Concepts and Facilities › Special Considerations

Special Considerations

Not every VSAM data set is a good candidate for compression. An ESDS cluster should be compressed only under certain processing conditions. If an ESDS is compressed and the programs accessing it read a record, update the record and then write the record out, the ESDS is probably not a good candidate for compression because a record in an ESDS, once written, cannot change in size. Any request to change size is failed by VSAM record management. If any of the data in the record is changed, the record length probably changes too, unless the data being changed is defined as noncompressible. However, a read-only ESDS data set or an ESDS where data is only added to the end of the data set is a good candidate for compression.

Due to the nature of sequential access, and because EXCP access cannot be allowed for SORT, high overhead is unavoidable in processing compressed physical sequential data sets. As in the case of a VSAM ESDS, updates in place cannot be supported. If you need to update, or if the data set is read a great deal, compression cannot justify the overhead. Archival tapes and other large data sets which are seldom read are ideal selections.

Use the following guidelines when selecting VSAM data sets for compression:

• Random primary access is preferred, as is the normal case in an online operation.
• Sequential primary access can be used, but must be carefully considered. The runtime can be unacceptable, unless the file is seldom accessed in its compressed format.
• Files with very short records, less than 20 bytes of compressible area, often cost more in compression and expansion overhead than can be justified by the amount of DASD space saved.