utility is designed to change the dynamic parameters of a file system
which affect the layout policies.
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
The file system has several backups of the super-block.
this option will cause all backups to be modified as well as the
This is potentially dangerous - use with caution.
-a enable | disable
Turn on/off the administrative ACL enable flag.
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.
Specify the expected average file size.
-J enable | disable
Turn on/off GJournal flag.
Add/modify an optional file system volume label.
-l enable | disable
Turn on/off MAC multilabel flag.
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
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 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.
Show a summary of what the current tunable settings
are on the selected file system.
More detailed information can be
obtained from the
Specify the expected number of files per directory.
At least one of the above flags is required.
read this to determine the device file for a
specified mount point.