Becoming the Master of Disaster

Wednesday Apr 23rd 2003 by Jim Czuprynski

If a disaster happened to your database server, could you recover from it? This article discusses the questions a DBA can ask to probe the realm of what we all hate to think about - the possibility of the untimely demise of a production database server. Learn how to become a fearless 'master of disaster'.

A few Saturdays ago, I performed a planned viability test of my Oracle 9iR2 hot standby database. I terminated the transmission of archived redo logs from the primary site, activated the standby database, and compared results between primary and standby sites. As expected, row counts, dollar totals, and a few other measures matched up perfectly. Satisfied, I kicked off the process to copy RMAN backups from the primary site in preparation for restoring the standby site to its standby role, and went home until Monday, leaving the newly activated database running over the weekend.

When I arrived early on Monday to run a few more tests of our applications against the standby site, I was surprised to discover the instance had crashed. After investigation, I found out that all of the drives had failed on one of the standby site's two disk drive arrays. Since that array held drives that contained datafiles for the system rollback segments, the rollback segment tablespace was corrupted almost immediately. Further investigation revealed that the disk array had failed because the array had only one power supply, even though a second redundant power supply module could have been installed.

Even though this was a rather unexpected and reasonably unlikely failure, it could not have come at a better time. It caused me to review our entire disaster recovery plan for both the secondary and primary servers. I found out that none of the production servers had been outfitted with redundant power supplies for the disk arrays. And some further reevaluation of my disaster recovery scenarios proved that the loss of one of the arrays would have caused the loss of UNDO segments on the production database - because it turns out those datafiles weren't mirrored properly either.

Both the primary and standby sites are all repaired now, of course, and everything is copasetic. However, my cautionary tale underlines how a robust disaster recovery plan can be critical in preventing and surviving a potential disaster.

Developing disaster recovery scenarios.
A good disaster recovery planner isn't afraid to "think about the unthinkable." This entails developing the common disaster recovery scenarios that could happen to your database and server.

Based on my experiences over the past several years as an Oracle DBA, the most serious of these is media failure. A typical example of preventable media failure involves under-utilization of RAID-0+1 or RAID-1 redundancy for critical data files, log groups, and control files. Moreover, as I described in my earlier tale of woe, it is a good idea to remember those pesky and often-overlooked UNDO or rollback segments - it may be impossible to restart the database when those tablespaces are damaged or corrupted due to media failure.

Another set of disaster recovery scenarios with serious implications involves the partial or complete loss of the database server itself. This might include damage to the software needed to run the Oracle instance - for example, the loss of critical operating system files - as well as physical damage, such as a failed power supply, memory, or CPU module.

Hardware disasters can be more difficult to predict, and can be even harder to test, since realistically a "test to destruction" of the hardware might have to be performed to simulate some of the failures. However, even with robust modern service agreements available from major hardware suppliers, it could be hours or even days before the damaged server is repaired and ready to take the load of a production database again, so these scenarios should not be ignored.

Once you've uncovered potential single points of failure and have painted some grim pictures as to what might happen if those failures occurred, it's time to turn attention to the methods, practices, and hardware configurations that help prevent a disaster.

Alternate production server.
If you are using Oracle's DataGuard facilities to create and maintain either a logical or physical "hot standby" server site, then you've already got this angle covered. However, if you do not have an alternate server to which you could quickly restore your production database, the ability to recover from a serious hardware disaster will be much more in doubt.

One less robust alternative to a standby site is a quality-assurance (QA) database server. This server should ideally be a close match to the hardware for the production site to allow evaluation of the next set of application or database changes about to be released to production. On one occasion before getting our hot standby server in working order, I was forced to transfer our production database over to our QA site because we had noticed some "flaky" performance of the production server. As it turned out, we had guessed right - the production server's motherboard was facing an imminent failure, and failed shortly after the transfer of responsibility. Though the QA server had only half the memory and CPU power of the production site, having a QA server in my "back pocket" saved the day.

Regularly scheduled backups.
I won't harp on the obvious: The ability to access recent, consistent backup files and archived redo logs is the key to recovering from and surviving a disaster. Of course, I am assuming your production database is running in ARCHIVELOG mode for maximum flexibility for recovery. Moreover, if the database is running in ARCHIVELOG mode, I am assuming that Recovery Manager (RMAN) is being used for creating backups and recovery.

As in most shops, we have designed our production backup scheme to run overnight during off-peak hours. We have the luxury of a relatively small production database (330GB) at about 20% utilization, so nightly Incremental Level 0 RMAN backups only consume about 45-50 GB of disk space, and they are completed in approximately 4 hours. However, this gives me extreme flexibility in rolling forward from a potential disaster, including point-in-time incomplete recovery via RMAN.

Alternatives to RMAN backups.
My Oracle University teacher repeated this in class over and over again: "RMAN is the best way to back up a database - but it's not the only way." I thank him every day for the reminder! Even though our RMAN backups are created nightly, as a second line of defense against data loss I create a full set of exports every night. If I should need to recover just one table, or a portion of the table, it is a lot easier to recover it from an export than from a full tablespace backup. In addition, if a disaster does arise, and my backups are damaged as well, I have a chance of recovering at least some of the data from an export.

Alternate media storage of backup files.
While writing to disk media is probably the speediest and easiest mechanism for backup files retention, in many cases the disk space required is a luxury. Even though I do have the advantage of sufficient disk space, however, I have worked out a scheme of alternate media backups (tape) as a third line of defense against loss of the database server. In an absolute worst-case scenario - complete loss of the physical hardware - I still have a guaranteed method to recover a significant portion of my production database, albeit limited by the most recent available set of archived redo logs on tape.

Another word about alternate media backups: Offsite storage is strongly recommended for at least some of the backup tapes. We currently send a complete set of backups off to a remote site once a week for vaulted archival with guaranteed turnaround of one hour for any particular tape (for a small fee, of course).

If you're having a hard time imagining why you'd ever need offsite storage for backups, here's a classic Oracle "urban legend" I heard at a recent seminar. A panicked DBA called Oracle for help because his production server had been destroyed when a truck backed up through his company's loading dock, which was on the other side of the server room. Part of the collapsed wall crashed down directly on top of the production server, destroying it. The DBA had an alternate server available, and had been backing up his database to tape.

Unfortunately, the backup tapes were stored - you guessed it - on top of the production server.

Evaluating the disaster recovery plan.
Once all the disaster recovery pieces I have discussed previously are in place, I have found it is important to determine if the disaster recovery plan will work by actually simulating at least the most critical disaster scenarios.

After my experiences a few Saturdays ago, I reviewed all the media failure possibilities, including the loss of one or more datafiles containing SYSTEM, UNDO/rollback, index, and data segments. Then I constructed scenarios under which they might fail, and my expected course of action. Finally, I constructed methods to simulate the failure.

To simulate media failures of the various segment types, for example, I configured a RAID-0 drive on one of our development servers and then restored copies of a test database so that the appropriate datafiles were installed on that drive. While our QA manager simulated activity against that datafile by running application code that accessed that datafile's tablespace, I simply pulled that drive out of the disk array. I compared the expected results from the simulated failure against my expectations, and then attempted to restore and recover the damaged datafile using appropriate RMAN scripts.

I ran into some unexpected challenges with my initial attempts at RMAN recovery scripts, since some of the commands to rename and switch datafiles during restoration are slightly different from those used when restoring from "hot" or "cold" backups of datafiles and tablespaces. However, I have considered the lessons I learned during the evaluations of these scenarios to be invaluable, since I now have working examples of RMAN scripts for each specific scenario.

The result? I am now fully confident that in the worst-case scenarios of a partial or complete media failure of my production databases, I can easily restore and recover the appropriate datafiles from an RMAN backup set - something I do not ever want to have to do under the gun with one hand on the manual and one hand on the keyboard!

Jim Czuprynski is an Oracle DBA for a telecommunications company in Schaumburg, IL. He can be contacted at jczuprynski@zerodefectcomputing.com.

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