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 lay-out layout
out 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 stan-dard standard
dard superblock. If this option is not used, then use of a backup superblock by fsck(8) will lose any-thing anything
thing 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 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.
-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 allocating blocks, or it can attempt
to minimize the space fragmentation on the disk. Optimization for space has much higher over-head overhead
head for file writes. If the value of minfree (see above) is less than 10%, then the file sys-tem system
tem 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 sys-tem system
tem 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
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