In order to better utilize the capacity of backup volumes, HSMS provides the option of compressing the data before it is written to a save file. The COMPRESS-FILES operand in the HSMS statements ARCHIVE-FILES, BACKUP-FILES, BACKUP-FILE-VERSIONS, EXPORT-FILES and MIGRATE-FILES tells HSMS to start software compression.
The archive owner can use the archive default setting to define the default value for the appropriate HSMS statements and also determine that save versions are compressed only when written to storage level S1 but not when written to S2 (COMPRESS=*S1-ONLY).
No entry need be made when reading the data, e.g. during restore. HSMS automatically decompresses compressed backups.
The degree of compression depends on the saved files, e.g. text files are more heavily compressed than are load modules. Although compression increases load on the CPU, it also reduces input/output operations and the number of backup volumes. Therefore, compression does not save time during backup, but rather reduces the number of volumes required for backup.
Modern MTC devices already perform compression using their hardware. In this case HSMS does not perform software-based compression and ignores instructions to do so in the COMPRESS operand.