GPFS: Administration and Programming Reference - IRA Home

See the “mmdf Command” on page 165 for complete usage information.
Querying and reducing file system fragmentation
Disk fragmentation within a file system is an unavoidable condition. When a file is closed after it has been
written to, the last logical block of data is reduced to the actual number of subblocks required, thus
creating a fragmented block. In order to write to a file system, free full blocks of disk space are required.
Due to fragmentation, it is entirely possible to have the situation where the file system is not full, but an
insufficient number of free full blocks are available to write to the file system. Replication can also cause
the copy of the fragment to be distributed among disks in different failure groups. The mmdefragfs
command can be used to query the current fragmented state of the file system and reduce the
fragmentation of the file system.
In order to reduce the fragmentation of a file system, the mmdefragfs command migrates fragments to
free space in another fragmented disk block of sufficient space, thus creating a free full block. There is no
requirement to have a free full block in order to run the mmdefragfs command. The execution time of the
mmdefragfs command depends on the size and allocation pattern of the file system. For a file system
with a large number of disks, the mmdefragfs command will run through several iterations of its algorithm,
each iteration compressing a different set of disks. Execution time is also dependent on how fragmented
the file system is. The less fragmented a file system, the shorter time for the mmdefragfs command to
execute.
The fragmentation of a file system can be reduced on all disks which are not suspended or stopped. If a
disk is suspended or stopped, the state of the disk, not the utilization information, will be displayed as
output for the mmdefragfs command.
The mmdefragfs command can be run on both a mounted or an unmounted file system, but achieves
best results on an unmounted file system. Running the command on a mounted file system can cause
conflicting allocation information and consequent retries to find a new free subblock of the correct size to
store the fragment in.
If the mmdefragfs command is issued on a file that is locked by SANergy, the file is not de-fragmented.
Querying file system fragmentation
To query the current status of the amount of fragmentation for a file system, specify the file system name
along with the -i option on the mmdefragfs command. For example, to display the current fragmentation
information for file system fs0, enter:
mmdefragfs fs0 -i
The system displays information similar to:
"fs0" 10304 inodes: 457 allocated / 9847 free
free subblk free
disk disk size in full subblk in % %
name in nSubblk blocks fragments free blk blk util
--------------- --------- --------- --------- -------- -------
gpfs68nsd 4390912 4270112 551 97.249 99.544
gpfs69nsd 4390912 4271360 490 97.277 99.590
--------- --------- --------- -------
(total) 8781824 8541472 1041 99.567
See the “mmdefragfs Command” on page 146 for complete usage information.
22 GPFS: Administration and Programming Reference