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 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 -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 sev-
eral 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 cylin-
der 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 cylin-
der 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.
-m minfree
Specify 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. 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 space | time
The file system can either try to minimize the time spent allo-
cating blocks, or it can attempt to minimize the space fragmenta-
tion on the disk. Optimization for space has much higher over-
head for file writes. 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.
-s avgfpdir
Specify the expected number of files per directory.
-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.
At least one of the above flags is required.
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.
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
Mac OS X 10.6 - Generated Thu Sep 17 20:26:28 CDT 2009