fs, inode - Specifies the format of the file system volume
Every file system storage volume (disk, nine-track tape,
for instance) has a common format for certain vital information.
Each such volume is divided into a certain number
of blocks. The block size is a parameter of the file system.
Sectors beginning at BBLOCK and continuing for BBSIZE
are used to contain a label and for some hardware primary
and secondary bootstrapping programs.
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 superblock, which in
turn describes the cylinder groups. The superblock is
critical data and is replicated in each cylinder group to
protect against loss of data. This is done at file system
creation time and the critical superblock data does not
change, so the copies need not be referenced further until
Addresses stored in inodes are capable of addressing fragments
of blocks. File system blocks of at most MAXBSIZE
size 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 exclusively of large data blocks. To
avoid 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 determined 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
The root inode is the root of the file system. Inode 0
(zero) 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,
but numerous dump tapes make this assumption).
Some fields to the fs structure are as follows: 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 field may be set to 0 (zero) 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
the fs_minfree field is 10%.
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. 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 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 fs_minfree is
greater than or equal to 10%, fragmentation is
unlikely to be problematical, and the file system
defaults to optimizing for time.
Cylinder group related limits: Each cylinder keeps
track of the availability of blocks at different
positions of rotation, 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 2 milliseconds for a typical 3600 rpm
drive. Gives the minimum number of milliseconds to
initiate another disk transfer on the same cylinder.
The fs_rotdelay field is used in determining
the rotationally optimal layout for disk blocks
within a file; the default value for fs_rotdelay is
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.
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
superblocks are never more than size SBSIZE.
The pathname 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. The limit on
the amount of summary information per file system is
defined by MAXCSBUFS. For a 4096 byte block size, it is
currently parameterized for a maximum of two million
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 superblock.
Superblock for a file system: The size of the rotational
layout tables is limited by the fact that the superblock
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
Inode: 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
sizeof (struct csum) must be a power of two in order for
the fs_cs macro to work.
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