When you enter the DEFRAG command, DFSMSdss relocates data set extents on a DASD volume to reduce or eliminate free space fragmentation. You can specify which data sets, if any, should be excluded from relocation. When the DEFRAG operation completes, a summary report is created to show the before and after statistics of the volume. The report includes the fragmentation index, the size of the largest free space extent, and so on.

The amount of time it takes for a DEFRAG operation to complete depends on the size and fragmentation of the volume being processed. In general, larger or more fragmented volumes take longer to process.

Before using DEFRAG on a volume, you might want to use the CONSOLIDATE command to consolidate the extents of any multiple extent data sets on the volume.