Understanding the Basic Components › Virtual Volume Compression

Virtual Volume Compression

Virtual Volume compression lets you simulate Improved Data Recording Capability (IDRC) for the CA Vtape Virtual Volumes. Achieve IDRC simulation by compressing the data:

• when the Virtual Device Engine receives it from the application, and
• prior to writing the data to a file on the Storage Gateway Appliance

Compression has multiple advantages:

• Compressed Virtual Volumes occupy less DASD 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. The expense is CPU overhead to perform the actual data compression.

Note: The CPU overhead and the compression rate are a function of the data. While our benchmark testing commonly indicated a 2-to-1 compression rate, your rate can vary.

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
• LZ compression through hardware instructions, software instructions, or both

When Virtual Volume Compression is enabled, the Virtual Device Engine periodically analyzes the data that is written to a Virtual Volume to select the most efficient compression technique dynamically.