Sizing a tape library for unattended operation
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One of the major advantages of a tape library rather than a stand-alone tape drive for backup is that the library can run longer without swapping tapes. This makes libraries easier to administer than single tape drives, which should have the tape changed for every backup. Deciding how long you want your backups to go before they need manual intervention has an important effect on choosing the size of your library.
In general, you want to remove at least every third full backup to a secure location, so you want to change tapes in the library at a minimum of once every three full backups. That means the library should be capable of handling at least three times the total amount of tape space used in a full backup, including the space occupied by any incremental backups done between the full backup. This isn't necessarily the same as three times the size of the data set (plus space for incremental backups) because it often makes sense to partition the backup data by application, function or in some other fashion and that increases overhead.
Many storage administrators find it makes sense to change the tapes in the library weekly, especially if the system is set to do a full backup once a week and incremental backups daily.
Overland Data Inc., a San Diego, CA, maker of tape libraries, has a white paper on selecting a library that is fairly vendor-neutral. It is available in the "news and information" section of its Web site at www.overlanddata.com.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
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