MirBSD manpage: tunefs(8)

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


     tunefs - tune up an existing filesystem


     tunefs [-'] [-a maxcontig] [-d rotdelay] [-e maxbpg] [-f avgfilesize]
            [-m minfree] [-n avgfpdir] [-o optimize_preference]
            [special | filesys]


     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. Specifying
             this option will cause all backups to be modified as well as the
             primary super-block. This is potentially dangerous - use with

     -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 depends on the block size (4 for 16KB blocks, 8 for
             8KB blocks and 16 for 4KB blocks). Most device drivers can chain
             several buffers together in a single transfer. For optimal per-
             formance, the value of maxcontig should be chosen based on the
             maximum chain length supported by the device driver.

     -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 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 avgfilesize
             Specifies the expected average file size in bytes. This value
             could be used for various optimizations, but for now it is only
             used together with avgfpdir to optimize the directory allocation
             policy. To take effect, both avgfpdir and avgfilesize must be
             greater than zero. (Also see avgfpdir.)

     -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, a factor of
             up to three in throughput will be lost over the performance ob-
             tained 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

     -n avgfpdir
             Specifies the expected average number of files per directory in
             the filesystem. This value is used only if both avgfilesize and
             avgfpdir are greater than zero. It is used to limit number of
             directories which may be allocated one after another in the same
             cylinder group without intervention by regular files. This value
             does not affect most filesystem operations but is useful for ap-
             plications which at first create a directory structure and then
             populate with files later. (Also see avgfilesize.)

     -o optimize_preference
             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. If the value of minfree (see above) is less than 5%,
             then the filesystem 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 problematical, and
             the filesystem can be optimized for time.

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


     fs(5), dumpfs(8), growfs(8), newfs(8)

     M. McKusick, W. Joy, S. Leffler, and R. Fabry, "A Fast Filesystem 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 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.

MirBSD #10-current            December 11, 1993                              1

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