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OpenDarwin 7.2.1 - man page for tunefs (opendarwin section 8)

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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] [-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 updated in all the alternate superblocks instead of just the stan-
     dard superblock.  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).  The default value is one, since most
	     device drivers require an interrupt per disk transfer.  Device drivers that can
	     chain several buffers together in a single transfer should set this to the maximum
	     chain length.

     -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 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 subse-
	     quently 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 exclu-
	     sively large files, this parameter should be set higher.

     -m minfree
	     This value specifies the percentage of space held back from normal users; the mini-
	     mum free space threshold.	The default value used is 10%.	This value can be set to
	     zero, however up to a factor of three in throughput will be lost over the perfor-
	     mance obtained at a 10% threshold.  Note that if the value is raised above the cur-
	     rent usage level, users will be unable to allocate files until enough files have
	     been deleted to get under the higher threshold.

     -o optimize_preference
	     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.  If the value of min-
	     free (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 problematical, 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 dis-
     mounted 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		   May 3, 1995			4.2 Berkeley Distribution
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