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

     tunefs -- tune up an existing filesystem

     tunefs [-A] [-a maxcontig] [-d rotdelay] [-e maxbpg] [-f avgfilesize]
            [-m minfree] [-n enable | disable] [-o space | time] [-p]
            [-s avgfpdir] [special | filesystem]

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

     -A      The filesystem 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
             Specify 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 inter-
             rupt per disk transfer.  Device drivers that can chain several
             buffers together in a single transfer should set this to the max-
             imum chain length.

     -d rotdelay
             Specify 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
             Indicate the maximum number of blocks any single file can allo-
             cate out of a cylinder group before it is forced to begin allo-
             cating 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
             filesystems with exclusively large files, this parameter should
             be set higher.

     -f avgfilezsize
             Specify the expected average file size.

     -m minfree
             Specify the percentage of space held back from normal users; the
             minimum free space threshold.  The default value used is 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 10%
             threshold.  Settings of 5% and less force space optimization to
             always be used which will greatly increase the overhead for file
             writes.  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 enable | disable
             Turn on/off soft updates.

     -o space | time
             The filesystem can either try to minimize the time spent allocat-
             ing blocks, or it can attempt to minimize the space fragmentation
             on the disk.  Optimization for space has much higher overhead for
             file writes.  The kernel normally changes the preference automat-
             ically as the percent fragmentation changes on the filesystem.

     -p      Show a summary of what the current tunable settings are on the
             selected filesystem.  More detailed information can be obtained
             in the dumpfs(8) manual page.

     -s avgfpdir
             Specify the expected number of files per directory.

     /etc/fstab  read this to determine the device file for a specified mount

     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).

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

     You can tune a filesystem, but you can't tune a fish.

     The tunefs command appeared in 4.2BSD.

FreeBSD 4.10                   December 11, 1993                  FreeBSD 4.10


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