Monitoring fragmentation


Monitoring fragmentation
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Fragmentation reduces performance and availability. Regular use of fsadm's fragmentation reporting and reorganization facilities is therefore advisable.

The easiest way to ensure that fragmentation does not become a problem is to schedule regular defragmentation runs using the cron command.

Defragmentation scheduling should range from weekly (for frequently used file systems) to monthly (for infrequently used file systems). Extent fragmentation should be monitored with fsadm command.

To determine the degree of fragmentation, use the following factors:

Percentage of free space in extents of less than 8 blocks in length
Percentage of free space in extents of less than 64 blocks in length
Percentage of free space in extents of length 64 blocks or greater

An unfragmented file system has the following characteristics:

Less than 1 percent of free space in extents of less than 8 blocks in length
Less than 5 percent of free space in extents of less than 64 blocks in length
More than 5 percent of the total file system size available as free extents in lengths of 64 or more blocks

A badly fragmented file system has one or more of the following characteristics:

Greater than 5 percent of free space in extents of less than 8 blocks in length
More than 50 percent of free space in extents of less than 64 blocks in length
Less than 5 percent of the total file system size available as free extents in lengths of 64 or more blocks

The optimal period for scheduling of extent reorganization runs can be determined by choosing a reasonable interval, scheduling fsadm runs at the initial interval, and running the extent fragmentation report feature of fsadm before and after the reorganization.

The "before" result is the degree of fragmentation prior to the reorganization. If the degree of fragmentation is approaching the figures for bad fragmentation, reduce the interval between fsadm runs. If the degree of fragmentation is low, increase the interval between fsadm runs.

The "after" result is an indication of how well the reorganizer has performed. The degree of fragmentation should be close to the characteristics of an unfragmented file system. If not, it may be a good idea to resize the file system; full file systems tend to fragment and are difficult to defragment. It is also possible that the reorganization is not being performed at a time during which the file system in question is relatively idle.

Directory reorganization is not nearly as critical as extent reorganization, but regular directory reorganization improves performance. It is advisable to schedule directory reorganization for file systems when the extent reorganization is scheduled. The following is a sample script that is run periodically at 3:00 A.M. from cron for a number of file systems:

  outfile=/usr/spool/fsadm/out.'/bin/date +'%m%d''
  for i in /home /home2 /project /db
  do
    /bin/echo "Reorganizing $i"
    /bin/timex fsadm -F vxfs -e -E -s $i
    /bin/timex fsadm -F vxfs -s -d -D $i
  done > $outfile 2>&1