Intel AS/400 RISC Server Save Command Parameters that Affect Performance, Data Compression Dtacpr

15.2 Save Command Parameters that Affect Performance

Use Optimum Block Size (USEOPTBLK)

The USEOPTBLK parameter is used to send a larger block of data to backup devices that can take advantage of the larger block size. Every block of data that is sent has a certain amount of overhead that goes with it. This overhead includes block transfer time, IOA overhead, and backup device overhead. The block size does not change the IOA overhead and backup device overhead, but the number of blocks does. For example, sending 8 small blocks will result in 8 times as much IOA overhead and backup device overhead. This allows the actual transfer time of the data to become the gating factor. In this example, 8 software operations with 8 hardware operations essentially become 1 software operation with 1 hardware operation when USEOPTBLK(*YES) is specified. The usual results are significantly lower CPU utilization and the backup device will perform more efficiently.

Data Compression (DTACPR)

Data compression is the ability to compress strings of identical characters and mark the beginning of the compressed string with a control byte. Strings of blanks from 2 to 63 bytes are compressed to a single byte. Strings of identical characters between 3 and 63 bytes are compressed to 2 bytes. If a string cannot be compressed a control character is still added which will actually expand the data. This parameter is usually used to conserve storage media. If the backup device does not support data compaction, the system i software can be used to compress the data. This situation can require a considerable amount of CPU.

Data Compaction (COMPACT)

Data compaction is the same concept as software compression but available at the hardware level. If you wish to use data compaction, the backup device you choose must support it.

IBM i 6.1 Performance Capabilities Reference - January/April 2008
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