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 standard 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 buf-
	     fers 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 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
	     This value specifies the percentage of space held back from normal users; the minimum 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 performance obtained 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 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 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 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 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					    May 3, 1995 					 4.2 Berkeley Distribution