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] [-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.
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 enable | disable
Turn on/off the administrative ACL enable flag.
-e maxbpg
Indicate 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.
-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
performance:
· 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 soft updates.
-o space | time
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 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), 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.