tunefs man page on FreeBSD

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

NAME
     tunefs — tune up an existing file system

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

DESCRIPTION
     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
     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 enable | disable
	     Turn on/off the administrative POSIX.1e ACL enable flag.

     -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.

     -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 perfor‐
	     mance:

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

	     ·	 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 the administrative NFSv4 ACL enable flag.

     -n enable | disable
	     Turn on/off soft updates.

     -o space | time
	     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 over‐
	     head 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.

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

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

HISTORY
     The tunefs utility appeared in 4.2BSD.

BUGS
     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.

BSD				August 13, 2007				   BSD
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