TUNEFS(8)               OpenBSD System Manager's Manual              TUNEFS(8)

NAME
     tunefs - tune up an existing file system

SYNOPSIS
     tunefs [-A] [-a maxcontig] [-d rotdelay] [-e maxbpg] [-m minfree]
            [-o optimize_preference] [special | filesys]

DESCRIPTION
     tunefs is designed to change the dynamic parameters of a file system
     which affect the layout policies.  The parameters which are to be changed
     are indicated by the flags given below:

     -A      The file system has several backups of the super-block.  Specify-
             ing this option will cause all backups to be modified as well as
             the primary super-block.  This is potentially dangerous - use
             with caution.

     -a maxcontig
             This specifies the maximum number of contiguous blocks that will
             be laid out before forcing a rotational delay (see -d below).
             The default value is one, since most device drivers require an
             interrupt per disk transfer.  Device drivers that can chain sev-
             eral buffers together in a single transfer should set this to the
             maximum chain length.

     -d rotdelay
             This specifies the expected time (in milliseconds) to service a
             transfer completion interrupt and initiate a new transfer on the
             same disk.  It is used to decide how much rotational spacing to
             place between successive blocks in a file.

     -e maxbpg
             This indicates the maximum number of blocks any single file can
             allocate out of a cylinder group before it is forced to begin al-
             locating blocks from another cylinder group.  Typically this val-
             ue is set to about one quarter of the total blocks in a cylinder
             group.  The intent is to prevent any single file from using up
             all the blocks in a single cylinder group, thus degrading access
             times for all files subsequently allocated in that cylinder
             group.  The effect of this limit is to cause big files to do long
             seeks more frequently than if they were allowed to allocate all
             the blocks in a cylinder group before seeking elsewhere.  For
             file systems with exclusively large files, this parameter should
             be set higher.

     -f avgfilesize
             Specifies the expected average file size in bytes.  This value
             could be used for various optimizations, but for now it is only
             used together with avgfpdir to optimize the directory allocation
             policy.  To take effect, both avgfpdir and avgfilesize must be
             greater than zero.  (Also see avgfpdir.)

     -m minfree
             This value specifies the percentage of space held back from nor-
             mal users; the minimum free space threshold.  The default value
             used is 5%.  This value can be set to zero; however, a factor of
             up to three in throughput will be lost over the performance ob-
             tained at a 5% threshold.  Note that if the value is raised above
             the current usage level, users will be unable to allocate files
             until enough files have been deleted to get under the higher
             threshold.

     -n avgfpdir
             Specifies the expected average number of files per directory in
             the file system.  This value is used only if both avgfilesize and
             avgfpdir are greater than zero.  It is used to limit number of
             directories which may be allocated one after another in the same
             cylinder group without intervention by regular files.  This value
             does not affect most file system operations but is useful for ap-
             plications which at first create a directory structure and then
             populate with files later.  (Also see avgfilesize.)

     -o optimize_preference
             The file system can either try to minimize the time spent allo-
             cating blocks, or it can attempt to minimize the space fragmenta-
             tion on the disk.  If the value of minfree (see above) is less
             than 5%, then the file system should optimize for space to avoid
             running out of full sized blocks.  For values of minfree greater
             than or equal to 5%, fragmentation is unlikely to be problemati-
             cal, and the file system can be optimized for time.

     -p      This option shows a summary of what the current tuneable settings
             are on the selected file system.  More detailed information can
             be obtained in the dumpfs(8) manual page.

SEE ALSO
     fs(5), dumpfs(8), newfs(8)

     M. McKusick, W. Joy, S. Leffler, and R. Fabry, "A Fast File System for
     UNIX", ACM Transactions on Computer Systems 2, 3, pp 181-197, August
     1984, (reprinted in the BSD System Manager's Manual, SMM:5).

HISTORY
     The tunefs command appeared in 4.2BSD.

BUGS
     This program should work on mounted and active file systems.  Because the
     super-block is not kept in the buffer cache, the changes will only take
     effect if the program is run on dismounted file systems.  To change the
     root file system, the system must be rebooted after the file system is
     tuned.

     You can tune a file system, but you can't tune a fish.

OpenBSD 2.9                    December 11, 1993                             2
