Compressed image copy

A compressed image copy has the format of database records that are compressed by one of the IMS HP Image Copy compression routines. The advantage of this format is that it reduces allocation space for output image copy.

The IMS HP Image Copy functions support data sets that are in compressed format. This feature enables you to reduce the number of tapes that are needed to record the image copies.

The Image Copy function generates image copies in compressed format. Once image copies are compressed, you can decompress them with the Recovery function for input to a database recovery job or with the Create Image Copy function for input to create new copy data sets.

You can create image copy data sets in compressed image copy format by using the following image copy options of IMS HP Image Copy:

  • The standard image copy process
  • The FASTIC DUMP process of Advanced Image Copy Services
  • The FASTIC FDUMP process of Advanced Image Copy Services
  • The FASTIC COPY process of Advanced Image Copy Services and the OFFLDTYPE=S option, to create secondary image copies in compressed image copy format
  • IMS compatible JCL

You can request to compress image copies by specifying the COMP keyword parameter and the COMPRTN keyword parameter in the ICEIN control statement.

Compressed image copy data sets can be created by IMS HP Image Copy and can be used for recovery with IMS HP Image Copy and IMS Database Recovery Facility.

IMS HP Image Copy provides the following compression routines:

FABJCMP1
Runs repeated character compression. Free space can also be compressed.
FABJCMP2
Runs only free space compression. Segmented data is not compressed.
FABJCMP3
Compresses repeating-characters in segment data by the z/OS® program compression method. Free space might also be compressed.
FABJCMP4
Compresses data set records by the z/OS program compression method. The segment data and free space are not distinguished.
Attention: If the database is a DEDB that has SDEP segments, use FABJCMP4. Other compression routines might damage the SDEP segments.

The following factors are important in selecting a compression method:

  • The amount of CPU time used.
  • The compression ratio.
  • IDRC capability on tape subsystem.
  • For DEDBs that have SDEP defined, you must use FABJCMP4.

Generally, FABJCMP1, FABJCMP3, and FABJCMP4 use more CPU time than FABJCMP2, but FABJCMP3 achieves better compression and uses less CPU time than FABJCMP1. FABJCMP4 uses less CPU time than FABJCMP3, but FABJCMP3 achieves better compression than FABJCMP4.

Data compression for IDRC enabled tape drives

A magnetic tape subsystem that uses tape cartridges and has the Improved Data Recording Capability (IDRC) can record data in compacted format. The typical compaction ratio is 2 to 3 depending on the data that is being compressed.

When the IDRC is active on the native IDRC enabled tape drive, software compression is unnecessary; it uses CPU cycles without providing benefit. To prevent unnecessary use of CPU cycles, IMS HP Image Copy behaves as follows for IDRC enabled tape devices:
  • When the compression routine is not FABJCMP2, IMS HP Image Copy does not compress database records. Data is compressed by IDRC while it is transmitted to the tape drive. When IDRC is active on the tape device specified for the primary image copy data set, the compression routine is ignored for the other image copies that are taken from the same database data set.
  • When the compression routine is FABJCMP2, IMS HP Image Copy uses FABJCMP2 to compress database records. Data is then compressed by IDRC while it is transmitted to the tape drive.
By specifying the COMPMODE keyword, you can deactivate IDRC and use a compression routine instead. Because IDRC is a feature provided by the hardware, a large number of uncompressed database records go through the channel and could lead to a system-wide slowdown. IMS HP Image Copy optimizes I/O buffers for output image copy data sets and can improve the speed of the image copy process.

Use of IDRC does not require a change to software compression parameters for your image copies jobs.