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
Each disk drive contains some number of file systems. A file system con-
sists of a number of cylinder groups. Each cylinder group has inodes and
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 sys-
tem creation time and the critical superblock data does not change, so the
copies need not be referenced further until necessary.
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 frag-
ments 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 deter-
mined from information in the inode, using the blksize(fs, ip, lbn) macro.
The file system records space availability at the fragment level; to deter-
mine block availability, aligned fragments are examined.
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 frag-
mentation of 8, thus the default fragment size is an eighth of
the block size.
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 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 posi-
tions, the resolution of the summary information is 2 mil-
liseconds 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 2 mil-
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 232 with only two levels of indirec-
tion. 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 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 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 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 sys-
tem. 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 rota-
tional 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
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 direc-
tory, each mounted-on file, text file, and the root. An inode is `named'
by its device/i-number pair.
sizeof (struct csum) must be a power of two in order for the fs_cs macro to