Compression and fragmentation

The Journaled File System supports fragmented and compressed file systems as a means of saving disk space.

On average, data compression saves disk space by about a factor of two. However, the fragmentation and compression might incur a performance loss associated with increased allocation activity. For a description of how compression and fragmentation might affect performance, see Logical volume and disk I/O performance.

To enhance performance, both JFS and Enhanced JFS allow for online defragmentation. The file system can be mounted and is accessible while the defragmentation process is underway.