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author | Theo de Raadt <deraadt@cvs.openbsd.org> | 1995-10-18 08:53:40 +0000 |
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committer | Theo de Raadt <deraadt@cvs.openbsd.org> | 1995-10-18 08:53:40 +0000 |
commit | d6583bb2a13f329cf0332ef2570eb8bb8fc0e39c (patch) | |
tree | ece253b876159b39c620e62b6c9b1174642e070e /share/man/man5/fs.5 |
initial import of NetBSD tree
Diffstat (limited to 'share/man/man5/fs.5')
-rw-r--r-- | share/man/man5/fs.5 | 344 |
1 files changed, 344 insertions, 0 deletions
diff --git a/share/man/man5/fs.5 b/share/man/man5/fs.5 new file mode 100644 index 00000000000..5d7c2b04d82 --- /dev/null +++ b/share/man/man5/fs.5 @@ -0,0 +1,344 @@ +.\" $NetBSD: fs.5,v 1.3 1994/11/30 19:31:17 jtc Exp $ +.\" +.\" Copyright (c) 1983, 1991, 1993 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 3. All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" This product includes software developed by the University of +.\" California, Berkeley and its contributors. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)fs.5 8.2 (Berkeley) 4/19/94 +.\" +.Dd April 19, 1994 +.Dt FS 5 +.Os BSD 4.2 +.Sh NAME +.Nm fs , +.Nm inode +.Nd format of file system volume +.Sh SYNOPSIS +.Fd #include <sys/types.h> +.Fd #include <ufs/fs.h> +.Fd #include <ufs/inode.h> +.Sh DESCRIPTION +The files +.Aq Pa fs.h +and +.Aq Pa inode.h +declare several structures, defined variables and macros +which are used to create and manage the underlying format of +file system objects on random access devices (disks). +.Pp +The block size and number of blocks which +comprise a file system are parameters of the file system. +Sectors beginning at +.Dv BBLOCK +and continuing for +.Dv BBSIZE +are used +for a disklabel and for some hardware primary +and secondary bootstrapping programs. +.Pp +The actual file system begins at sector +.Dv SBLOCK +with the +.Em super-block +that is of size +.Dv SBSIZE . +The following structure described the super-block and is +from the file +.Aq Pa ufs/fs.h : +.Bd -literal +#define FS_MAGIC 0x011954 +struct fs { + struct fs *fs_link; /* linked list of file systems */ + struct fs *fs_rlink; /* used for incore super blocks */ + daddr_t fs_sblkno; /* addr of super-block in filesys */ + daddr_t fs_cblkno; /* offset of cyl-block in filesys */ + daddr_t fs_iblkno; /* offset of inode-blocks in filesys */ + daddr_t fs_dblkno; /* offset of first data after cg */ + long fs_cgoffset; /* cylinder group offset in cylinder */ + long fs_cgmask; /* used to calc mod fs_ntrak */ + time_t fs_time; /* last time written */ + long fs_size; /* number of blocks in fs */ + long fs_dsize; /* number of data blocks in fs */ + long fs_ncg; /* number of cylinder groups */ + long fs_bsize; /* size of basic blocks in fs */ + long fs_fsize; /* size of frag blocks in fs */ + long fs_frag; /* number of frags in a block in fs */ +/* these are configuration parameters */ + long fs_minfree; /* minimum percentage of free blocks */ + long fs_rotdelay; /* num of ms for optimal next block */ + long fs_rps; /* disk revolutions per second */ +/* these fields can be computed from the others */ + long fs_bmask; /* ``blkoff'' calc of blk offsets */ + long fs_fmask; /* ``fragoff'' calc of frag offsets */ + long fs_bshift; /* ``lblkno'' calc of logical blkno */ + long fs_fshift; /* ``numfrags'' calc number of frags */ +/* these are configuration parameters */ + long fs_maxcontig; /* max number of contiguous blks */ + long fs_maxbpg; /* max number of blks per cyl group */ +/* these fields can be computed from the others */ + long fs_fragshift; /* block to frag shift */ + long fs_fsbtodb; /* fsbtodb and dbtofsb shift constant */ + long fs_sbsize; /* actual size of super block */ + long fs_csmask; /* csum block offset */ + long fs_csshift; /* csum block number */ + long fs_nindir; /* value of NINDIR */ + long fs_inopb; /* value of INOPB */ + long fs_nspf; /* value of NSPF */ +/* yet another configuration parameter */ + long fs_optim; /* optimization preference, see below */ +/* these fields are derived from the hardware */ + long fs_npsect; /* # sectors/track including spares */ + long fs_interleave; /* hardware sector interleave */ + long fs_trackskew; /* sector 0 skew, per track */ + long fs_headswitch; /* head switch time, usec */ + long fs_trkseek; /* track-to-track seek, usec */ +/* sizes determined by number of cylinder groups and their sizes */ + daddr_t fs_csaddr; /* blk addr of cyl grp summary area */ + long fs_cssize; /* size of cyl grp summary area */ + long fs_cgsize; /* cylinder group size */ +/* these fields are derived from the hardware */ + long fs_ntrak; /* tracks per cylinder */ + long fs_nsect; /* sectors per track */ + long fs_spc; /* sectors per cylinder */ +/* this comes from the disk driver partitioning */ + long fs_ncyl; /* cylinders in file system */ +/* these fields can be computed from the others */ + long fs_cpg; /* cylinders per group */ + long fs_ipg; /* inodes per group */ + long fs_fpg; /* blocks per group * fs_frag */ +/* this data must be re-computed after crashes */ + struct csum fs_cstotal; /* cylinder summary information */ +/* these fields are cleared at mount time */ + char fs_fmod; /* super block modified flag */ + char fs_clean; /* file system is clean flag */ + char fs_ronly; /* mounted read-only flag */ + char fs_flags; /* currently unused flag */ + char fs_fsmnt[MAXMNTLEN]; /* name mounted on */ +/* these fields retain the current block allocation info */ + long fs_cgrotor; /* last cg searched */ + struct csum *fs_csp[MAXCSBUFS]; /* list of fs_cs info buffers */ + long fs_cpc; /* cyl per cycle in postbl */ + short fs_opostbl[16][8]; /* old rotation block list head */ + long fs_sparecon[56]; /* reserved for future constants */ + quad fs_qbmask; /* ~fs_bmask - for use with quad size */ + quad fs_qfmask; /* ~fs_fmask - for use with quad size */ + long fs_postblformat; /* format of positional layout tables */ + long fs_nrpos; /* number of rotational positions */ + long fs_postbloff; /* (short) rotation block list head */ + long fs_rotbloff; /* (u_char) blocks for each rotation */ + long fs_magic; /* magic number */ + u_char fs_space[1]; /* list of blocks for each rotation */ +/* actually longer */ +}; +.Ed +.Pp +Each disk drive contains some number of file systems. +A file system consists of a number of cylinder groups. +Each cylinder group has inodes and data. +.Pp +A file system is described by its super-block, which in turn +describes the cylinder groups. The super-block is critical +data and is replicated in each cylinder group to protect against +catastrophic loss. This is done at file system creation +time and the critical +super-block data does not change, so the copies need not be +referenced further unless disaster strikes. +.Pp +Addresses stored in inodes are capable of addressing fragments +of `blocks'. File system blocks of at most size +.Dv MAXBSIZE +can +be optionally broken into 2, 4, or 8 pieces, each of which is +addressable; these pieces may be +.Dv DEV_BSIZE , +or some multiple of +a +.Dv DEV_BSIZE +unit. +.Pp +Large files consist of exclusively large data blocks. To avoid +undue wasted disk space, the last data block of a small file is +allocated as only as many fragments of a large block as are +necessary. The file system format retains only a single pointer +to such a fragment, which is a piece of a single large block that +has been divided. The size of such a fragment is determinable from +information in the inode, using the +.Fn blksize fs ip lbn +macro. +.Pp +The file system records space availability at the fragment level; +to determine block availability, aligned fragments are examined. +.Pp +The root inode is the root of the file system. +Inode 0 can't be used for normal purposes and +historically bad blocks were linked to inode 1, +thus the root inode is 2 (inode 1 is no longer used for +this purpose, however numerous dump tapes make this +assumption, so we are stuck with it). +.Pp +The +.Fa fs_minfree +element gives the minimum acceptable percentage of file system +blocks that may be free. If the freelist drops below this level +only the super-user may continue to allocate blocks. +The +.Fa fs_minfree +element +may be set to 0 if no reserve of free blocks is deemed necessary, +however severe performance degradations will be observed if the +file system is run at greater than 90% full; thus the default +value of +.Fa fs_minfree +is 10%. +.Pp +Empirically the best trade-off between block fragmentation and +overall disk utilization at a loading of 90% comes with a +fragmentation of 8, thus the default fragment size is an eighth +of the block size. +.Pp +The element +.Fa fs_optim +specifies whether the file system should try to minimize the time spent +allocating blocks, or if it should attempt to minimize the space +fragmentation on the disk. +If the value of fs_minfree (see above) is less than 10%, +then the file system defaults to optimizing for space to avoid +running out of full sized blocks. +If the value of minfree is greater than or equal to 10%, +fragmentation is unlikely to be problematical, and +the file system defaults to optimizing for time. +.Pp +.Em Cylinder group related limits : +Each cylinder keeps track of the availability of blocks at different +rotational positions, so that sequential blocks can be laid out +with minimum rotational latency. With the default of 8 distinguished +rotational positions, the resolution of the +summary information is 2ms for a typical 3600 rpm drive. +.Pp +The element +.Fa fs_rotdelay +gives the minimum number of milliseconds to initiate +another disk transfer on the same cylinder. +It is used in determining the rotationally optimal +layout for disk blocks within a file; +the default value for +.Fa fs_rotdelay +is 2ms. +.Pp +Each file system has a statically allocated number of inodes. +An inode is allocated for each +.Dv NBPI +bytes of disk space. +The inode allocation strategy is extremely conservative. +.Pp +.Dv MINBSIZE +is the smallest allowable block size. +With a +.Dv MINBSIZE +of 4096 +it is possible to create files of size +2^32 with only two levels of indirection. +.Dv MINBSIZE +must be big enough to hold a cylinder group block, +thus changes to +.Pq Fa struct cg +must keep its size within +.Dv MINBSIZE . +Note that super-blocks are never more than size +.Dv SBSIZE . +.Pp +The path name on which the file system is mounted is maintained in +.Fa fs_fsmnt . +.Dv MAXMNTLEN +defines the amount of space allocated in +the super-block for this name. +The limit on the amount of summary information per file system +is defined by +.Dv MAXCSBUFS. +For a 4096 byte block size, it is currently parameterized for a +maximum of two million cylinders. +.Pp +Per cylinder group information is summarized in blocks allocated +from the first cylinder group's data blocks. +These blocks are read in from +.Fa fs_csaddr +(size +.Fa fs_cssize ) +in addition to the super-block. +.Pp +.Sy N.B.: +.Xr sizeof Pq Fa struct csum +must be a power of two in order for +the +.Fn fs_cs +macro to work. +.Pp +The +.Em "Super-block for a file system" : +The size of the rotational layout tables +is limited by the fact that the super-block is of size +.Dv SBSIZE . +The size of these tables is +.Em inversely +proportional to the block +size of the file system. The size of the tables is +increased when sector sizes are not powers of two, +as this increases the number of cylinders +included before the rotational pattern repeats +.Pq Fa fs_cpc . +The size of the rotational layout +tables is derived from the number of bytes remaining in +.Pq Fa struct fs . +.Pp +The number of blocks of data per cylinder group +is limited because cylinder groups are at most one block. +The inode and free block tables +must fit into a single block after deducting space for +the cylinder group structure +.Pq Fa struct cg . +.Pp +The +.Em Inode : +The inode is the focus of all file activity in the +.Tn UNIX +file system. +There is a unique inode allocated +for each active file, +each current directory, each mounted-on file, +text file, and the root. +An inode is `named' by its device/i-number pair. +For further information, see the include file +.Aq Pa sys/inode.h . +.Sh HISTORY +A super-block structure named filsys appeared in +.At v6 . +The file system described in this manual appeared +in +.Bx 4.2 . |