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

NAME
     tunefs - tune up an existing file system

SYNOPSIS
     tunefs [-AFN] [-e maxbpg] [-g avgfilesize] [-h avgfpdir] [-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 options are as follows:

     -A      Cause the values to be updated in all the alternate superblocks
             instead of just the standard superblock.  If this option is not
             used, then use of a backup superblock by fsck(8) will lose any-
             thing changed by tunefs.  -A is ignored when -N is specified.

     -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      Indicates that special is a file system image, rather than a de-
             vice name or file system mount point.  special will be accessed
             `as-is'.

     -g avgfilesize
             This specifies the expected average file size.

     -h avgfpdir
             This specifies the expected number of files per directory.

     -m minfree
             This value specifies the percentage of space held back from nor-
             mal users; the minimum free space threshold.  The default value
             is set during creation of the filesystem; see newfs(8).  This
             value can be set to zero, however up to a factor of three in
             throughput will be lost over the performance obtained 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      Display all the settable options (after any changes from the tun-
             ing options) but do not cause any of them to be changed.

     -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.

             optimize_preference can be specified as either space or time.

SEE ALSO
     fs(5), dumpfs(8), fsck_ffs(8), growfs(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 unmounted 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 4.7                      May 31, 2007                                2
