In many ways, backups are the heart of any design of critical systems. Handled properly, they represent the last line of defense against just about any catastrophe. Even if your building or your entire city is wiped out, your business can be restored on other computers from properly generated and protected backup tapes. But there are several "if" conditions that must be satisfied for everything to work out properly and data to be recoverable.
There are a number of basic backup guidelines. By keeping them in mind as you design your backup environment, you will make the best advantage of your backups, and they will serve you best when you need them:
- Mirroring does not replace backups. This hard fact runs counter to a very old and unfortunate myth. Mirroring protects against the failure of storage hardware, but it does nothing at all to protect you from a deleted or corrupted file. If a file is deleted on one side of a mirror, it is gone from both sides and therefore must be retrieved through some external means. The most common (but by no means the only) external means of recovery is restoring a backup tape.
- The most common use of restores isn't after a catastrophe. Yes, of course catastrophes do happen, but it is much more likely that a user will accidentally delete or damage a single file, or even a directory, than it is for the contents of both sides of a mirror, or two disks in a RAID stripe, to fail at the same time, or for a sitewide
- disaster to occur. So, design your backups to optimize restore times for a single file.
- Regularly test your ability to restore. Backups are very important, of course, but if you cannot restore them, you have wasted a lot of time and effort creating useless tapes. You shouldn't need to test every backup tape that you create, but certainly every tape drive should be tested regularly to ensure that the backups it makes are readable, and a random sampling of tapes should be tested to ensure that they can be read and restored properly.
- Keep those tape heads clean. Dirty tape heads may cause backups to appear to complete successfully when, in fact, only garbage has been written to the tape. Clean the tape heads as often as the manufacturer recommends, if not slightly more often. If your backup utility reports tape read and/or write errors, clean the heads immediately. If errors continue, see if they are present on other tapes, and if they are, then contact the vendors.
- Beware of dirty tapes. No, not adult videos, but rather the tapes themselves. Sometimes a single bad spot -- a crease, a fold, or a smudge of dirt -- on a tape can prevent the entire tape from being read. Store the tapes in their cases in relatively clean rooms, and test a sample from time to time.
- Pay attention to Mean Time Between Failure (MTBF) numbers for tapes. If the manufacturer suggests that a tape's useful life is 1,000 backups, then use it 1,000 times, and get rid of it. The time to find out that a tape has gone bad is NOT during a restore. Good backup management software will keep track of tape access counts for you and tell you when it's time to dispose of a tape.
- Tapes decompose over time. Do not assume that the tape you made five or six years ago is still readable today. Tapes break down; they get exposed to weak magnetic fields that affect their quality, and some simply age badly. It is very difficult for manufacturers who are introducing new tape formats to accurately determine the shelf life of such a tape. If they were to perform definitive tests, introduction of such tapes could be delayed for several years. Test your tapes on a regular basis, and copy them to new media from time to time. To help extend the shelf life of your tapes, pay attention to the tape's storage requirements; avoid extremes in temperature and humidity.
- Make two copies of critical tapes. Tapes are much less expensive to purchase and maintain than is recreating the data that is on the tapes. To better ensure the longevity and safety of your tapes and data, it is wise to store one copy of your tapes off-site.
- Make sure you can still read old media. It's nice having a record album collection, but if you don't have a turntable with a good needle, the records are pretty useless. The same is true of old magnetic tapes; if you can't read your old nine-track tapes, there's no point in keeping them. When new technology comes along, copy the old tapes that you need onto new media.
Content in this tip has been excerpted by permission from the book, ""Blueprints for high availability, Second edition," authored by Evan Marcus and Hal Stern, Wiley Publishing, Inc. All rights reserved.
About the authors: Evan Marcus is a frequent SearchStorage.com contributor and an expert at answering readers' questions related to availability, backup and disaster recovery-related issues. He is also a principal engineer for Veritas Software and the industry's data availability maven, with over 12 years of experience in this area. He is also a frequent speaker at industry technical conferences.
Hal Stern is the vice president and chief technology officer for the Services business unit of Sun Microsystems. He has worked on reliability and availability issues for some of the largest online trading and sports information as well as several network service providers.
Do you have a question for Evan Marcus? You can find him in our High Availability category.