Performing trial compression

You can run the Trial utility by using an image copy, an expanded unload file, or a database as input, and you can specify that all compression techniques be used for the trial.

You must run the Trial utility if you want to use Custom Huffman, Shared Table, or Custom Hardware Dictionary compression. During trial compression, the utility samples the segment data and creates the necessary registration, table, and dictionary records. You can register Custom Huffman records in the DPICDS only through the Trial utility, and if you want to use Shared Tables or Custom Hardware Dictionaries, you must run the Trial utility at least once to build the table or dictionary.

You should, however, always run the Trial utility before compressing your data to determine the best compression technique to use. The Trial Compression Analysis report shown in Trial-Compression-Analysis-report is produced for each segment type. The report indicates the compression percentage achieved for each compression technique selected. You can specify all compression methods and obtain a percentage for each technique.

You should also run the Trial utility if you want to use product features such as MSL, FPS, or FSS. If you specify MSL or FPS as an option for the Trial utility, the utility calculates a new average segment length based on the MSL or FPS specified. If you specify an MSL, for example, all segments that compress smaller than the MSL are set equal to the MSL and a new average segment length and MSL are calculated. When you register your compression options, you can specify the MSL calculated during execution of the Trial utility.

The Trial Compression Analysis report also indicates the average segment length. When specifying an FSS for your segments, you should know which segment types are added to the database most frequently and what the average length of those segments are. Choose a value for FSS that is close to the average length of your most frequently inserted or replaced segments. When you register your compression options, you can specify an FSS that approximates the average segment length indicated in the report.

You can run the Trial utility by using the online panels to build the JCL; the online procedures are explained in ISPF-Trial-compression. You can also use the batch procedures explained in Batch-Trial-compression.