DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

8.2.2 Conclusion
The purpose of the locking Protocol Level 2 enhancement is to avoid the cost of global
contention processing whenever possible. DB2 no longer needs to wait for global lock
contention processing to determine that a new parent IX or IS L-lock is compatible with
existing parent IX or IS L-locks which is reasonably common in a data sharing environment.
For our workload, our testing has shown that the new locking protocol dramatically reduces
both XES and false contention. This results in shorter lock and global contention suspension
times. We also observed less IRLM SRB time with an improved overall transaction
throughput. However, we see more synchronous XES requests to the coupling facility and
more lock structure traffic with slightly higher coupling facility utilization. These increases do
not appear to be excessive and should not impact overall performance.
The new locking protocol also reduces the performance gap between RELEASE(COMMIT)
and RELEASE(DEALLOCATE) for data sharing workloads.
Another consequence of this enhancement is that, since child L-lock propagation is no longer
dependent upon the parent L-lock, parent L-locks will no longer be held in retained state
following a system failure. This means, for example, that an IX L-lock will no longer be held as
a retained X-lock after a system failure. This can provide an important availability benefit in a
data sharing environment. Because there is no longer a retained X-lock on the page set most
of the data in the page set remains available to applications running on other members. Only
the pages (assuming page locking is used) with a retained X-lock will be unavailable.
With the change to the LOCKPART parameter behavior, you may see additional locks
acquired on individual partitions even though LOCKPART NO is specified.
When DB2 decides it must propagate its locks to the coupling facility for a given page set,
DB2 must collect and propagate all the locks it currently owns for that page set to the coupling
facility. This can cause some overhead, particularly when a page set is not used often enough
for lock propagation to occur all the time. Page set P-locks are long duration locks and tend to
be more static than L-locks, so the chances are higher that lock propagation will continue for
longer.
8.2.3 Recommendations
The new mapping in the coupling facility lock structure to support the Protocol 2 Level locking,
takes effect after the restart of the first member, after successful quiesce of all members in
the DB2 data sharing group. To enable this feature, a group-wide outage is required after you
have entered NFM. This obviously has some impact on availability.
However, we recommend you plan to schedule a group-wide restart of DB2 soon after you
enter NFM to enable the new locking protocol as soon as possible.
If you currently are in the practice of starting table spaces in RO to avoid locking contention in
a data sharing environment, you may wish to revise this strategy.
In data sharing, the recommendation for RELEASE(DEALLOCATE) (and thread reuse) to
reduce XES messaging for L-locks is no longer required. This is good news because using
RELEASE(DEALLOCATE):
► Can cause increased EDM pool consumption, because plans and packages stay allocated
longer in the EDM pool.
► May also cause availability concerns due to parent L-locks being held for longer. This can
potentially prevent DDL from running, or cause applications using the LOCK TABLE
statement and some utilities to fail.
Chapter 8. Data sharing enhancements 331