Overview - compression and maintenance procedures
This chapter describes the steps for performing the following procedures:
- Compressing full-function databases and DEDBs
- Changing compression techniques after your data are compressed
- Performing maintenance on DPICDS and DPITBL data sets
For the processes described, optional steps are presented for running the Trial utility and registering the segments being compressed. If you perform only the required steps (that is, you do not run the Trial utility and register your segments), the DATA PACKER/IMS defaults are used. These defaults are listed in the following table:
Table 1. DATA PACKER/IMS defaults
Default item | Default value |
|---|---|
Compression technique | Static Huffman |
Information and error message destination |
|
Statistics | Stored in DPICDS |
Static Huffman is the DATA PACKER/IMS default compression technique. If you do not register your segments and specify compression options, all segments are compressed using Static Huffman compression, statistics are stored in the DATA PACKER/IMS control data set (DPICDS), and messages are routed to the master terminal operator console (MTO) and to the default system console (WTO).
DATA PACKER/IMS compresses all non-key data after the segment key in database segments. For fixed-length segments, the position of the key field after compression is offset by a two-byte length field because compressed segments are stored as variable-length segments. For variable-length segments, the length is already in the segment, so the actual key position is not changed by compression.
DATA PACKER/IMS compresses a segment if it has 20 or more compressible bytes. Compressible bytes are those bytes after the key. For example, a 30-byte segment with a 12-byte key beginning in position one is not compressed because it contains only 18 compressible bytes.