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
-A is ignored when
- 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.
- Indicates that special is a file system image,
rather than a device name or file system mount point.
special will be accessed ‘as-is’.
- This specifies the expected average file size.
- This specifies the expected number of files per directory.
- 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
- 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.