Three compression techniques are available for compressed format data sets. They are DBB-based compression, tailored compression, and zEnterprise® data compression (zEDC). These techniques determine the method used to derive a compression dictionary for the data sets:

• DBB-based compression (also referred to as GENERIC). With DBB-based compression (the original form of compression used with both sequential and VSAM KSDS compressed format data sets), the system selects a set of dictionary building blocks (DBBs), found in SYS1.DBBLIB, which best reflects the initial data written to the data set. The system can later reconstruct the dictionary by using the information in the dictionary token stored in the catalog.
• Tailored compression. With tailored compression, the system attempts to derive a compression dictionary tailored specifically to the initial data written to the data set. Once derived, the compression dictionary is stored in system blocks which are imbedded within the data set itself. An OPEN for input reconstructs the dictionary by reading in the system blocks containing the dictionary.

  This form of compression is not supported for VSAM KSDSs.

• zEnterprise data compression (zEDC). With zEDC compression, no separate dictionary needs to be created, as zEDC compression hides the dictionary in the data stream. A new dictionary starts in each compression unit. The system can decompress the segment as is.
  When creating a zEDC-compressed format data set, you can specify either "zEDC Required" or "zEDC Preferred," which indicates how the system should proceed if the zEDC function cannot be used for the data set being created, as follows:

  • "zEDC Required" indicates that the system should fail the allocation request if the zEDC function is not supported by the system (regardless if the zEDC feature is installed), or if the minimum allocation amount requirement is not met.
  • "zEDC Preferred" indicates that the system should not fail the allocation request, but rather create either a tailored compressed data set if the zEDC function is not supported by the system (regardless if the zEDC feature is installed), or a non-compressed extended format data set if the minimum allocation amount requirement is not met.

  For more information on zEDC compression, see z/OS MVS Programming: Callable Services for High-Level Languages.

  

The form of compression the system is to use for newly created compressed format data sets can be specified at either or both the data set level and at the installation level. At the data set level, the storage administrator can specify T (for tailored), G (for generic), ZR (for "zEDC Required"), or ZP (for "zEDC Preferred") on the COMPACTION option in the data class. When the data class is not specified at the data set level (i.e. when the COMPACTION option is specified as Y), it is based on the COMPRESS(TAILORED|GENERIC|ZEDC_R|ZEDC_P) parameter found in the IGDSMSxx member of SYS1.PARMLIB. If the data class specifies the compression form, this takes precedence over that which is specified in SYS1.PARMLIB. COMPRESS(GENERIC) refers to generic DBB-based compression. This is the default. COMPRESS(TAILORED) refers to tailored compression, COMPRESS(ZEDC_R) refers to "zEDC Required" compression, and COMPRESS(ZEDC_P) refers to "zEDC Preferred" compression. When this member is activated using SET SMS=xx or IPL, new compressed format data sets are created in the form specified. The COMPRESS parameter in PARMLIB is ignored for VSAM KSDSs. For a complete description of this parameter see z/OS DFSMSdfp Storage Administration.