1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)fs.5 8.2 (Berkeley) 4/19/94 33.\" $FreeBSD$ 34.\" 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 ufs/ufs/quota.h 49.In ufs/ufs/inode.h 50.Sh DESCRIPTION 51The files 52.In fs.h 53and 54.In inode.h 55declare several structures, defined variables and macros 56which are used to create and manage the underlying format of 57file system objects on random access devices (disks). 58.Pp 59The block size and number of blocks which 60comprise a file system are parameters of the file system. 61Sectors beginning at 62.Dv BBLOCK 63and continuing for 64.Dv BBSIZE 65are used 66for a disklabel and for some hardware primary 67and secondary bootstrapping programs. 68.Pp 69The actual file system begins at sector 70.Dv SBLOCK 71with the 72.Em super-block 73that is of size 74.Dv SBLOCKSIZE . 75The following structure describes the super-block and is 76from the file 77.In ufs/ffs/fs.h : 78.Bd -literal 79/* 80 * Super block for an FFS filesystem. 81 */ 82struct fs { 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 */ 115 /* old fs_csshift */ 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 */ 184}; 185 186/* 187 * Filesystem identification 188 */ 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 */ 194 195/* 196 * Preference for optimization. 197 */ 198#define FS_OPTTIME 0 /* minimize allocation time */ 199#define FS_OPTSPACE 1 /* minimize disk fragmentation */ 200.Ed 201.Pp 202Each disk drive contains some number of file systems. 203A file system consists of a number of cylinder groups. 204Each cylinder group has inodes and data. 205.Pp 206A file system is described by its super-block, which in turn 207describes the cylinder groups. 208The super-block is critical 209data and is replicated in each cylinder group to protect against 210catastrophic loss. 211This is done at file system creation 212time and the critical 213super-block data does not change, so the copies need not be 214referenced further unless disaster strikes. 215.Pp 216Addresses stored in inodes are capable of addressing fragments 217of `blocks'. 218File system blocks of at most size 219.Dv MAXBSIZE 220can 221be optionally broken into 2, 4, or 8 pieces, each of which is 222addressable; these pieces may be 223.Dv DEV_BSIZE , 224or some multiple of 225a 226.Dv DEV_BSIZE 227unit. 228.Pp 229Large files consist of exclusively large data blocks. 230To avoid 231undue wasted disk space, the last data block of a small file is 232allocated as only as many fragments of a large block as are 233necessary. 234The file system format retains only a single pointer 235to such a fragment, which is a piece of a single large block that 236has been divided. 237The size of such a fragment is determinable from 238information in the inode, using the 239.Fn blksize fs ip lbn 240macro. 241.Pp 242The file system records space availability at the fragment level; 243to determine block availability, aligned fragments are examined. 244.Pp 245The root inode is the root of the file system. 246Inode 0 cannot be used for normal purposes and 247historically bad blocks were linked to inode 1, 248thus the root inode is 2 (inode 1 is no longer used for 249this purpose, however numerous dump tapes make this 250assumption, so we are stuck with it). 251.Pp 252The 253.Fa fs_minfree 254element gives the minimum acceptable percentage of file system 255blocks that may be free. 256If the freelist drops below this level 257only the super-user may continue to allocate blocks. 258The 259.Fa fs_minfree 260element 261may be set to 0 if no reserve of free blocks is deemed necessary, 262however severe performance degradations will be observed if the 263file system is run at greater than 90% full; thus the default 264value of 265.Fa fs_minfree 266is 10%. 267.Pp 268Empirically the best trade-off between block fragmentation and 269overall disk utilization at a loading of 90% comes with a 270fragmentation of 8, thus the default fragment size is an eighth 271of the block size. 272.Pp 273The element 274.Fa fs_optim 275specifies whether the file system should try to minimize the time spent 276allocating blocks, or if it should attempt to minimize the space 277fragmentation on the disk. 278If the value of fs_minfree (see above) is less than 10%, 279then the file system defaults to optimizing for space to avoid 280running out of full sized blocks. 281If the value of minfree is greater than or equal to 10%, 282fragmentation is unlikely to be problematical, and 283the file system defaults to optimizing for time. 284.Pp 285.Em Cylinder group related limits : 286Each cylinder keeps track of the availability of blocks at different 287rotational positions, so that sequential blocks can be laid out 288with minimum rotational latency. 289With the default of 8 distinguished 290rotational positions, the resolution of the 291summary information is 2ms for a typical 3600 rpm drive. 292.Pp 293The element 294.Fa fs_old_rotdelay 295gives the minimum number of milliseconds to initiate 296another disk transfer on the same cylinder. 297It is used in determining the rotationally optimal 298layout for disk blocks within a file; 299the default value for 300.Fa fs_old_rotdelay 301is 2ms. 302.Pp 303Each file system has a statically allocated number of inodes. 304An inode is allocated for each 305.Dv NBPI 306bytes of disk space. 307The inode allocation strategy is extremely conservative. 308.Pp 309.Dv MINBSIZE 310is the smallest allowable block size. 311With a 312.Dv MINBSIZE 313of 4096 314it is possible to create files of size 3152^32 with only two levels of indirection. 316.Dv MINBSIZE 317must be big enough to hold a cylinder group block, 318thus changes to 319.Pq Fa struct cg 320must keep its size within 321.Dv MINBSIZE . 322Note that super-blocks are never more than size 323.Dv SBLOCKSIZE . 324.Pp 325The path name on which the file system is mounted is maintained in 326.Fa fs_fsmnt . 327.Dv MAXMNTLEN 328defines the amount of space allocated in 329the super-block for this name. 330The limit on the amount of summary information per file system 331is defined by 332.Dv MAXCSBUFS . 333For a 4096 byte block size, it is currently parameterized for a 334maximum of two million cylinders. 335.Pp 336Per cylinder group information is summarized in blocks allocated 337from the first cylinder group's data blocks. 338These blocks are read in from 339.Fa fs_csaddr 340(size 341.Fa fs_cssize ) 342in addition to the super-block. 343.Pp 344.Sy N.B. : 345.Fn sizeof "struct csum" 346must be a power of two in order for 347the 348.Fn fs_cs 349macro to work. 350.Pp 351The 352.Em "Super-block for a file system" : 353The size of the rotational layout tables 354is limited by the fact that the super-block is of size 355.Dv SBLOCKSIZE . 356The size of these tables is 357.Em inversely 358proportional to the block 359size of the file system. 360The size of the tables is 361increased when sector sizes are not powers of two, 362as this increases the number of cylinders 363included before the rotational pattern repeats 364.Pq Fa fs_cpc . 365The size of the rotational layout 366tables is derived from the number of bytes remaining in 367.Pq Fa struct fs . 368.Pp 369The number of blocks of data per cylinder group 370is limited because cylinder groups are at most one block. 371The inode and free block tables 372must fit into a single block after deducting space for 373the cylinder group structure 374.Pq Fa struct cg . 375.Pp 376The 377.Em Inode : 378The inode is the focus of all file activity in the 379.Ux 380file system. 381There is a unique inode allocated 382for each active file, 383each current directory, each mounted-on file, 384text file, and the root. 385An inode is `named' by its device/i-number pair. 386For further information, see the include file 387.In ufs/ufs/inode.h . 388.Sh HISTORY 389A super-block structure named filsys appeared in 390.At v6 . 391The file system described in this manual appeared 392in 393.Bx 4.2 . 394