Db2 for z/OS Version 13 introduces a significant improvement related to compression and
log processing:
the ability to store Huffman-based compression dictionaries in the Compression Dictionary
Data Set (CDDS).
This enhancement, delivered through APAR PH61310 and available from Function Level
500 onward, strengthens
replication workflows, log-based processing, and continuous-availability environments.
CDDS is a VSAM key‑sequenced data set used to store expansion dictionaries that Db2
needs to decompress
compressed log records. Previously, CDDS supported only fixed-length dictionaries.
Environments using
Huffman dictionaries—favored for their better compression efficiency—were unable to rely
on CDDS for
dictionary storage, creating challenges for replication and log‑reading technologies.
With this enhancement, Db2 now supports storing Huffman-type compression dictionaries
in CDDS. This allows
log readers, replication engines, and high-availability proxy configurations to access the
correct
dictionary when decompressing log records, even for workloads using Huffman
compression.
Key benefits include:
• Improved replication reliability for compressed table spaces
• Simplified log handling, especially in GDPS Continuous Availability or Active‑Active
environments
• Better administrative control with centralized dictionary storage
• Ability to use more efficient compression without operational trade-offs
To enable the enhancement, apply the PH61310 PTF and ensure the subsystem is running
at Function Level 500
or higher. For existing table spaces, a REORG, LOAD REPLACE, or other dictionary‑building
utility may be
required to generate Huffman dictionaries eligible for CDDS storage.
This enhancement aligns compression efficiency with modern replication needs, offering
improved
availability and operational stability for Db2 environments.