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34 .\" @(#)fs.5 8.2 (Berkeley) 4/19/94
42 .Nd format of file system volume
44 .Fd #include <sys/types.h>
45 .Fd #include <ufs/fs.h>
46 .Fd #include <ufs/inode.h>
52 declare several structures, defined variables and macros
53 which are used to create and manage the underlying format of
54 file system objects on random access devices (disks).
56 The block size and number of blocks which
57 comprise a file system are parameters of the file system.
63 for a disklabel and for some hardware primary
64 and secondary bootstrapping programs.
66 The actual file system begins at sector
72 The following structure described the super-block and is
76 #define FS_MAGIC 0x011954
78 struct fs *fs_link; /* linked list of file systems */
79 struct fs *fs_rlink; /* used for incore super blocks */
80 daddr_t fs_sblkno; /* addr of super-block in filesys */
81 daddr_t fs_cblkno; /* offset of cyl-block in filesys */
82 daddr_t fs_iblkno; /* offset of inode-blocks in filesys */
83 daddr_t fs_dblkno; /* offset of first data after cg */
84 long fs_cgoffset; /* cylinder group offset in cylinder */
85 long fs_cgmask; /* used to calc mod fs_ntrak */
86 time_t fs_time; /* last time written */
87 long fs_size; /* number of blocks in fs */
88 long fs_dsize; /* number of data blocks in fs */
89 long fs_ncg; /* number of cylinder groups */
90 long fs_bsize; /* size of basic blocks in fs */
91 long fs_fsize; /* size of frag blocks in fs */
92 long fs_frag; /* number of frags in a block in fs */
93 /* these are configuration parameters */
94 long fs_minfree; /* minimum percentage of free blocks */
95 long fs_rotdelay; /* num of ms for optimal next block */
96 long fs_rps; /* disk revolutions per second */
97 /* these fields can be computed from the others */
98 long fs_bmask; /* ``blkoff'' calc of blk offsets */
99 long fs_fmask; /* ``fragoff'' calc of frag offsets */
100 long fs_bshift; /* ``lblkno'' calc of logical blkno */
101 long fs_fshift; /* ``numfrags'' calc number of frags */
102 /* these are configuration parameters */
103 long fs_maxcontig; /* max number of contiguous blks */
104 long fs_maxbpg; /* max number of blks per cyl group */
105 /* these fields can be computed from the others */
106 long fs_fragshift; /* block to frag shift */
107 long fs_fsbtodb; /* fsbtodb and dbtofsb shift constant */
108 long fs_sbsize; /* actual size of super block */
109 long fs_csmask; /* csum block offset */
110 long fs_csshift; /* csum block number */
111 long fs_nindir; /* value of NINDIR */
112 long fs_inopb; /* value of INOPB */
113 long fs_nspf; /* value of NSPF */
114 /* yet another configuration parameter */
115 long fs_optim; /* optimization preference, see below */
116 /* these fields are derived from the hardware */
117 long fs_npsect; /* # sectors/track including spares */
118 long fs_interleave; /* hardware sector interleave */
119 long fs_trackskew; /* sector 0 skew, per track */
120 long fs_headswitch; /* head switch time, usec */
121 long fs_trkseek; /* track-to-track seek, usec */
122 /* sizes determined by number of cylinder groups and their sizes */
123 daddr_t fs_csaddr; /* blk addr of cyl grp summary area */
124 long fs_cssize; /* size of cyl grp summary area */
125 long fs_cgsize; /* cylinder group size */
126 /* these fields are derived from the hardware */
127 long fs_ntrak; /* tracks per cylinder */
128 long fs_nsect; /* sectors per track */
129 long fs_spc; /* sectors per cylinder */
130 /* this comes from the disk driver partitioning */
131 long fs_ncyl; /* cylinders in file system */
132 /* these fields can be computed from the others */
133 long fs_cpg; /* cylinders per group */
134 long fs_ipg; /* inodes per group */
135 long fs_fpg; /* blocks per group * fs_frag */
136 /* this data must be re-computed after crashes */
137 struct csum fs_cstotal; /* cylinder summary information */
138 /* these fields are cleared at mount time */
139 char fs_fmod; /* super block modified flag */
140 char fs_clean; /* file system is clean flag */
141 char fs_ronly; /* mounted read-only flag */
142 char fs_flags; /* currently unused flag */
143 char fs_fsmnt[MAXMNTLEN]; /* name mounted on */
144 /* these fields retain the current block allocation info */
145 long fs_cgrotor; /* last cg searched */
146 struct csum *fs_csp[MAXCSBUFS]; /* list of fs_cs info buffers */
147 long fs_cpc; /* cyl per cycle in postbl */
148 short fs_opostbl[16][8]; /* old rotation block list head */
149 long fs_sparecon[56]; /* reserved for future constants */
150 quad fs_qbmask; /* ~fs_bmask - for use with quad size */
151 quad fs_qfmask; /* ~fs_fmask - for use with quad size */
152 long fs_postblformat; /* format of positional layout tables */
153 long fs_nrpos; /* number of rotational positions */
154 long fs_postbloff; /* (short) rotation block list head */
155 long fs_rotbloff; /* (u_char) blocks for each rotation */
156 long fs_magic; /* magic number */
157 u_char fs_space[1]; /* list of blocks for each rotation */
158 /* actually longer */
162 Each disk drive contains some number of file systems.
163 A file system consists of a number of cylinder groups.
164 Each cylinder group has inodes and data.
166 A file system is described by its super-block, which in turn
167 describes the cylinder groups. The super-block is critical
168 data and is replicated in each cylinder group to protect against
169 catastrophic loss. This is done at file system creation
170 time and the critical
171 super-block data does not change, so the copies need not be
172 referenced further unless disaster strikes.
174 Addresses stored in inodes are capable of addressing fragments
175 of `blocks'. File system blocks of at most size
178 be optionally broken into 2, 4, or 8 pieces, each of which is
179 addressable; these pieces may be
186 Large files consist of exclusively large data blocks. To avoid
187 undue wasted disk space, the last data block of a small file is
188 allocated as only as many fragments of a large block as are
189 necessary. The file system format retains only a single pointer
190 to such a fragment, which is a piece of a single large block that
191 has been divided. The size of such a fragment is determinable from
192 information in the inode, using the
193 .Fn blksize fs ip lbn
196 The file system records space availability at the fragment level;
197 to determine block availability, aligned fragments are examined.
199 The root inode is the root of the file system.
200 Inode 0 can't be used for normal purposes and
201 historically bad blocks were linked to inode 1,
202 thus the root inode is 2 (inode 1 is no longer used for
203 this purpose, however numerous dump tapes make this
204 assumption, so we are stuck with it).
208 element gives the minimum acceptable percentage of file system
209 blocks that may be free. If the freelist drops below this level
210 only the super-user may continue to allocate blocks.
214 may be set to 0 if no reserve of free blocks is deemed necessary,
215 however severe performance degradations will be observed if the
216 file system is run at greater than 90% full; thus the default
221 Empirically the best trade-off between block fragmentation and
222 overall disk utilization at a loading of 90% comes with a
223 fragmentation of 8, thus the default fragment size is an eighth
228 specifies whether the file system should try to minimize the time spent
229 allocating blocks, or if it should attempt to minimize the space
230 fragmentation on the disk.
231 If the value of fs_minfree (see above) is less than 10%,
232 then the file system defaults to optimizing for space to avoid
233 running out of full sized blocks.
234 If the value of minfree is greater than or equal to 10%,
235 fragmentation is unlikely to be problematical, and
236 the file system defaults to optimizing for time.
238 .Em Cylinder group related limits :
239 Each cylinder keeps track of the availability of blocks at different
240 rotational positions, so that sequential blocks can be laid out
241 with minimum rotational latency. With the default of 8 distinguished
242 rotational positions, the resolution of the
243 summary information is 2ms for a typical 3600 rpm drive.
247 gives the minimum number of milliseconds to initiate
248 another disk transfer on the same cylinder.
249 It is used in determining the rotationally optimal
250 layout for disk blocks within a file;
251 the default value for
255 Each file system has a statically allocated number of inodes.
256 An inode is allocated for each
259 The inode allocation strategy is extremely conservative.
262 is the smallest allowable block size.
266 it is possible to create files of size
267 2^32 with only two levels of indirection.
269 must be big enough to hold a cylinder group block,
272 must keep its size within
274 Note that super-blocks are never more than size
277 The path name on which the file system is mounted is maintained in
280 defines the amount of space allocated in
281 the super-block for this name.
282 The limit on the amount of summary information per file system
285 For a 4096 byte block size, it is currently parameterized for a
286 maximum of two million cylinders.
288 Per cylinder group information is summarized in blocks allocated
289 from the first cylinder group's data blocks.
290 These blocks are read in from
294 in addition to the super-block.
297 .Xr sizeof Pq Fa struct csum
298 must be a power of two in order for
304 .Em "Super-block for a file system" :
305 The size of the rotational layout tables
306 is limited by the fact that the super-block is of size
308 The size of these tables is
310 proportional to the block
311 size of the file system. The size of the tables is
312 increased when sector sizes are not powers of two,
313 as this increases the number of cylinders
314 included before the rotational pattern repeats
316 The size of the rotational layout
317 tables is derived from the number of bytes remaining in
320 The number of blocks of data per cylinder group
321 is limited because cylinder groups are at most one block.
322 The inode and free block tables
323 must fit into a single block after deducting space for
324 the cylinder group structure
329 The inode is the focus of all file activity in the
331 There is a unique inode allocated
332 for each active file,
333 each current directory, each mounted-on file,
334 text file, and the root.
335 An inode is `named' by its device/i-number pair.
336 For further information, see the include file
339 A super-block structure named filsys appeared in
341 The file system described in this manual appeared
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