Data Compression

When deciding whether to compress data, consider the following guidelines and rules:

Compress when an existing data set is approaching the 4 gigabyte VSAM size limit or when you have capacity constraints
Only SMS-managed data is eligible for compression
The data set must be an extended format key-sequenced data set
Control interval access is not permitted
Compression could require excessive amounts of storage when processed in locate mode (OPTCD=LOC)
The GSR option is not permitted for compressed data sets
Compression processing can negatively impact CPU and elapsed times.

You can convert an application to compression processing if the application uses data that can be highly compressible based on the structure or type of data. One consideration could be the length of the data records:

The records can be large relative to the size of a control interval.
Smaller control interval sizes can be desirable because of the random structure of the data.
When a smaller control interval size is used without compressing data records, the length of the records can require a spanned data set. Records placed in a spanned data set are less likely to span control intervals when compression is used. The result could improve performance when VSAM processes the data because the amount of input/output required to GET or PUT the record is reduced.

Any program other than DFSMSdss, REPRO, and any other physical data copy/move program that does direct input/output to DASD for data sets which have data in compressed format can compromise data integrity. These programs must be modified to access the data using VSAM keyed access to permit expansion of compressed data.