; /* cylinders per group */
137 int32_t fs_ipg; /* inodes per group */
138 int32_t fs_fpg; /* blocks per group * fs_frag */
139 /* this data must be re-computed after crashes */
140 struct csum fs_old_cstotal; /* cylinder summary information */
141 /* these fields are cleared at mount time */
142 int8_t fs_fmod; /* super block modified flag */
143 int8_t fs_clean; /* filesystem is clean flag */
144 int8_t fs_ronly; /* mounted read-only flag */
145 int8_t fs_old_flags; /* old FS_ flags */
146 u_char fs_fsmnt[MAXMNTLEN]; /* name mounted on */
147 u_char fs_volname[MAXVOLLEN]; /* volume name */
148 u_int64_t fs_swuid; /* system-wide uid */
149 int32_t fs_pad; /* due to alignment of fs_swuid */
150 /* these fields retain the current block allocation info */
151 int32_t fs_cgrotor; /* last cg searched */
152 void *fs_ocsp[NOCSPTRS]; /* padding; was list of fs_cs buffers */
153 u_int8_t *fs_contigdirs; /* # of contiguously allocated dirs */
154 struct csum *fs_csp; /* cg summary info buffer for fs_cs */
155 int32_t *fs_maxcluster; /* max cluster in each cyl group */
156 u_int *fs_active; /* used by snapshots to track fs */
157 int32_t fs_old_cpc; /* cyl per cycle in postbl */
158 int32_t fs_maxbsize; /* maximum blocking factor permitted */
159 int64_t fs_unrefs; /* number of unreferenced inodes */
160 int64_t fs_sparecon64[16]; /* old rotation block list head */
161 int64_t fs_sblockloc; /* byte offset of standard superblock */
162 struct csum_total fs_cstotal; /* cylinder summary information */
163 ufs_time_t fs_time; /* last time written */
164 int64_t fs_size; /* number of blocks in fs */
165 int64_t fs_dsize; /* number of data blocks in fs */
166 ufs2_daddr_t fs_csaddr; /* blk addr of cyl grp summary area */
167 int64_t fs_pendingblocks; /* blocks in process of being freed */
168 int32_t fs_pendinginodes; /* inodes in process of being freed */
169 int32_t fs_snapinum[FSMAXSNAP]; /* list of snapshot inode numbers */
170 int32_t fs_avgfilesize; /* expected average file size */
171 int32_t fs_avgfpdir; /* expected # of files per directory */
172 int32_t fs_save_cgsize; /* save real cg size to use fs_bsize */
173 int32_t fs_sparecon32[26]; /* reserved for future constants */
174 int32_t fs_flags; /* see FS_ flags below */
175 int32_t fs_contigsumsize; /* size of cluster summary array */
176 int32_t fs_maxsymlinklen; /* max length of an internal symlink */
177 int32_t fs_old_inodefmt; /* format of on-disk inodes */
178 u_int64_t fs_maxfilesize; /* maximum representable file size */
179 int64_t fs_qbmask; /* ~fs_bmask for use with 64-bit size */
180 int64_t fs_qfmask; /* ~fs_fmask for use with 64-bit size */
181 int32_t fs_state; /* validate fs_clean field */
182 int32_t fs_old_postblformat; /* format of positional layout tables */
183 int32_t fs_old_nrpos; /* number of rotational positions */
184 int32_t fs_spare5[2]; /* old fs_postbloff */
185 /* old fs_rotbloff */
186 int32_t fs_magic; /* magic number */
187 };
189 /*
190 * Filesystem identification
191 */
192 #define FS_UFS1_MAGIC 0x011954 /* UFS1 fast filesystem magic number */
193 #define FS_UFS2_MAGIC 0x19540119 /* UFS2 fast filesystem magic number */
194 #define FS_OKAY 0x7c269d38 /* superblock checksum */
195 #define FS_42INODEFMT -1 /* 4.2BSD inode format */
196 #define FS_44INODEFMT 2 /* 4.4BSD inode format */
198 /*
199 * Preference for optimization.
200 */
201 #define FS_OPTTIME 0 /* minimize allocation time */
202 #define FS_OPTSPACE 1 /* minimize disk fragmentation */
203 .Ed
204 .Pp
205 Each disk drive contains some number of file systems.
206 A file system consists of a number of cylinder groups.
207 Each cylinder group has inodes and data.
208 .Pp
209 A file system is described by its super-block, which in turn
210 describes the cylinder groups.
211 The super-block is critical
212 data and is replicated in each cylinder group to protect against
213 catastrophic loss.
214 This is done at file system creation
215 time and the critical
216 super-block data does not change, so the copies need not be
217 referenced further unless disaster strikes.
218 .Pp
219 Addresses stored in inodes are capable of addressing fragments
220 of `blocks'.
221 File system blocks of at most size
222 .Dv MAXBSIZE
223 can
224 be optionally broken into 2, 4, or 8 pieces, each of which is
225 addressable; these pieces may be
226 .Dv DEV_BSIZE ,
227 or some multiple of
228 a
229 .Dv DEV_BSIZE
230 unit.
231 .Pp
232 Large files consist of exclusively large data blocks.
233 To avoid
234 undue wasted disk space, the last data block of a small file is
235 allocated as only as many fragments of a large block as are
236 necessary.
237 The file system format retains only a single pointer
238 to such a fragment, which is a piece of a single large block that
239 has been divided.
240 The size of such a fragment is determinable from
241 information in the inode, using the
242 .Fn blksize fs ip lbn
243 macro.
244 .Pp
245 The file system records space availability at the fragment level;
246 to determine block availability, aligned fragments are examined.
247 .Pp
248 The root inode is the root of the file system.
249 Inode 0 cannot be used for normal purposes and
250 historically bad blocks were linked to inode 1,
251 thus the root inode is 2 (inode 1 is no longer used for
252 this purpose, however numerous dump tapes make this
253 assumption, so we are stuck with it).
254 .Pp
255 The
256 .Fa fs_minfree
257 element gives the minimum acceptable percentage of file system
258 blocks that may be free.
259 If the freelist drops below this level
260 only the super-user may continue to allocate blocks.
261 The
262 .Fa fs_minfree
263 element
264 may be set to 0 if no reserve of free blocks is deemed necessary,
265 however severe performance degradations will be observed if the
266 file system is run at greater than 90% full; thus the default
267 value of
268 .Fa fs_minfree
269 is 10%.
270 .Pp
271 Empirically the best trade-off between block fragmentation and
272 overall disk utilization at a loading of 90% comes with a
273 fragmentation of 8, thus the default fragment size is an eighth
274 of the block size.
275 .Pp
276 The element
277 .Fa fs_optim
278 specifies whether the file system should try to minimize the time spent
279 allocating blocks, or if it should attempt to minimize the space
280 fragmentation on the disk.
281 If the value of fs_minfree (see above) is less than 10%,
282 then the file system defaults to optimizing for space to avoid
283 running out of full sized blocks.
284 If the value of minfree is greater than or equal to 10%,
285 fragmentation is unlikely to be problematical, and
286 the file system defaults to optimizing for time.
287 .Pp
288 .Em Cylinder group related limits :
289 Each cylinder keeps track of the availability of blocks at different
290 rotational positions, so that sequential blocks can be laid out
291 with minimum rotational latency.
292 With the default of 8 distinguished
293 rotational positions, the resolution of the
294 summary information is 2ms for a typical 3600 rpm drive.
295 .Pp
296 The element
297 .Fa fs_old_rotdelay
298 gives the minimum number of milliseconds to initiate
299 another disk transfer on the same cylinder.
300 It is used in determining the rotationally optimal
301 layout for disk blocks within a file;
302 the default value for
303 .Fa fs_old_rotdelay
304 is 2ms.
305 .Pp
306 Each file system has a statically allocated number of inodes.
307 An inode is allocated for each
308 .Dv NBPI
309 bytes of disk space.
310 The inode allocation strategy is extremely conservative.
311 .Pp
312 .Dv MINBSIZE
313 is the smallest allowable block size.
314 With a
315 .Dv MINBSIZE
316 of 4096
317 it is possible to create files of size
318 2^32 with only two levels of indirection.
319 .Dv MINBSIZE
320 must be big enough to hold a cylinder group block,
321 thus changes to
322 .Pq Fa struct cg
323 must keep its size within
324 .Dv MINBSIZE .
325 Note that super-blocks are never more than size
326 .Dv SBLOCKSIZE .
327 .Pp
328 The path name on which the file system is mounted is maintained in
329 .Fa fs_fsmnt .
330 .Dv MAXMNTLEN
331 defines the amount of space allocated in
332 the super-block for this name.
333 The limit on the amount of summary information per file system
334 is defined by
335 .Dv MAXCSBUFS .
336 For a 4096 byte block size, it is currently parameterized for a
337 maximum of two million cylinders.
338 .Pp
339 Per cylinder group information is summarized in blocks allocated
340 from the first cylinder group's data blocks.
341 These blocks are read in from
342 .Fa fs_csaddr
343 (size
344 .Fa fs_cssize )
345 in addition to the super-block.
346 .Pp
347 .Sy N.B. :
348 .Fn sizeof "struct csum"
349 must be a power of two in order for
350 the
351 .Fn fs_cs
352 macro to work.
353 .Pp
354 The
355 .Em "Super-block for a file system" :
356 The size of the rotational layout tables
357 is limited by the fact that the super-block is of size
358 .Dv SBLOCKSIZE .
359 The size of these tables is
360 .Em inversely
361 proportional to the block
362 size of the file system.
363 The size of the tables is
364 increased when sector sizes are not powers of two,
365 as this increases the number of cylinders
366 included before the rotational pattern repeats
367 .Pq Fa fs_cpc .
368 The size of the rotational layout
369 tables is derived from the number of bytes remaining in
370 .Pq Fa struct fs .
371 .Pp
372 The number of blocks of data per cylinder group
373 is limited because cylinder groups are at most one block.
374 The inode and free block tables
375 must fit into a single block after deducting space for
376 the cylinder group structure
377 .Pq Fa struct cg .
378 .Pp
379 The
380 .Em Inode :
381 The inode is the focus of all file activity in the
382 .Ux
383 file system.
384 There is a unique inode allocated
385 for each active file,
386 each current directory, each mounted-on file,
387 text file, and the root.
388 An inode is `named' by its device/i-number pair.
389 For further information, see the include file
390 .In ufs/ufs/inode.h .
391 .Pp
392 The format of an external attribute is defined by the extattr structure:
393 .Bd -literal
394 struct extattr {
395 int32_t ea_length; /* length of this attribute */
396 int8_t ea_namespace; /* name space of this attribute */
397 int8_t ea_contentpadlen; /* padding at end of attribute */
398 int8_t ea_namelength; /* length of attribute name */
399 char ea_name[1]; /* null-terminated attribute name */
400 /* extended attribute content follows */
401 };
402 .Ed
403 .Pp
404 Several macros are defined to manipulate these structures.
405 Each macro takes a pointer to an extattr structure.
406 .Bl -tag -width ".Dv EXTATTR_SET_LENGTHS(eap, size)"
407 .It Dv EXTATTR_NEXT(eap)
408 Returns a pointer to the next extended attribute following
409 .Fa eap .
410 .It Dv EXTATTR_CONTENT(eap)
411 Returns a pointer to the extended attribute content referenced by
412 .Fa eap .
413 .It Dv EXTATTR_CONTENT_SIZE(eap)
414 Returns the size of the extended attribute content referenced by
415 .Fa eap .
416 .It Dv EXTATTR_SET_LENGTHS(eap, size)
417 Called with the size of the attribute content after initializing
418 the attribute name to calculate and set the
419 .Fa ea_length ,
420 .Fa ea_namelength ,
421 and
422 .Fa ea_contentpadlen
423 fields of the extended attribute structure.
424 .El
425 .Pp
426 The following code identifies an ACL:
427 .Bd -literal
428 if (eap->ea_namespace == EXTATTR_NAMESPACE_SYSTEM &&
429 !strcmp(eap->ea_name, POSIX1E_ACL_ACCESS_EXTATTR_NAME) {
430 aclp = EXTATTR_CONTENT(eap);
431 acllen = EXTATTR_CONTENT_SIZE(eap);
432 ...
433 }
434 .Ed
435 .Pp
436 The following code creates an extended attribute
437 containing a copy of a structure
438 .Fa mygif :
439 .Bd -literal
440 eap->ea_namespace = EXTATTR_NAMESPACE_USER;
441 strcpy(eap->ea_name, "filepic.gif");
442 EXTATTR_SET_LENGTHS(eap, sizeof(struct mygif));
443 memcpy(EXTATTR_CONTENT(eap), &mygif, sizeof(struct mygif));
444 .Ed
445 .Sh HISTORY
446 A super-block structure named filsys appeared in
447 .At v6 .
448 The file system described in this manual appeared
449 in
450 .Bx 4.2 .