Introduction to disk compression

Disk compression is a technology that increases the apparent capacity of disk storage devices by encoding the data to take up less physical storage space on disk.

Disk compression is performed in the disk subsystem controller and does not affect the IBM® i processor. The compression of data is performed automatically on each write command. The decompression of data is performed automatically on each read command. With the exception of a performance impact, disk compression is transparent to applications. The performance of compressed disk drives is slower than the performance of non-compressed disk drives. This is due to the overhead of compression and decompression, and the variations in the length of the data that is written to disk.

Typically, data that is found on disk units has a wide range of access requirements. You can choose to move data that is accessed infrequently, or data that does not require high performance input/output (I/O) rates, to compressed disk units. Disk compression is intended to make infrequently accessed data available online at a lower cost. This storage alternative is positioned between non-compressed disk unit storage and optical or tape storage.

Compressed disks have the same disk subsystem availability options of device parity protection and mirrored protection as non-compressed disks. Disk compression is only supported in user auxiliary storage pools (ASPs).