tunefs(8)

NAME

tunefs - tune up an existing file system

SYNOPSIS

tunefs  [-A]  [-a  enable  |  disable]   [-e   maxbpg]   [-f
avgfilesize]
       [-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 parame

ters 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 sys

tem, 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 modi

fied as well as
the primary super-block. This is potentially dan

gerous - 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. Typical

ly 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 de

grading 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 else

where. For
file systems with exclusively large files, this pa

rameter should
be set higher.

-f avgfilesize

Specify the expected average file size.

-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 nor

mal users; the
minimum free space threshold. The default value

used is 8%.
Note that lowering the threshold can adversely af

fect performance:

+o Settings of 5% and less force space optimization

to always be

used which will greatly increase the overhead

for file
writes.

+o 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 lev

el, 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.

You can tune a file system, but you cannot tune a fish.

BSD May 18, 2002

BSD