Monday, August 25, 2014

DB2 for z/OS: Can a Pooled DBAT Hold Locks?

The answer to the question in the title of this post is, "No, it can't -- unless it can." How's that for a hook?

OK, a little background. A DBAT is a database access thread. It's the kind of thread that a network-attached application process uses to interact with a DB2 for z/OS system (such a process, also referred to as a DRDA requester, issues SQL statements that get to DB2 by way of the distributed data facility, aka DDF). When a transaction using a "regular" DBAT completes, the DBAT goes into a pool so that it can be used in the execution of another transaction (a high-performance DBAT does not go into the DBAT pool upon transaction completion; instead, it remains dedicated to the connection through which it was instantiated until it has been reused 200 times). 

Last month, a DB2 for z/OS DBA asked me if a pooled DBAT can hold locks. Why did he ask that question? Because of some wording he saw in the DB2 for z/OS Installation and Migration Guide. The text to which he referred, in the description of the ZPARM parameter POOLINAC, is as follows (emphasis added by me):

"A database access thread in the pool counts as an active thread against MAX
REMOTE ACTIVE and can hold locks."


That "and can hold locks" bit looked funny to me. How could a pooled DBAT hold locks when a prerequisite for going into the pool is a "clean commit" of the transaction using the DBAT -- something that implies, among other things, a release of all locks owned by the thread? [Again, we're talking about "regular" -- not high-performance -- DBATs here, and that means RELEASE(COMMIT) packages, and that means all locks are released by a clean commit.]

I ran this by a colleague in the DB2 for z/OS development organization, and he said that the "and can hold locks" wording in the Installation and Migration Guide is probably outdated -- a leftover from the days, relatively early in the life of DB2 for z/OS V6, when RELEASE(DEALLOCATE) was honored for packages executed via DBATs. Of course, with that behavior in effect, a transaction using a DBAT could end with a "clean commit" (i.e., with no "hanging resources" such as WITH HOLD cursors, not-dropped declared global temporary tables, and held LOB locators) -- thereby allowing the DBAT to be pooled -- and still the thread could hold table space locks because those are retained until thread deallocation when RELEASE(DEALLOCATE) is in effect for a package. In fact, this aspect of RELEASE(DEALLOCATE) is a key reason why a DB2 V6 APAR was created to make DB2 stop honoring that bind option for packages executed via DBATs: DBATs can last a long time, and having table space locks retained for such long periods of time could lead to contention problems in the system. With the fix to this DB2 V6 APAR applied, DB2 treated a RELEASE(DEALLOCATE) package executed via a DBAT as though it had been bound with RELEASE(COMMIT).

So, I was right: a pooled DBAT can't hold locks, because only non-high-performance DBATs can be pooled, and non-high-performance DBATs are associated only with RELEASE(COMMIT) packages, and with a RELEASE(COMMIT) package you get a release of all locks when a transaction completes with a clean commit (something that has to happen before a DBAT can be pooled).

Here's the deal, though: I was right only briefly. Just a short time ago, a fix for DB2 APAR PI20352 came out, and with that fix applied in a DB2 10 or 11 for z/OS system, a pooled DBAT can hold locks. How so? Pretty simple: PI20352 allows a high-performance DBAT to be pooled, and a high-performance DBAT can hold locks (specifically, table space locks) after a transaction using the thread completes with a clean commit, because that's what happens for a RELEASE(DEALLOCATE) package, and a high-performance DBAT will be associated with at least one RELEASE(DEALLOCATE) package -- instantiation of a high-performance DBAT depends on a DBAT being used in the execution of such a package.

But why would you want a high-performance DBAT to be pooled? What's wrong with the original high-performance DBAT behavior, which eschews pooling in favor of staying dedicated to the instantiating connection for 200 units of work? Well, that behavior is fine when DDF-using applications stay connected to a DB2 for z/OS system for long periods of time, but it can be sub-optimal for environments in which applications stay connected to DB2 for shorter periods of time. That situation means more disconnect and reconnect activity. Prior to APAR PI20352, if an application connected to DB2 and executed a RELEASE(DEALLOCATE) package -- thereby instantiating a high-performance DBAT -- and then disconnected before the high-performance DBAT had been reused 200 times, the high-performance DBAT would have to be terminated because it couldn't be pooled. That would end up causing a good bit of DBAT create and terminate activity, and that is CPU overhead you'd rather avoid.

With the fix for APAR PI20352 applied, when an application using a high-performance DBAT disconnects from DB2 before the thread has been used for 200 units of work, the high-performance DBAT will be pooled rather than terminated. The pooled high-performance DBAT can then be used by any DDF connection needing a thread to process a request. Once it's used to service a request from a connection, the high-performance DBAT that had been pooled will do as high-performance DBATs do: it will stay dedicated to that connection so that it can be reused (a CPU efficiency booster when paired with RELEASE(DEALLOCATE) packages). Note that a particular high-performance DBAT can be used by a maximum of 200 units of work. That re-use limit is not reset when a thread is pooled, so if a high performance DBAT has been used 50 times and the associated application disconnects from DB2, resulting in the pooling of the DBAT, it can be reused 150 more times before being terminated (that termination, which is normal for high-performance DBATs, is done to free up resources allocated to the DBAT).

This is good stuff, but you might be thinking of a potential fly in the ointment: what if POOLINAC (the ZPARM that specifies the amount of time that a pooled DBAT can sit around waiting to be used -- exceeding that time results in DBAT termination) at a DB2 site has been set to zero, which would disable termination of pooled DBATs due to inactivity? Couldn't that lead to pooled high-performance DBATs sitting around indefinitely, holding table space locks (and holding RELEASE(DEALLOCATE) packages)? As it turns out, this is not something about which you need to be concerned. The DB2 developers anticipated that situation, and when the fix for APAR PI20352 is applied, a POOLINAC setting of 0 will be changed to the default POOLINAC value of 120 seconds.

So, to return to the question in this post's title: can a pooled DBAT hold locks? The answer depends on whether or not you've applied to fix to APAR PI20352 to your system. If you haven't applied that fix, the answer to the question is, "no." If you have applied that fix, the answer is, "yes," but it's a qualified yes. Only a pooled high-performance DBAT can hold locks; and these will only be table space-level locks (which are almost always of the non-exclusive "intent" variety), versus page or row locks. Keep in mind that it's good that a high-performance DBAT can be pooled if the application using it disconnects from DB2 before the thread has been used by 200 transactions (this cuts down on DBAT termination and creation activity that would otherwise occur). And remember the POOLINAC reset that will change a specification of 0 to the default of 120 seconds -- a nice enhancement that will keep people from shooting themselves in the foot.

Pooled DBATs have long been a mainstay of DB2 client-server efficiency and scalability. With APAR PI20352, high-performance DBATs get their invitation to the DBAT pool party. Seems appropriate that this was a summertime APAR (speaking from a northern hemisphere perspective).

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