TUNEFS(8) BSD System Manager’s Manual TUNEFS(8)


tunefs — tune up an existing file system


tunefs [−AN] [−a maxcontig] [−d rotdelay] [−e maxbpg] [−m minfree] [−o optimize_preference] [−t trackskew] [special filesys]


The tunefs program is designed to change the dynamic parameters of a file system which affect the layout policies. 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 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.

−m minfree

This value specifies 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. 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 threshold.

−o optimize_preference

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 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.

−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.


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) .


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 dismounted 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.


The tunefs command appeared in 4.2BSD.

4.2 Berkeley Distribution May 3, 1995 4.2 Berkeley Distribution