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

     tunefs - tune up an existing file system

     tunefs [-AN] [-a maxcontig] [-d rotdelay] [-e maxbpg] [-m minfree]
            [-o optimize_preference] [-t trackskew] [special | filesys]

     The tunefs program is designed to change the dynamic parameters of a file
     system which affect the layout policies.  The -N flag displays all the
     settable options (after any changes from the tuning options) but does not
     cause any of them to be changed.  The -A flag causes the values to be up-
     dated in all the alternate superblocks instead of just the standard su-
     perblock.  If this option is not used, then use of a backup superblock by
     fsck(8) will lose anything changed by tunefs. The -A flag is ignored when
     the -N flag is specified.

     The parameters which are to be changed are indicated by the flags given

     -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.

     -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 10%.  This value can be set to zero, however up to a fac-
             tor of three in throughput will be lost over the performance ob-
             tained at a 10% 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 high-
             er 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 10%, 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 10%, fragmentation is unlikely to be problemati-
             cal, and the file system can be optimized for time.

     -t trackskew
             This specifies the skew in sectors from one track to the next in
             a cylinder.  The default value is zero, indicating that each
             track in a cylinder begins at the same rotational position.

     fs(5),  dumpfs(8),  fsck(8),  newfs(8),  mkfs(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).

     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

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

     The tunefs command appeared in 4.2BSD.

4.2 Berkeley Distribution         May 3, 1995                                2