343 lines
13 KiB
Groff
343 lines
13 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)fs.5 8.2 (Berkeley) 4/19/94
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.\"
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.Dd April 19, 1994
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.Dt FS 5
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.Os BSD 4.2
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.Sh NAME
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.Nm fs ,
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.Nm inode
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.Nd format of file system volume
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <ufs/ffs/fs.h>
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.Fd #include <ufs/ufs/inode.h>
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.Sh DESCRIPTION
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The files
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.Aq Pa fs.h
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and
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.Aq Pa inode.h
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declare several structures, defined variables and macros
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which are used to create and manage the underlying format of
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file system objects on random access devices (disks).
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.Pp
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The block size and number of blocks which
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comprise a file system are parameters of the file system.
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Sectors beginning at
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.Dv BBLOCK
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and continuing for
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.Dv BBSIZE
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are used
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for a disklabel and for some hardware primary
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and secondary bootstrapping programs.
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.Pp
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The actual file system begins at sector
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.Dv SBLOCK
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with the
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.Em super-block
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that is of size
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.Dv SBSIZE .
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The following structure described the super-block and is
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from the file
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.Aq Pa ufs/ffs/fs.h :
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.Bd -literal
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#define FS_MAGIC 0x011954
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struct fs {
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struct fs *fs_link; /* linked list of file systems */
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struct fs *fs_rlink; /* used for incore super blocks */
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daddr_t fs_sblkno; /* addr of super-block in filesys */
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daddr_t fs_cblkno; /* offset of cyl-block in filesys */
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daddr_t fs_iblkno; /* offset of inode-blocks in filesys */
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daddr_t fs_dblkno; /* offset of first data after cg */
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long fs_cgoffset; /* cylinder group offset in cylinder */
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long fs_cgmask; /* used to calc mod fs_ntrak */
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time_t fs_time; /* last time written */
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long fs_size; /* number of blocks in fs */
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long fs_dsize; /* number of data blocks in fs */
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long fs_ncg; /* number of cylinder groups */
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long fs_bsize; /* size of basic blocks in fs */
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long fs_fsize; /* size of frag blocks in fs */
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long fs_frag; /* number of frags in a block in fs */
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/* these are configuration parameters */
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long fs_minfree; /* minimum percentage of free blocks */
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long fs_rotdelay; /* num of ms for optimal next block */
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long fs_rps; /* disk revolutions per second */
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/* these fields can be computed from the others */
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long fs_bmask; /* ``blkoff'' calc of blk offsets */
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long fs_fmask; /* ``fragoff'' calc of frag offsets */
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long fs_bshift; /* ``lblkno'' calc of logical blkno */
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long fs_fshift; /* ``numfrags'' calc number of frags */
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/* these are configuration parameters */
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long fs_maxcontig; /* max number of contiguous blks */
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long fs_maxbpg; /* max number of blks per cyl group */
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/* these fields can be computed from the others */
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long fs_fragshift; /* block to frag shift */
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long fs_fsbtodb; /* fsbtodb and dbtofsb shift constant */
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long fs_sbsize; /* actual size of super block */
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long fs_csmask; /* csum block offset */
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long fs_csshift; /* csum block number */
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long fs_nindir; /* value of NINDIR */
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long fs_inopb; /* value of INOPB */
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long fs_nspf; /* value of NSPF */
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/* yet another configuration parameter */
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long fs_optim; /* optimization preference, see below */
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/* these fields are derived from the hardware */
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long fs_npsect; /* # sectors/track including spares */
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long fs_interleave; /* hardware sector interleave */
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long fs_trackskew; /* sector 0 skew, per track */
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long fs_headswitch; /* head switch time, usec */
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long fs_trkseek; /* track-to-track seek, usec */
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/* sizes determined by number of cylinder groups and their sizes */
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daddr_t fs_csaddr; /* blk addr of cyl grp summary area */
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long fs_cssize; /* size of cyl grp summary area */
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long fs_cgsize; /* cylinder group size */
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/* these fields are derived from the hardware */
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long fs_ntrak; /* tracks per cylinder */
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long fs_nsect; /* sectors per track */
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long fs_spc; /* sectors per cylinder */
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/* this comes from the disk driver partitioning */
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long fs_ncyl; /* cylinders in file system */
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/* these fields can be computed from the others */
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long fs_cpg; /* cylinders per group */
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long fs_ipg; /* inodes per group */
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long fs_fpg; /* blocks per group * fs_frag */
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/* this data must be re-computed after crashes */
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struct csum fs_cstotal; /* cylinder summary information */
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/* these fields are cleared at mount time */
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char fs_fmod; /* super block modified flag */
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char fs_clean; /* file system is clean flag */
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char fs_ronly; /* mounted read-only flag */
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char fs_flags; /* currently unused flag */
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char fs_fsmnt[MAXMNTLEN]; /* name mounted on */
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/* these fields retain the current block allocation info */
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long fs_cgrotor; /* last cg searched */
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struct csum *fs_csp[MAXCSBUFS]; /* list of fs_cs info buffers */
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long fs_cpc; /* cyl per cycle in postbl */
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short fs_opostbl[16][8]; /* old rotation block list head */
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long fs_sparecon[56]; /* reserved for future constants */
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quad fs_qbmask; /* ~fs_bmask - for use with quad size */
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quad fs_qfmask; /* ~fs_fmask - for use with quad size */
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long fs_postblformat; /* format of positional layout tables */
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long fs_nrpos; /* number of rotational positions */
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long fs_postbloff; /* (short) rotation block list head */
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long fs_rotbloff; /* (u_char) blocks for each rotation */
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long fs_magic; /* magic number */
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u_char fs_space[1]; /* list of blocks for each rotation */
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/* actually longer */
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};
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.Ed
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.Pp
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Each disk drive contains some number of file systems.
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A file system consists of a number of cylinder groups.
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Each cylinder group has inodes and data.
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.Pp
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A file system is described by its super-block, which in turn
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describes the cylinder groups. The super-block is critical
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data and is replicated in each cylinder group to protect against
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catastrophic loss. This is done at file system creation
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time and the critical
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super-block data does not change, so the copies need not be
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referenced further unless disaster strikes.
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.Pp
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Addresses stored in inodes are capable of addressing fragments
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of `blocks'. File system blocks of at most size
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.Dv MAXBSIZE
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can
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be optionally broken into 2, 4, or 8 pieces, each of which is
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addressable; these pieces may be
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.Dv DEV_BSIZE ,
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or some multiple of
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a
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.Dv DEV_BSIZE
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unit.
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.Pp
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Large files consist of exclusively large data blocks. To avoid
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undue wasted disk space, the last data block of a small file is
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allocated as only as many fragments of a large block as are
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necessary. The file system format retains only a single pointer
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to such a fragment, which is a piece of a single large block that
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has been divided. The size of such a fragment is determinable from
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information in the inode, using the
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.Fn blksize fs ip lbn
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macro.
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.Pp
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The file system records space availability at the fragment level;
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to determine block availability, aligned fragments are examined.
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.Pp
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The root inode is the root of the file system.
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Inode 0 can't be used for normal purposes and
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historically bad blocks were linked to inode 1,
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thus the root inode is 2 (inode 1 is no longer used for
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this purpose, however numerous dump tapes make this
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assumption, so we are stuck with it).
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.Pp
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The
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.Fa fs_minfree
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element gives the minimum acceptable percentage of file system
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blocks that may be free. If the freelist drops below this level
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only the super-user may continue to allocate blocks.
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The
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.Fa fs_minfree
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element
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may be set to 0 if no reserve of free blocks is deemed necessary,
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however severe performance degradations will be observed if the
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file system is run at greater than 90% full; thus the default
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value of
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.Fa fs_minfree
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is 10%.
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.Pp
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Empirically the best trade-off between block fragmentation and
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overall disk utilization at a loading of 90% comes with a
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fragmentation of 8, thus the default fragment size is an eighth
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of the block size.
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.Pp
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The element
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.Fa fs_optim
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specifies whether the file system should try to minimize the time spent
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allocating blocks, or if it should attempt to minimize the space
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fragmentation on the disk.
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If the value of fs_minfree (see above) is less than 10%,
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then the file system defaults to optimizing for space to avoid
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running out of full sized blocks.
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If the value of minfree is greater than or equal to 10%,
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fragmentation is unlikely to be problematical, and
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the file system defaults to optimizing for time.
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.Pp
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.Em Cylinder group related limits :
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Each cylinder keeps track of the availability of blocks at different
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rotational positions, so that sequential blocks can be laid out
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with minimum rotational latency. With the default of 8 distinguished
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rotational positions, the resolution of the
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summary information is 2ms for a typical 3600 rpm drive.
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.Pp
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The element
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.Fa fs_rotdelay
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gives the minimum number of milliseconds to initiate
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another disk transfer on the same cylinder.
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It is used in determining the rotationally optimal
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layout for disk blocks within a file;
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the default value for
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.Fa fs_rotdelay
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is 2ms.
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.Pp
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Each file system has a statically allocated number of inodes.
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An inode is allocated for each
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.Dv NBPI
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bytes of disk space.
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The inode allocation strategy is extremely conservative.
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.Pp
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.Dv MINBSIZE
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is the smallest allowable block size.
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With a
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.Dv MINBSIZE
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of 4096
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it is possible to create files of size
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2^32 with only two levels of indirection.
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.Dv MINBSIZE
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must be big enough to hold a cylinder group block,
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thus changes to
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.Pq Fa struct cg
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must keep its size within
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.Dv MINBSIZE .
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Note that super-blocks are never more than size
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.Dv SBSIZE .
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.Pp
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The path name on which the file system is mounted is maintained in
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.Fa fs_fsmnt .
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.Dv MAXMNTLEN
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defines the amount of space allocated in
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the super-block for this name.
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The limit on the amount of summary information per file system
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is defined by
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.Dv MAXCSBUFS.
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For a 4096 byte block size, it is currently parameterized for a
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maximum of two million cylinders.
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.Pp
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Per cylinder group information is summarized in blocks allocated
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from the first cylinder group's data blocks.
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These blocks are read in from
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.Fa fs_csaddr
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(size
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.Fa fs_cssize )
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in addition to the super-block.
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.Pp
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.Sy N.B.:
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.Xr sizeof Pq Fa struct csum
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must be a power of two in order for
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the
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.Fn fs_cs
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macro to work.
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.Pp
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The
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.Em "Super-block for a file system" :
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The size of the rotational layout tables
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is limited by the fact that the super-block is of size
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.Dv SBSIZE .
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The size of these tables is
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.Em inversely
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proportional to the block
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size of the file system. The size of the tables is
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increased when sector sizes are not powers of two,
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as this increases the number of cylinders
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included before the rotational pattern repeats
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.Pq Fa fs_cpc .
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The size of the rotational layout
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tables is derived from the number of bytes remaining in
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.Pq Fa struct fs .
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.Pp
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The number of blocks of data per cylinder group
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is limited because cylinder groups are at most one block.
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The inode and free block tables
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must fit into a single block after deducting space for
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the cylinder group structure
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.Pq Fa struct cg .
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.Pp
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The
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.Em Inode :
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The inode is the focus of all file activity in the
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.Tn UNIX
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file system.
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There is a unique inode allocated
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for each active file,
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each current directory, each mounted-on file,
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text file, and the root.
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An inode is `named' by its device/i-number pair.
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For further information, see the include file
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.Aq Pa sys/inode.h .
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.Sh HISTORY
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A super-block structure named filsys appeared in
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.At v6 .
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The file system described in this manual appeared
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in
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.Bx 4.2 .
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