TUNEFS(8)               386BSD System Manager's Manual               TUNEFS(8)

NAME
     tunefs - tune up an existing file system

SYNOPSIS
     tunefs [-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 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
             several 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
             allocating blocks from another cylinder group.  Typically this
             value 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
             normal users; the minimum free space threshold.  The default
             value used is 10%.  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 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
             higher threshold.

     -o optimize_preference
             The file system can either try to minimize the time spent
             allocating blocks, or it can attempt minimize the space
             fragmentation 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
             problematical, and the file system can be optimized for time.

SEE ALSO
     fs(5),  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:14).

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.

HISTORY
     The tunefs command appeared in 4.2BSD.

4.2 Berkeley Distribution       March 16, 1991                               2