Troubleshooting backups: Tape compression

Although nearly all backup utilities and most tape drives offer the ability to compress data "up to 50 percent" when storing it, that percentage is almost never achieved in the real world. Here are the reasons why, and what you can do about it.

Compression is a standard feature on backup systems. Nearly all backup utilities and most tape drives offer the ability to compress data "up to 50 percent" when storing it. . .at least according to the vendors of these products.

While 50 percent compression is theoretically possible, it is almost never achieved. More common is a figure of 40 percent, but the exact percentage depends on the types of data being backed up and may be less. . .sometimes a lot less.

Data types aside, there are many reasons why you don't get the expected compression ratio. Here are five of them.

  1. The data has already been compressed. If the data has already been compressed, the compression figure may actually be negative. Some data, including graphics files or archived disk partitions, is compressed as it is written. Not only can't tape systems further compress already compressed data, but the attempt can result in files that are larger than the originals. Likewise, although both backup software and tape drives can compress data, the rule is that you only use one compression method at a time. In general, you can expect the tape hardware to do a slightly better job of compression than the backup software.
  2. The system may not be set to produce maximum compression. Since compression increases the time it takes to do the backup, some systems provide a choice of compression settings which let you trade time for space. Check to see that the backup system is set to the appropriate compression level.
  3. Throughput issues. If your server can't meet the tape system's demands for data, or if it feeds data faster than the tape can handle it, compressed capacity is likely to suffer. In either case, the computer has to stop and restart writing data to the tape, whjch reduces the tape's capacity.
  4. The tape may be going bad. A flaky tape can have the same effects as incorrect throughput, since the tape drive has to work around defective areas on the tape. Make sure you replace your tapes regularly, according to the tape manufacturer's specifications.
  5. The tape drive may simply need cleaning. Modern tape drives don't need cleaning nearly as often as they used to, but you still have to clean the drives occasionally. Run the tape cleaner cartridge at least as often as the drive manufacturer specifies. If you suspect the drive heads are dirty, run the cleaner. Don't forget to follow the drive maker's specifications for the rest of the cleaning process. Note: Keep track of the number of uses of the cleaning cartridge so you can replace it as needed.

About the author: Rick Cook specializes in writing about issues related to storage and storage management.

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This was first published in September 2006
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