Choosing compression type

You can use compression and other capabilities to improve save performance and also use less media for your save operation.

Data compression compresses data on the media when you perform the save operations. Data decompression reconstructs data when you perform a restore operation. The system ensures that information saved can be reconstructed exactly. No data is lost as a result of compression and decompression.

The two main types of compression are hardware compression and software compression. Most tape media devices use hardware compression, which is normally faster than software compression. Software compression takes considerable processing unit resources and might increase your save and restore time.

In addition to data compression, you can use compaction and optimum block size features to streamline your save operation. These features are available through parameters on all save commands:

Data Compression (DTACPR)
Data Compaction (COMPACT)
Use Optimum Block Size (USEOPTBLK)

You can see examples of the parameter values in the SAVSYS command description.

If you save to save files or optical media, you also have three choices available for software compression: low, medium, and high. If you choose a higher form of compression, your save will take longer, but the resulting save data will usually be smaller. The following choices are available on the Data Compression (DTACPR) parameter of the save commands and through the Save Object (QsrSave) and Save Object List (QSRSAVO) APIs:

Low: This is the default form of compression for save files and optical media. Low compression is usually faster than medium or high compression. The compressed data is usually larger than if medium or high compression is used.
Medium: This is the default form of compression for optical-DVD media. Medium compression is usually slower than low compression but faster than high compression. The compressed data is usually smaller than if low compression is used and larger than if high compression is used.
High: This form of compression is meant to be used when maximum compression is desired. High compression is usually noticeably slower than low and medium compression. The compressed data is usually smaller than if low or medium compression is used.

You can also find more information about compression, compaction, and optimum block size in System i® Performance Capabilities Reference Link to PDF . The "Saves and restores using save files" chapter contains information about the compression ratios for *LOW, *MEDIUM, and *HIGH compression.