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How Virtual Volume Compression Works

When compression is active, CA Vtape will always compress the first 5 MBs of every 50 MBs of data written to ta Virtual Volume. The first 5 MBs is always compressed to determine the best method to use for the remaining 45 MBs of data. If the compression rate falls below the MinimumCompressionRate setting, then compression is discontinued until the next 50 MB segment is written.

When the MinimumCompressionRate is set to a low value, a larger percentage of the Virtual Volumes will be compressed and more CPU will be used for compression. When the MinimumCompressionRate is set to a high value a smaller percentage of the Virtual Volumes will be compressed and less CPU will be used for compression.

Using compression reduces I/O and storage space at the cost of additional CPU. The compression routines in CA Vtape rely heavily on the use of certain hardware instructions to perform compression. The CPU expense for using these instructions varies based on the CPU hardware in your environment. In some instances, the additional CPU expense might not justify the use of compression.

MaximumCompressionCPU lets you control this additional CPU expense by controlling the percentage of data on which compression is performed.

Specifying a value of 100 would allow all data written to a Virtual Volume to be compressed, which would incur the highest additional CPU cost while producing the maximum amount of compression.

Specifying a value of 50 would only allow 50 percent of the data written to a Virtual Volume to be compressed. On average this would result in approximately 50 percent less compression, but also at 50 percent less CPU cost.

Specifying a value of 0 would result in no compression because no data would be subject to compression except for the first 5 MBs of each 50 MBs of data written.

A low MaximumCompressionCPU value can drop the actual compression rate below the MinimumCompressionRate specified, effectively turning off compression except for the 5 MBs of test compression for each 50 MBs of data written to a Virtual Volume.

If a zIIP Engine is installed and the zIIPExploitation and PercentRunOnzIIP parmlib attributes are set appropriately, CA Vtape depending on the availability of the zIIP engine, the amount of compression work performed on your general processors will be reduced, lowering the CPU cost of compression.