— tune up
an existing file system
[-o optimize_preference] special
is designed to change the dynamic parameters
of a file system which affect the layout policies.
The options are as follows:
- Cause 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.
-A is ignored when
-N is specified.
- 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
- Indicates that special is
a file system image, rather than a device name or file system mount point.
special will be accessed
- This specifies the expected average file size.
- This specifies the expected number of files per
- This value specifies the percentage of space held back from
normal users; the minimum free space threshold. The default value is set
during creation of the filesystem; see
newfs(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 5% threshold. 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
- Display all the settable options (after any changes from
the tuning options) but do not cause any of them to be changed.
- 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. If the value of minfree (see
above) is less than 5%, 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 5%,
fragmentation is unlikely to be problematical, and the file system can be
optimized for time.
optimize_preference can be specified as
W. Joy, S. Leffler, and
R. Fabry, A Fast File System for
UNIX, ACM Transactions on Computer Systems 2,
3, pp 181-197,
command appeared in
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 unmounted 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.