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

     tunefs -- tune up an existing file system

     tunefs [-AFN] [-e maxbpg] [-g avgfilesize] [-h avgfpdir] [-m minfree]
            [-o optimize_preference] special | filesys

     tunefs is designed to change the dynamic parameters of a file system
     which affect the layout policies.

     The following options are supported by tunefs:

     -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.

     -F      Indicates that special is a file system image, rather than a
             device name or file system mount point.  special will be accessed

     -N      Display all the settable options (after any changes from the tun-
             ing options) but do not cause any of them to be changed.

     -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
             allocating blocks from another cylinder group.  Typically this
             value is set to about one quarter of the total blocks in a cylin-
             der 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 cylin-
             der 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.

     -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
             used is 5%.  This value can be set to zero, however up to a fac-
             tor 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.

     -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.

     fs(5), dumpfs(8), fsck_ffs(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).

     The tunefs command appeared in 4.2BSD.

     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

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

BSD                             April 26, 2004                             BSD