fs, inode - format of file system volume
#include <sys/types.h>
#include <ufs/ffs/fs.h>
#include <ufs/ufs/inode.h>
The files <ufs/ffs/fs.h> and <ufs/ufs/inode.h> declare several structures
and define variables and macros which are used to create and
manage the
underlying format of file system objects on random access
devices
(disks).
The block size and number of blocks which comprise a file
system are parameters
of the file system. Sectors beginning at BBLOCK
and continuing
for BBSIZE are used for a disklabel and for some hardware
primary and
secondary bootstrapping programs.
The actual file system begins at sector SBLOCK with the
super-block that
is of size SBSIZE. The following structure describes the
super-block and
is from the file <ufs/ffs/fs.h>:
#define FS_MAGIC 0x011954
struct fs {
int32_t fs_firstfield; /* historic file system
linked list, */
int32_t fs_unused_1; /* used for incore super
blocks */
int32_t fs_sblkno; /* addr of super-block in
filesys */
int32_t fs_cblkno; /* offset of cyl-block in
filesys */
int32_t fs_iblkno; /* offset of inode-blocks in
filesys */
int32_t fs_dblkno; /* offset of first data after cg */
int32_t fs_cgoffset; /* cylinder group offset in
cylinder */
int32_t fs_cgmask; /* used to calc mod fs_ntrak
*/
time_t fs_time; /* last time written */
int32_t fs_size; /* number of blocks in fs */
int32_t fs_dsize; /* number of data blocks in
fs */
int32_t fs_ncg; /* number of cylinder groups
*/
int32_t fs_bsize; /* size of basic blocks in
fs */
int32_t fs_fsize; /* size of frag blocks in fs
*/
int32_t fs_frag; /* number of frags in a
block in fs */
/* these are configuration parameters */
int32_t fs_minfree; /* minimum percentage of
free blocks */
int32_t fs_rotdelay; /* num of ms for optimal
next block */
int32_t fs_rps; /* disk revolutions per second */
/* these fields can be computed from the others */
int32_t fs_bmask; /* ``blkoff'' calc of blk
offsets */
int32_t fs_fmask; /* ``fragoff'' calc of frag
offsets */
int32_t fs_bshift; /* ``lblkno'' calc of logical blkno */
int32_t fs_fshift; /* ``numfrags'' calc number
of frags */
/* these are configuration parameters */
int32_t fs_maxcontig; /* max number of contiguous
blks */
int32_t fs_maxbpg; /* max number of blks per
cyl group */
/* these fields can be computed from the others */
int32_t fs_fragshift; /* block to frag shift */
int32_t fs_fsbtodb; /* fsbtodb and dbtofsb shift
constant */
int32_t fs_sbsize; /* actual size of super
block */
int32_t fs_csmask; /* csum block offset (now
unused) */
int32_t fs_csshift; /* csum block number (now
unused) */
int32_t fs_nindir; /* value of NINDIR */
int32_t fs_inopb; /* value of INOPB */
int32_t fs_nspf; /* value of NSPF */
/* yet another configuration parameter */
int32_t fs_optim; /* optimization preference,
see below */
/* these fields are derived from the hardware */
int32_t fs_npsect; /* # sectors/track including
spares */
int32_t fs_interleave; /* hardware sector interleave */
int32_t fs_trackskew; /* sector 0 skew, per track
*/
/* fs_id takes the space of the unused fs_headswitch and
fs_trkseek */
int32_t fs_id[2]; /* unique filesystem id */
/* sizes determined by number of cylinder groups and their
sizes */
int32_t fs_csaddr; /* blk addr of cyl grp summary area */
int32_t fs_cssize; /* size of cyl grp summary
area */
int32_t fs_cgsize; /* cylinder group size */
/* these fields are derived from the hardware */
int32_t fs_ntrak; /* tracks per cylinder */
int32_t fs_nsect; /* sectors per track */
int32_t fs_spc; /* sectors per cylinder */
/* this comes from the disk driver partitioning */
int32_t fs_ncyl; /* cylinders in file system
*/
/* these fields can be computed from the others */
int32_t fs_cpg; /* cylinders per group */
int32_t fs_ipg; /* inodes per group */
int32_t 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 */
int8_t fs_fmod; /* super block modified flag
*/
int8_t fs_clean; /* file system is clean flag
*/
int8_t fs_ronly; /* mounted read-only flag */
int8_t fs_flags; /* see FS_ below */
u_char fs_fsmnt[MAXMNTLEN]; /* name mounted on */
/* these fields retain the current block allocation info */
int32_t fs_cgrotor; /* last cg searched */
void *fs_ocsp[NOCSPTRS]; /* padding; was list of
fs_cs bufs */
u_int8_t *fs_contigdirs; /* # of contiguously allocated dirs */
struct csum *fs_csp; /* cg summary info buffer
for fs_cs */
int32_t *fs_maxcluster; /* max cluster in each cyl
group */
int32_t fs_cpc; /* cyl per cycle in postbl
*/
int16_t fs_opostbl[16][8]; /* old rotation block
list head */
int32_t fs_snapinum[20];/* reserved for snapshot
inode nums */
int32_t fs_avgfilesize; /* expected average file
size */
int32_t fs_avgfpdir; /* expected # of files per
directory */
int32_t fs_sparecon[27];/* reserved for future constants */
time_t fs_fscktime; /* last time fsck(8)ed */
int32_t fs_contigsumsize; /* size of cluster summary array */
int32_t fs_maxsymlinklen; /* max length of internal
symlink */
int32_t fs_inodefmt; /* format of on-disk inodes
*/
u_int64_t fs_maxfilesize;/* maximum representable
file size */
int64_t fs_qbmask; /* ~fs_bmask - for use with
quad size */
int64_t fs_qfmask; /* ~fs_fmask - for use with
quad size */
int32_t fs_state; /* validate fs_clean field
*/
int32_t fs_postblformat;/* format of positional
layout tables */
int32_t fs_nrpos; /* number of rotational positions */
int32_t fs_postbloff; /* (u_int16) rotation block
list head */
int32_t fs_rotbloff; /* (u_int8) blocks for each
rotation */
int32_t fs_magic; /* magic number */
u_int8_t fs_space[1]; /* list of blocks for each
rotation */
/* actually longer */
};
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.
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 superblock data does
not change, so the copies need not be referenced further unless disaster
strikes.
Addresses stored in inodes are capable of addressing fragments of
``blocks''. File system blocks of at most size MAXBSIZE can
be optionally
broken into 2, 4, or 8 pieces, each of which is addressable; these
pieces may be DEV_BSIZE, or some multiple of a DEV_BSIZE
unit.
Large files consist of exclusively large data blocks. To
avoid undue
wasted disk space, the last data block of a small file is
allocated 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 blksize(fs,
ip, lbn) macro.
The file system records space availability at the fragment
level; to determine
block availability, aligned fragments are examined.
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 (inode
1 is no longer used for this purpose; however, numerous dump
tapes make
this assumption, so we are stuck with it). Thus the root
inode is 2.
The 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 superuser may continue to allocate blocks. The
fs_minfree element
may be set to 0 if no reserve of free blocks is deemed
necessary,
although severe performance degradations will be observed if
the file
system is run at greater than 95% full; thus the default
value of
fs_minfree is 5%.
Empirically the best trade-off between block fragmentation
and overall
disk utilization at a loading of 95% comes with a fragmentation of 8;
thus the default fragment size is an eighth of the block
size.
The element fs_optim specifies whether the file system
should try to minimize
the time spent allocating blocks (FS_OPTTIME), or if
it should attempt
to minimize the space fragmentation on the disk
(FS_OPTSPACE). If
the value of fs_minfree (see above) is less than 5%, then
the file system
defaults to optimizing for space to avoid running out of
full sized
blocks. If the value of fs_minfree is greater than or equal
to 5%, fragmentation
is unlikely to be problematical, and the file system defaults
to optimizing for time.
The fs_flags element specifies how the filesystem was mounted:
FS_DOSOFTDEP The filesystem was mounted using soft dependencies.
FS_UNCLEAN The filesystem was mounted uncleanly.
Cylinder group related limits [Toc] [Back]
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 1 distinct rotational position,
the resolution of the summary information is 16ms for
a typical
3600 RPM drive.
The element fs_rotdelay was once used to tweak block layout.
Each file system has a statically allocated number of inodes, determined
by its size and the desired number of file data bytes per
inode at the
time it was created. See newfs(8) for details on how to set
this (and
other) filesystem parameters. By default, the inode allocation strategy
is extremely conservative.
MINBSIZE is the smallest allowable block size. With a MINBSIZE of 4096
it is possible to create files of size 2^32 with only two
levels of indirection.
MINBSIZE must be big enough to hold a cylinder
group block,
thus changes to struct cg must keep its size within MINBSIZE. Note that
super-blocks are never more than size SBSIZE.
The path name on which the file system is mounted is maintained in
fs_fsmnt. MAXMNTLEN defines the amount of space allocated
in the superblock
for this name.
Per cylinder group information is summarized in blocks allocated from the
first cylinder group's data blocks. These blocks are read
in from
fs_csaddr (of size fs_cssize) in addition to the superblock.
Note that sizeof(struct csum) must be a power of two in order for the
fs_cs() macro to work.
Super-block for a file system [Toc] [Back]
The size of the rotational layout tables is limited by the
fact that the
super-block is of size SBSIZE. The size of these tables is
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 (fs_cpc). The size of the rotational layout tables
is derived
from the number of bytes remaining in struct fs.
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 struct cg.
Inodes [Toc] [Back]
The inode is the focus of all file activity in the 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 <ufs/ufs/inode.h>.
A super-block structure named filsys appeared in Version 6
AT&T UNIX.
The file system described in this manual appeared in 4.2BSD.
OpenBSD 3.6 April 19, 1994
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