Active Data Guard

Like Snapshot Standby, Active Data Guard lets you have your standby and use it to. A new option introduced in Database 11g, Active Data Guard allows you to throw a standby instance into read mode to support real-time queries, solving one of the biggest issues businesses have with their OLTP databases: namely, how to separate OLTP and read activity while providing near-real-time reporting.

Active Data Guard not only allows you to make the standby readable, but you can do so with minimal configuration changes on your client. You can point reporting applications at the standby directly, or you can create what is essentially a read service on the primary server that will act as a broker that routes workload based on the type of activity. When the application connects to one service, it writes to the local OLTP database, and when it connects to the other service, the queries get routed to the standby to fulfill the reporting requests. This is powerful functionality that makes it easy for shops to report from their OLTP data.

The downside is that you can't make any permanent changes to the standby, so you're stuck with the indexing and other configurations of the OLTP database. As anyone who has ever put a reporting database together knows, OLTP index requirements are rarely the same as reporting index requirements.

Plus, configuring Active Data Guard, at least on Windows, is no picnic. I spent weeks in e-mail, on the phone, and in online conference with Oracle reps trying to get Active Data Guard up and running. The directions given online are incomplete, and there's no guidance on troubleshooting at all. Had I been alone trying to set this up, it never would have happened.

That said, there's nothing in the Active Data Guard setup that couldn't be done by a wizard, and in fact a wizard will be coming in Grid Control 11g, according to Oracle. Built-in monitoring and alerts would also be an improvement. As it stands, you have to set up your own scripts and alerts to monitor whether your logs are being shipped and applied.

Caveats aside, Active Data Guard is a huge leap forward, and DBAs will love what it can do. After I got the standby up and running, it seemed to perform very well and did exactly what it's supposed to do. I could hit it with queries, write to it, and it would resync with the primary when I switched back. It's also easy to use. It takes just a single statement at the command line to switch between modes.

Real Application Testing

Real Application Testing, an option comprising Database Replay and SQL Performance Analyzer, is a new feature in Oracle Database 11g that allows you to capture a workload, replay it on the same system or a different one, and then compare the results. Database Replay will replay your workload exactly as it happened complete with concurrency and timing, allowing you to fully test system changes against your actual production workload. Thus, you can see the true impact of changes to the database (index changes, percentage free, table partitioning, and so on) before introducing those changes into production.

In my tests, Database Replay was easy to configure, and it performed exactly as expected. You have to learn just a couple simple concepts related to setting it up -- such as how to create the directory object in the database to capture the workload and how to start the replay from the command line -- but once you get past them, it's smooth sailing.

My test was a 50-user read/write mixed workload. In between tests, I deleted indexes from my tables so that I could see changes in the report numbers. As expected, dropping the indexes did increase the performance of the writes, while decreasing the performance of the reads. I was able to verify that the replay mechanism re-created all my threads and ran them flawlessly.

Setting up a capture is a simple four- or five-step process. You typically restart the database, set up options for the capture (which parts of the workload to include or exclude), create the directory to save the capture files, and set the capture start time and duration or manually start it and stop it. If you don't want to capture the system-level activity such as background processes or indexing operations, you can easily filter them out. Or if you want to limit the capture to just a specific application or a specific piece of code you were having a problem with, you can isolate them to make it easier to look at the deltas. The GUI makes configuring exclusion or inclusion filters easy to do.

You can replay the workload on the host system itself (as I did) or from one or more clients. If you capture a large workload, Database Replay has a calibration tool that will tell you how many replay clients you will need.

The reports produced are replete with result numbers comparing the capture workload and the replay workload. You can get literally hundreds of different calculations, but while all of these different metrics are available, they're not very pretty. In fact, they're just one table of numbers after another. It would be nice to have some high-level graphs and such to point you in the right direction. You can export the data, however, so you should be able to do something else with it if you wish.

Another way to go about analyzing your workload, complete with graphs, is to simply record the data in SQL Performance Analyzer as you run your replay workloads. The database doesn't know the difference between a workload being fed by 100 users or by a generator hitting it with 100 virtual users, so you can simply capture the live data in Performance Analyzer while you're doing your replay tests.

The easiest way to get bang for your Real Application Testing buck is to use Database Replay in conjunction with Data Guard's Snapshot Standby feature, discussed above, which saves the state of your standby database and allows you to restore it later (along with logs that have been shipped in the meantime). The combination of Database Replay and Snapshot Standby gives you a powerful and flexible way to test changes to your applications. You simply capture the workload on the primary using Database Replay, set your standby in read/write mode, make the changes you need to make, and then replay the workload on the standby. When you've finished testing, you can record your results and put the standby back into a consistent state with the primary.

Figuring out how to accurately test changes in your own real-world environment without doing it in production is one of the biggest problems in database management. Database Replay solves this problem in an easy and elegant way.