Using the Basic Components › Virtual Volume Compression

Virtual Volume Compression

Virtual Volume compression lets you simulate Improved Data Recording Capability (IDRC) for CA Vtape Virtual Volumes by compressing the data as soon as the Virtual Device Engine receives it from the application and prior to writing it to a cache LDS.

There are multiple advantages to compression:

• Compressed Virtual Volumes occupy less DASD buffer space.
• The effective increase in Virtual Volume capacity allows CA Vtape to manage more application data.
• Compressed data requires fewer I/Os to read or write at the expense of some additional CPU overhead needed to perform the actual compression or expansion of that data.
• The Backstore Engine and the RECYCLE function do not expand or compress the copied data. Their runtimes and resource consumption are reduced correspondingly by the percentage of compression achieved.

Note: The additional CPU overhead required as well as the compression rate achieved is a function of the data. While it was common during our own benchmarks to see 2-to-1 compression rates, you may not get the same amount of compression.

The CA Vtape compression routines use a combination of techniques including the run length limited (RLL) processing facility, the hardware compression facility as described in the IBM publication Enterprise Systems Architecture/390 Data Compression, SA22-7208, and LZ compression through hardware instructions, software instructions, or both. When Virtual Volume Compression is enabled, the Virtual Device Engine periodically analyzes the data written to a Virtual Volume to dynamically select the most efficient compression technique.