Requirements for Compression

The following requirements apply for compression of extended format data sets:

The data set must already have met the requirements for extended format.
Compression Management Services requires the data set to have a primary allocation of at least five megabytes in order to be allocated as a compressed data set.
Compression Management Services requires the data set to have a primary allocation of eight megabytes if no secondary allocation is specified, in order to be allocated as a compressed data set.
Compression Management Services requires that the data set have a minimum record length of forty bytes, not including the key offset or key length.

For VSAM KSDS, the CMS requirement to have a primary allocation of five megabytes is due to the amount of sampling needed to develop a dictionary token. The five MB allocation must be for the data component only. If the amount is specified at the cluster level, then the index component will use a portion of the 5 megabyte allocation. This will result in the data set not meeting the CMS space requirement and the data set will not be eligible for compression processing. This also true for the eight megabyte primary allocation, if no secondary is specified.

The CISIZE for a nonspanned compressed KSDS must be at least 10 bytes larger than the maximum record length.

The VSAM KSDS data set is not eligible for compression because the CISIZE is not large enough to contain the data record's key field. When a data record spans CIs, the record's key field must be contained within the first CI. The CISIZE for a spanned compressed KSDS must be at least fifteen bytes larger than the key field.