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Backup Compression

Most backup systems support some type of compression. The idea behind compression is simple: to save space and allow the backup of more data onto a given media set. Most software supports compression, and in fact many backup devices even quote their capacity on the assumption that compression will be used during backup.

Normally there is nothing wrong with using compression, and in fact I use it myself in many cases. You do need to bear in mind a few things, however:

• Compressability: Not all files will compress equally well, as explained in this section on hard disk compression. If you are backing up a large number of files that are already in a compressed format, it may make sense to turn off compression, since it isn't going to do much for you anyway.
• Proprietary Formats: Each software program (or in many cases, family of software programs made by the same company) will use its own compression algorithm. This means that the tape written by one program may not be readable by a different software package. This is not normally a problem since most people only use one package on one PC, but it is something to bear in mind. The backup formats themselves are reasonably universal if compression is not used.
• Processing Power Requirements: In order to write some types of backup devices, especially tape drives, it is necessary for the software to provide to the device a steady stream of data. This is because tape can only be written to when it is streaming at constant speed. Compression algorithms take time to run and can sometimes interfere with this steady flow of data, causing problems like shoe-shining (where the tape has to repeatedly back up and restart sections of the backup). Compression should be turned off in most cases if doing the backup on a slower PC or if it is suspected that compression overhead is causing problems.
• Exaggerated Compression Ratios: Watch out for overly-optimistic estimated compression ratios. Many tape manufacturers like to claim that their 1.6 GB tapes for example will hold 3.2 GB "with compression". In practice, I rarely if ever see any full disk volumes (as opposed to collections of certain types of files) that will compress at a 2:1 ratio.

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