TUNEFS(8)

NAME
tunefs - tune up an existing file system

SYNOPSIS
tunefs [-A] [-a maxcontig] [-d rotdelay] [-e maxbpg] [-m minfree] [-p] [-o optimize_preference] [special | filesys]

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. Specifying 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
    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 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 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.

-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. 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  This option shows a summary of what the current tuneable 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, 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.