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     tunefs - tune up an existing file system


     tunefs [-A] [-a enable | disable] [-e maxbpg] [-f avgfilesize]
            [-J enable | disable] [-L volname] [-l enable | disable]
            [-m minfree] [-n enable | disable] [-o space | time] [-p]
            [-s avgfpdir] special | filesystem


     The tunefs utility is designed to change the dynamic parameters of a file
     system which affect the layout policies.  The tunefs utility cannot be
     run on an active file system.  To change an active file system, it must
     be downgraded to read-only or unmounted.

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

     -A      The file system has several backups of the super-block.
             Specifying this option will cause all backups to be modified as
             well as the primary super-block.  This is potentially dangerous -
             use with caution.

     -a enable | disable
             Turn on/off the administrative ACL enable flag.

     -e maxbpg
             Indicate 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.

     -f avgfilesize
             Specify the expected average file size.

     -J enable | disable
             Turn on/off GJournal flag.

     -L volname
             Add/modify an optional file system volume label.

     -l enable | disable
             Turn on/off MAC multilabel flag.

     -m minfree
             Specify the percentage of space held back from normal users; the
             minimum free space threshold.  The default value used is 8%.
             Note that lowering the threshold can adversely affect

             ·   Settings of 5% and less force space optimization to always be
                 used which will greatly increase the overhead for file

             ·   The file system’s ability to avoid fragmentation will be
                 reduced when the total free space, including the reserve,
                 drops below 15%.  As free space approaches zero, throughput
                 can degrade by up to a factor of three over the performance
                 obtained at a 10% threshold.

             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 file system can either try to minimize the time spent
             allocating 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 automatically as the percent fragmentation changes on
             the file system.

     -p      Show a summary of what the current tunable settings are on the
             selected file system.  More detailed information can be obtained
             from the dumpfs(8) utility.

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

     At least one of the above flags is required.


     /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).


     The tunefs utility appeared in 4.2BSD.


     This utility should work on active 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 cannot tune a fish.