Friday, September 01, 2017

The Db2 12 for z/OS Blog Series - Part 16: Db2 Catalog Availability Improvements

IBM has improved the availability of accessing Db2 Catalog objects when maintenance is being run in Db2 12 for z/OS. This impacts access during CATMAINT and online REORG.

This change is largely being driven by dynamic SQL, which is more prevalent but can cause problems. When dynamic SQL statement is executed, Db2 must dynamically prepares the SQL to determine access paths in order to run it. During this dynamic SQL preparation process, Db2 acquires read claims on a handful of Db2 Catalog table spaces and their related indexes. Additionally, a DBD lock is acquired on the Db2 Catalog database. The DBD lock is needed to serialize catalog operations with CATMAINT and other DDL that may execute against the catalog, because CATMAINT might be making structural changes to the catalog.

Prior to Version 12, the DBD lock and the read claims were released at COMMIT points. All well and good, but for transactions issuing dynamic SQL but not committing frequently, CATMAINT and online REORG on the Db2 Catalog were blocked during that period period of time.

As of Db2 12, DBD locks on the Db2 Catalog and read claims against catalog objects are released as soon as PREPARE statement execution is complete. This will improve availability for CATMAINT and online REORG of Db2 Catalog objects.

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