Monday, June 02, 2014

Don't Neglect Your DB2 Rebind Strategy

We’re all busy. Frequently it can seem like you just got in to the office and already it is past quitting time! There is so much to do and so little time to do it all. And we all work more than 40 hours a week… these are some of the common complaints of the busy DBA.

And those are valid concerns, but it does not diminish the need to properly address DB2 database administration and performance management... with a special focus on proactive management. 

So please take a little bit of time to read about, and consider your organization's strategy for rebinding DB2 applications.

REBIND Strategy

One of the most important contributors to the on-going efficiency and health of your DB2 environment is proper management of DB2 access path changes. A thorough REBIND management process is a requirement for healthy DB2 applications.

But many shops do not do everything possible to keep access paths up-to-date with the current state of their data. Approaches vary, such as rebinding only when a new version of DB2 is installed, whenever PTFs are applied to DB2, or to rebind automatically after a regular period of time. Although these methods are workable, they are less than optimal. 

The worst approach though is the “if it ain’t broke don’t fix it” mentality. In other words, many DBA groups adopt “never REBIND unless you absolutely have to” as a firm policy. The biggest problem this creates is that it penalizes every program in your subsystem for fear of a few degraded access paths. This results in potentially many programs having sub-optimal performance because the optimizer never gets a chance to create better access paths as the data and environment change. Of course, the possibility of degraded performance after a REBIND is real – and that is why many sites avoid regularly rebinding their programs.

Even so, the best approach is to perform regular REBINDs as your data changes. To do so, you should follow the Three R’s. Regularly reorganizing to ensure optimal structure; followed by RUNSTATS to ensure that the reorganized state of the data is reflected in the DB2 Catalog; and finally, rebinding all of programs that access the reorganized structures. This technique can improve application performance because access paths will be better designed based on an accurate view of your data.

Of course, adopting the Three R’s approach raises questions, such as “When should you reorganize?” To properly determine when to reorganize you’ll have to examine statistics. This means looking at either RUNSTATS in the catalog or Real Time Statistics (RTS). So, the Three R’s become the Four 4 R’s – examine the Real Time Stats, REORG database objects as indicated by RTS, RUNSTATS to get the new statistics, then REBIND any impacted application programs.

Some organizations do not rely on statistics to schedule REORGs. Instead, they build REORG  JCL as they create each object – that is, create a table space, build and schedule a REORG job, and run it monthly or quarterly. This is better than no REORG at all, but it is not ideal because you are likely to be reorganizing too soon (wasting CPU cycles) or too late (causing performance degradation until REORG).

It is better to base your REORGs off of thresholds on catalog or real-time statistics. Statistics are the fuel that makes the optimizer function properly. Without accurate statistics the optimizer cannot formulate the best access path to retrieve your data because it does not know how your data is currently structured. So when should you run RUNSTATS? One answer is “as frequently as possible based on how often your data changes.” To succeed you need an understanding of data growth patterns – and these patterns will differ for every table space and index.

The looming question is this: why are we running all of these RUNSTATS and REORGs? To improve performance, right? But only with regular REBINDs will your programs take advantage of the new statistics to build more efficient access paths, at least for static SQL applications.

Without an automated method of comparing and contrasting access paths, DB2 program change management can be time-consuming and error-prone – especially when we deal with thousands of programs. And we always have to be alert for a rogue access path – that is, when the optimizer formulates a new access path that performs worse than the previous access path.

Regular rebinding means that you must regularly review access paths and correct any “potential” problems. Indeed, the Four R’s become the Five R’s because we need to review the access paths after rebinding to make sure that there are no problems. So, we should begin with RTS (or RUNSTATS) to determine when to REORG. After reorganizing we should run RUNSTATS again, followed by a REBIND. Then we need that fifth R – which is to Review the access paths generated by the REBIND.

The review process involves finding which statements might perform worse than before. Ideally, the DBAs would review all access path changes to determine if they are better or worse. But DB2 does not provide any systematic means of doing that. There are tools that can help you achieve this though.

The bottom line is that DB2 shops should implement best practices whereby access paths are tested to compare the before and after impact of the optimizer’s choices. By adopting best practices to periodically REBIND your DB2 programs you can achieve better overall application performance because programs will be using access paths generated from statistics that more accurately represent the data. And by implementing a quality review step there should be less need to reactively tune application performance because there will be fewer access path and SQL-related performance problems.

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