FS(5) COMMAND REFERENCE FS(5)
NAME
fs, inode - format of file system volume
SYNOPSIS
#include <sys/types.h>
#include <sys/fs.h>
#include <sys/inode.h>
#include <sys/param.h>
DESCRIPTION
Every file system storage volume has a common format for
certain vital information. Every such volume is divided
into a certain number of blocks. The block size is a
parameter of the file system. Sectors 0 to 15 on a file
system are used to contain primary and secondary
bootstrapping programs. Sectors are 512 bytes in length.
The actual file system begins at sector 16 with the super
block. The layout of the super block as defined by the
include file <sys/fs.h> is:
#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 */
Printed 3/13/89 1
FS(5) COMMAND REFERENCE FS(5)
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 */
long fs_sparecon[6]; /* reserved for future constants */
/* 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 should be 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_postbl[MAXCPG][NRPOS];/* head of blocks for each rotation */
long fs_magic; /* magic number */
u_char fs_rotbl[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 super-block data does not
change, so the copies need not be referenced further unless
disaster strikes.
Printed 3/13/89 2
FS(5) COMMAND REFERENCE FS(5)
Addresses stored in inodes are capable of addressing
fragments of ``blocks.'' File system blocks of at most size
MAXBSIZE (defined in <sys/param.h>) can be optionally broken
into 2, 4, or 8 pieces, each of which is addressable; these
pieces may be DEV_BSIZE (defined in <sys/dir.h>), 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 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 ``blksize(fs, ip, lbn)'' macro defined in
<sys/fs.h>.
The file system records space availability at the fragment
level; to determine block availability, aligned fragments
are examined.
The root inode, inode 2, is the root of the file system.
(Inode 0 can't be used for normal purposes and inode 1 was
once used for linking bad blocks, so inode 2 is used for the
root inode.) The lost+found directory is given the next
available inode when it is initially created by mkfs.
fs_minfree gives the minimum acceptable percentage of file
system blocks which may be free. If the freelist drops below
this level only the super-user may continue to allocate
blocks. This 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 fs_minfree is 10%.
Empirically the best trade-off between block fragmentation
and overall disk utilization at a loading of 90% comes with
a fragmentation of 4, thus the default fragment size is a
fourth of the block size.
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. NRPOS is the number of
rotational positions which are distinguished. With NRPOS 8
the resolution of the summary information is 2ms for a
typical 3600 rpm drive.
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 fs_rotdelay is
Printed 3/13/89 3
FS(5) COMMAND REFERENCE FS(5)
2ms.
Each file system has a statically allocated number of
inodes. An inode is allocated for each NBPI bytes of disk
space. The inode allocation strategy is extremely
conservative.
MAXIPG (defined in <sys/fs.h>) bounds the number of inodes
per cylinder group, and is needed only to keep the structure
simpler by having the only a single variable size element
(the free bit map).
N.B.: MAXIPG must be a multiple of INOPB(fs) (defined in
<sys/fs.h>).
MINBSIZE (defined in <sys/fs.h>) 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. MAXCPG (defined in <sys/fs.h>) is limited
only to dimension an array in (struct cg); it can be made
larger as long as that structure's size remains within the
bounds dictated by MINBSIZE. Note that super blocks are
never more than size SBSIZE (defined in <sys/fs.h>).
The path name on which the file system is mounted is
maintained in fs_fsmnt. MAXMNTLEN (defined in <sys/fs.h>)
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 MAXCSBUFS (defined in
<sys/fs.h>). It is currently parameterized for a maximum of
two million cylinders.
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 (size fs_cssize) in
addition to the super block.
N.B.: sizeof (struct csum) must be a power of two in order
for the ``fs_cs'' macro to work.
Super block for a file system: MAXBPC (defined in
<sys/fs.h>) bounds the size of the rotational layout tables
and 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).
Printed 3/13/89 4
FS(5) COMMAND REFERENCE FS(5)
MAXBPG (defined in <sys/fs.h>) bounds the number of blocks
of data per cylinder group, and is limited by the fact that
cylinder groups are at most one block. The size of the free
block table is derived from the size of blocks and the
number of remaining bytes in the cylinder group structure
(struct cg).
Inode: The inode is the focus of all file activity in the
UTek 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 <sys/inode.h>.
SEE ALSO
fstab(5).
Printed 3/13/89 5
%%index%%
na:192,92;
sy:284,1009;
de:1293,2510;4067,2900;7231,2933;10428,2680;13372,902;
se:14274,142;
%%index%%000000000126