+++ /dev/null
-.\" $NetBSD: fs.5,v 1.3 1994/11/30 19:31:17 jtc Exp $
-.\"
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-.\" @(#)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
-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 .
projects / apple / xnu.git / blobdiff