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

NAME
     tunefs - tune up an existing file system

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

DESCRIPTION
     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
     below:

     -a maxcontig
	     This specifies the maximum number of contiguous blocks that will
	     be laid out before forcing a rotational delay (see -d below).
	     Device drivers that can chain several buffers together in a sin-
	     gle transfer should set this to the maximum chain length.	The
	     default value is the largest transfer size that a typical disk
	     controller for a given architecture can transfer at once.

     -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.

     -n soft_update_enabling
	     The soft update code allows most filesystem I/O to be done asyn-
	     chronously by reordering disk writes to ensure that the on-disk
	     copy of the filesystem is always consistent.  Use of this facili-
	     ty does not require any changes to the filesystem, so it can be
	     enabled or disabled any time that the filesystem is unmounted.
	     This facility is turned on by default.  It is turned on by using
	     -n enable; it is turned off by using -n disable.

     -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.

SEE ALSO
     fs(5),  dumpfs(8),	 fsck(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).

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, it must be mounted read-only.

     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 30, 1998				     2
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