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

NAME
     tunefs -- tune up an existing file	system

SYNOPSIS
     tunefs [-A] [-a maxcontig]	[-d rotdelay] [-e maxbpg] [-m minfree]
	    [-n	enable | disable] [-o optimize_preference] [-p]
	    [special | filesystem]

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	     The file system 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
	     file systems with exclusively large files,	this parameter should
	     be	set higher.

     -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. An unmounted filesystem is required.

     -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. 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
	     in	the dumpfs(8) manual page.

SEE ALSO
     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).

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      December	11, 1993     4.2 Berkeley Distribution

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | BUGS | HISTORY

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