Testing the Effects of Changing Values for Extended Compression Parameters

Before you change the default values of the extended compression parameters when using the DGASACMP utility, search on Improving Performance in the IBM® Connect:Direct® for z/OS® Administration Guide and review the benchmark figures showing different test results. You can also use this utility offline to determine the benefits of changing the default values in a test environment before actually using compression on live data in your production environment.

Although extended compression using ZLIB is available on a global basis using the ECZ initialization parameters and on a Process basis using the EXT parameters in the COPY statement, performing compression online consumes significant CPU resources. The DGASACMP utility produces a report (see the sample report in DGASACMP Output), which shows how much the data was read, written, compressed, and how long it took to compress. Another report is produced when you use DGASACMP to decompress data and restore it to its original format.

To use DGASACMP to test compression, follow this procedure:

  1. Prepare a file containing the typical type of data you transfer and the average amount of data involved in a transfer.
  2. Transfer the file without using compression and record the time it takes for the transfer to complete.
  3. To test the results of compressing and decompressing the file, run DGASACMP three times on the test file using the following settings. After each compression run, run DGASACMP to decompress the file. Keep the output reports for each test to compare results.
    • CMP=1,WIN=13,MEM=4 (the default settings)
    • CMP=1,WIN=1,MEM=14)
    • CMP=8,WIN=1,MEM=14)
  4. To determine if it is worthwhile to use compression and what extended compression parameter values are most beneficial for your environment, examine the test results taking the following factors into consideration:
    • Amount of CPU and elapsed time it takes to compress and decompress the data
    • Amount of time it takes to send the data
    • Virtual memory space used to maintain the ZLIB internal control blocks
    • Virtual memory space used to allocate the compression window or history buffer
    • Type of data being sent, including its compressibility