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32 .\" @(#)fs.5 8.2 (Berkeley) 4/19/94
41 .Nd format of file system volume
55 declare several structures, defined variables and macros
56 which are used to create and manage the underlying format of
57 file system objects on random access devices (disks).
59 The block size and number of blocks which
60 comprise a file system are parameters of the file system.
66 for a disklabel and for some hardware primary
67 and secondary bootstrapping programs.
69 The actual file system begins at sector
75 The following structure describes the super-block and is
80 * Super block for an FFS filesystem.
83 int32_t fs_firstfield; /* historic filesystem linked list, */
84 int32_t fs_unused_1; /* used for incore super blocks */
85 int32_t fs_sblkno; /* offset of super-block in filesys */
86 int32_t fs_cblkno; /* offset of cyl-block in filesys */
87 int32_t fs_iblkno; /* offset of inode-blocks in filesys */
88 int32_t fs_dblkno; /* offset of first data after cg */
89 int32_t fs_old_cgoffset; /* cylinder group offset in cylinder */
90 int32_t fs_old_cgmask; /* used to calc mod fs_ntrak */
91 int32_t fs_old_time; /* last time written */
92 int32_t fs_old_size; /* number of blocks in fs */
93 int32_t fs_old_dsize; /* number of data blocks in fs */
94 int32_t fs_ncg; /* number of cylinder groups */
95 int32_t fs_bsize; /* size of basic blocks in fs */
96 int32_t fs_fsize; /* size of frag blocks in fs */
97 int32_t fs_frag; /* number of frags in a block in fs */
98 /* these are configuration parameters */
99 int32_t fs_minfree; /* minimum percentage of free blocks */
100 int32_t fs_old_rotdelay; /* num of ms for optimal next block */
101 int32_t fs_old_rps; /* disk revolutions per second */
102 /* these fields can be computed from the others */
103 int32_t fs_bmask; /* ``blkoff'' calc of blk offsets */
104 int32_t fs_fmask; /* ``fragoff'' calc of frag offsets */
105 int32_t fs_bshift; /* ``lblkno'' calc of logical blkno */
106 int32_t fs_fshift; /* ``numfrags'' calc number of frags */
107 /* these are configuration parameters */
108 int32_t fs_maxcontig; /* max number of contiguous blks */
109 int32_t fs_maxbpg; /* max number of blks per cyl group */
110 /* these fields can be computed from the others */
111 int32_t fs_fragshift; /* block to frag shift */
112 int32_t fs_fsbtodb; /* fsbtodb and dbtofsb shift constant */
113 int32_t fs_sbsize; /* actual size of super block */
114 int32_t fs_spare1[2]; /* old fs_csmask */
116 int32_t fs_nindir; /* value of NINDIR */
117 int32_t fs_inopb; /* value of INOPB */
118 int32_t fs_old_nspf; /* value of NSPF */
119 /* yet another configuration parameter */
120 int32_t fs_optim; /* optimization preference, see below */
121 int32_t fs_old_npsect; /* # sectors/track including spares */
122 int32_t fs_old_interleave; /* hardware sector interleave */
123 int32_t fs_old_trackskew; /* sector 0 skew, per track */
124 int32_t fs_id[2]; /* unique filesystem id */
125 /* sizes determined by number of cylinder groups and their sizes */
126 int32_t fs_old_csaddr; /* blk addr of cyl grp summary area */
127 int32_t fs_cssize; /* size of cyl grp summary area */
128 int32_t fs_cgsize; /* cylinder group size */
129 int32_t fs_spare2; /* old fs_ntrak */
130 int32_t fs_old_nsect; /* sectors per track */
131 int32_t fs_old_spc; /* sectors per cylinder */
132 int32_t fs_old_ncyl; /* cylinders in filesystem */
133 int32_t fs_old_cpg; /* cylinders per group */
134 int32_t fs_ipg; /* inodes per group */
135 int32_t fs_fpg; /* blocks per group * fs_frag */
136 /* this data must be re-computed after crashes */
137 struct csum fs_old_cstotal; /* cylinder summary information */
138 /* these fields are cleared at mount time */
139 int8_t fs_fmod; /* super block modified flag */
140 int8_t fs_clean; /* filesystem is clean flag */
141 int8_t fs_ronly; /* mounted read-only flag */
142 int8_t fs_old_flags; /* old FS_ flags */
143 u_char fs_fsmnt[MAXMNTLEN]; /* name mounted on */
144 u_char fs_volname[MAXVOLLEN]; /* volume name */
145 u_int64_t fs_swuid; /* system-wide uid */
146 int32_t fs_pad; /* due to alignment of fs_swuid */
147 /* these fields retain the current block allocation info */
148 int32_t fs_cgrotor; /* last cg searched */
149 void *fs_ocsp[NOCSPTRS]; /* padding; was list of fs_cs buffers */
150 u_int8_t *fs_contigdirs; /* # of contiguously allocated dirs */
151 struct csum *fs_csp; /* cg summary info buffer for fs_cs */
152 int32_t *fs_maxcluster; /* max cluster in each cyl group */
153 u_int *fs_active; /* used by snapshots to track fs */
154 int32_t fs_old_cpc; /* cyl per cycle in postbl */
155 int32_t fs_maxbsize; /* maximum blocking factor permitted */
156 int64_t fs_unrefs; /* number of unreferenced inodes */
157 int64_t fs_sparecon64[16]; /* old rotation block list head */
158 int64_t fs_sblockloc; /* byte offset of standard superblock */
159 struct csum_total fs_cstotal; /* cylinder summary information */
160 ufs_time_t fs_time; /* last time written */
161 int64_t fs_size; /* number of blocks in fs */
162 int64_t fs_dsize; /* number of data blocks in fs */
163 ufs2_daddr_t fs_csaddr; /* blk addr of cyl grp summary area */
164 int64_t fs_pendingblocks; /* blocks in process of being freed */
165 int32_t fs_pendinginodes; /* inodes in process of being freed */
166 int32_t fs_snapinum[FSMAXSNAP]; /* list of snapshot inode numbers */
167 int32_t fs_avgfilesize; /* expected average file size */
168 int32_t fs_avgfpdir; /* expected # of files per directory */
169 int32_t fs_save_cgsize; /* save real cg size to use fs_bsize */
170 int32_t fs_sparecon32[26]; /* reserved for future constants */
171 int32_t fs_flags; /* see FS_ flags below */
172 int32_t fs_contigsumsize; /* size of cluster summary array */
173 int32_t fs_maxsymlinklen; /* max length of an internal symlink */
174 int32_t fs_old_inodefmt; /* format of on-disk inodes */
175 u_int64_t fs_maxfilesize; /* maximum representable file size */
176 int64_t fs_qbmask; /* ~fs_bmask for use with 64-bit size */
177 int64_t fs_qfmask; /* ~fs_fmask for use with 64-bit size */
178 int32_t fs_state; /* validate fs_clean field */
179 int32_t fs_old_postblformat; /* format of positional layout tables */
180 int32_t fs_old_nrpos; /* number of rotational positions */
181 int32_t fs_spare5[2]; /* old fs_postbloff */
182 /* old fs_rotbloff */
183 int32_t fs_magic; /* magic number */
187 * Filesystem identification
189 #define FS_UFS1_MAGIC 0x011954 /* UFS1 fast filesystem magic number */
190 #define FS_UFS2_MAGIC 0x19540119 /* UFS2 fast filesystem magic number */
191 #define FS_OKAY 0x7c269d38 /* superblock checksum */
192 #define FS_42INODEFMT -1 /* 4.2BSD inode format */
193 #define FS_44INODEFMT 2 /* 4.4BSD inode format */
196 * Preference for optimization.
198 #define FS_OPTTIME 0 /* minimize allocation time */
199 #define FS_OPTSPACE 1 /* minimize disk fragmentation */
202 Each disk drive contains some number of file systems.
203 A file system consists of a number of cylinder groups.
204 Each cylinder group has inodes and data.
206 A file system is described by its super-block, which in turn
207 describes the cylinder groups.
208 The super-block is critical
209 data and is replicated in each cylinder group to protect against
211 This is done at file system creation
212 time and the critical
213 super-block data does not change, so the copies need not be
214 referenced further unless disaster strikes.
216 Addresses stored in inodes are capable of addressing fragments
218 File system blocks of at most size
221 be optionally broken into 2, 4, or 8 pieces, each of which is
222 addressable; these pieces may be
229 Large files consist of exclusively large data blocks.
231 undue wasted disk space, the last data block of a small file is
232 allocated as only as many fragments of a large block as are
234 The file system format retains only a single pointer
235 to such a fragment, which is a piece of a single large block that
237 The size of such a fragment is determinable from
238 information in the inode, using the
239 .Fn blksize fs ip lbn
242 The file system records space availability at the fragment level;
243 to determine block availability, aligned fragments are examined.
245 The root inode is the root of the file system.
246 Inode 0 cannot be used for normal purposes and
247 historically bad blocks were linked to inode 1,
248 thus the root inode is 2 (inode 1 is no longer used for
249 this purpose, however numerous dump tapes make this
250 assumption, so we are stuck with it).
254 element gives the minimum acceptable percentage of file system
255 blocks that may be free.
256 If the freelist drops below this level
257 only the super-user may continue to allocate blocks.
261 may be set to 0 if no reserve of free blocks is deemed necessary,
262 however severe performance degradations will be observed if the
263 file system is run at greater than 90% full; thus the default
268 Empirically the best trade-off between block fragmentation and
269 overall disk utilization at a loading of 90% comes with a
270 fragmentation of 8, thus the default fragment size is an eighth
275 specifies whether the file system should try to minimize the time spent
276 allocating blocks, or if it should attempt to minimize the space
277 fragmentation on the disk.
278 If the value of fs_minfree (see above) is less than 10%,
279 then the file system defaults to optimizing for space to avoid
280 running out of full sized blocks.
281 If the value of minfree is greater than or equal to 10%,
282 fragmentation is unlikely to be problematical, and
283 the file system defaults to optimizing for time.
285 .Em Cylinder group related limits :
286 Each cylinder keeps track of the availability of blocks at different
287 rotational positions, so that sequential blocks can be laid out
288 with minimum rotational latency.
289 With the default of 8 distinguished
290 rotational positions, the resolution of the
291 summary information is 2ms for a typical 3600 rpm drive.
295 gives the minimum number of milliseconds to initiate
296 another disk transfer on the same cylinder.
297 It is used in determining the rotationally optimal
298 layout for disk blocks within a file;
299 the default value for
303 Each file system has a statically allocated number of inodes.
304 An inode is allocated for each
307 The inode allocation strategy is extremely conservative.
310 is the smallest allowable block size.
314 it is possible to create files of size
315 2^32 with only two levels of indirection.
317 must be big enough to hold a cylinder group block,
320 must keep its size within
322 Note that super-blocks are never more than size
325 The path name on which the file system is mounted is maintained in
328 defines the amount of space allocated in
329 the super-block for this name.
330 The limit on the amount of summary information per file system
333 For a 4096 byte block size, it is currently parameterized for a
334 maximum of two million cylinders.
336 Per cylinder group information is summarized in blocks allocated
337 from the first cylinder group's data blocks.
338 These blocks are read in from
342 in addition to the super-block.
345 .Fn sizeof "struct csum"
346 must be a power of two in order for
352 .Em "Super-block for a file system" :
353 The size of the rotational layout tables
354 is limited by the fact that the super-block is of size
356 The size of these tables is
358 proportional to the block
359 size of the file system.
360 The size of the tables is
361 increased when sector sizes are not powers of two,
362 as this increases the number of cylinders
363 included before the rotational pattern repeats
365 The size of the rotational layout
366 tables is derived from the number of bytes remaining in
369 The number of blocks of data per cylinder group
370 is limited because cylinder groups are at most one block.
371 The inode and free block tables
372 must fit into a single block after deducting space for
373 the cylinder group structure
378 The inode is the focus of all file activity in the
381 There is a unique inode allocated
382 for each active file,
383 each current directory, each mounted-on file,
384 text file, and the root.
385 An inode is `named' by its device/i-number pair.
386 For further information, see the include file
387 .In ufs/ufs/inode.h .
389 A super-block structure named filsys appeared in
391 The file system described in this manual appeared