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fs(4)


NAME

  fs, inode - Specifies the format of the file system volume


SYNOPSIS

  #include <sys/types.h>

  #include <sys/fs.h>

  #include <ufs/inode.h>


DESCRIPTION

  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 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
  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 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:

  fs_minfree
      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.

  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 positions, the
      resolution of the summary information is 2 milliseconds for a typical
      3600 rpm drive.

  fs_rotdelay
      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 milliseconds.

  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 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
  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.

  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 device/i-number pair.


NOTES

  sizeof (struct csum) must be a power of two in order for the fs_cs macro to
  work.

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