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 upon 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 umounted 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.
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
--------------- --------- --------- --------- -------- -------
gpfs68n1 4390912 4270112 551 97.249 99.544
gpfs69n1 4390912 4271360 490 97.277 99.590
--------- --------- --------- -------
(total) 8781824 8541472 1041 99.567
See the mmdefragfs Command for complete usage information.
You can reduce the amount of fragmentation for a file system by issuing the mmdefragfs command, with or without a desired block utilization goal. For example, to reduce the amount of fragmentation for file system fs0 with a goal of 99% utilization, enter:
mmdefragfs fs0 -u 99
The system displays information similar to:
WARNING: "fs0" is mounted on 1 nodes(s) and in use on 1 node(s)
Start processing: iteration 1
Processing Pass 1 of 1
Disk Name gpfs57n7 gpfs60n8
6 % complete
12 % complete
17 % complete
22 % complete
27 % complete
32 % complete
38 % complete
43 % complete
49 % complete
56 % complete
63 % complete
70 % complete
75 % complete
81 % complete
87 % complete
92 % complete
98 % complete
free subblk free
disk in full subblk in % %
name blocks blk fragments free blk blk util
before after freed before after before after before after
--------------- ---------------- ------- ---------------- ------------ ------------
gpfs57n7 39424 40064 20 1347 589 68.75 69.87 92.48 96.59
gpfs60n8 50688 51456 24 1555 593 72.26 73.36 92.01 96.83
--------------------- ----------------
(total) 90112 91520 44 2902 1182
See the mmdefragfs Command for complete usage information.