SYNOPSIS

[-A ] [-a enable | disable ] [-e maxbpg ] [-f avgfilesize ] [-J enable | disable ] [-L volname ] [-l enable | disable ] [-m minfree ] [-N enable | disable ] [-n enable | disable ] [-o space | time ] [-p ] [-s avgfpdir ] special | filesystem

DESCRIPTION

The utility is designed to change the dynamic parameters of a file system which affect the layout policies. The 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 parameters which are to be changed are indicated by the flags given below:

-A

The file system has several backups of the super-block. Specifying this option will cause all backups to be modified as well as the primary super-block. This is potentially dangerous - use with caution.

-a enable | disable

Turn on/off the administrative POSIX.1e ACL enable flag.

-e maxbpg

Indicate 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.

-J enable | disable

Turn on/off gjournal flag.

-L volname

Add/modify an optional file system volume label.

-l enable | disable

Turn on/off MAC multilabel flag.

-m minfree

Specify the percentage of space held back from normal users; the minimum free space threshold. The default value used is 8%. Note that lowering the threshold can adversely affect performance:

• Settings of 5% and less force space optimization to always be used which will greatly increase the overhead for file writes.
• The file system's ability to avoid fragmentation will be reduced when the total free space, including the reserve, drops below 15%. As free space approaches zero, throughput can degrade by up to a factor of three 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.

-N enable | disable

Turn on/off the administrative NFSv4 ACL enable flag.

-n enable | disable

Turn on/off soft updates.

-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 overhead for file writes. The kernel normally changes the preference automatically as the percent fragmentation changes on the file system.

-p

Show a summary of what the current tunable settings are on the selected file system. More detailed information can be obtained from the dumpfs(8) utility.

-s avgfpdir

Specify the expected number of files per directory.

At least one of the above flags is required.

FILES

/etc/fstab

read this to determine the device file for a specified mount point.

HISTORY

The utility appeared in BSD 4.2