tunefs is designed to change the dynamic parameters of a file system which affect the layout policies. When using tunefs with filesystem, filesystem must be in /etc/vfstab. The parameters which are to be changed are indicated by the options given below.

Generally, one should optimize for time unless the file system is over 90% full.

The following options are supported:

-a maxcontig

Specifies the maximum number of contiguous logical blocks that will be laid out before forcing a rotational delay (see -d). The default value is determined from the disk drive's maximum transfer rate.

UFS supports no more than 1048576 byte (1MB) runs. With a logical block size of 4096, this gives a maxcontig limit of 256. With 8192, the limit is 128.

-d rotdelay

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

Indicates the maximum number of contiguous logical 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 approximately one quarter of the total contiguous logical 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

Specifies the percentage of space held back from normal users; the minimum free space threshold. This value can be set to 0; however, up to a factor of three in throughput will be lost over the performance obtained at a 10% threshold. Note: 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]

Changes optimization strategy for the file system.

space: conserves space

time: attempt to organize file layout to minimize access time

See largefile(5) for the description of the behavior of tunefs when encountering files greater than or equal to 2 Gbyte ( 2³¹ bytes).

See attributes(5) for descriptions of the following attributes:

mkfs(1M), attributes(5), largefile(5)