Lock structure size

The coupling facility lock structure contains two parts: a lock entry table and a list of update locks.

The lock entry table is used to determine if inter-Db2 read/write interest exists on a particular hash class. Hash classes are resources that hash to a particular place in the lock table. The second part is a list of the update locks that are currently held (sometimes called a modify lock list or record list table). You can control the division of the lock structure storage between these two components by using the IRLMPROC or by using an IRLM MODIFY command. If you do not specify how the structure is to be split, IRLM attempts to divide it with a 1:1 ratio between lock table entry (LTE) and record list entry (RLE) storage.

The total size of the lock structure must be large enough to limit hash contention, which prevents performance problems. The lock structure must also be large enough to prevent failures that result from a lack of record table storage to write a MODIFY entry (RLE). Proper specification for the number of LTEs can help avoid hash contention.

IRLM reserves 10% of the record table entries for must complete functions (such as rollback or commit processing), so that a shortage of storage does not cause a member to fail. However, if storage runs short in the record table, there can be an impact on availability (transactions are terminated), response time, and throughput.

The z/OS® system can automatically alter the size of a lock structure in the coupling facility if it needs storage space. The ALLOWAUTOALT option in the CFRM policy specifies whether system-initiated automatic alters are to be allowed for a structure.

If no competing resource is using the coupling facility, consider specifying ALLOWAUTOALT(YES) to enable the lock structure to change as the workload grows. However, if you use this setting, other resources can force the IRLM lock structure size to decrease. This situation can cause an IRLM out-of-storage condition in the lock table and an abend.

If you need the lock structure to be stable, specify ALLOWAUTOALT(NO). If you do not specify ALLOWAUTOALT, the default value of NO is used.