Tape rotation is an essential part of backing up to tape and creating an effective data backup strategy. Here's a look at the most common backup tape rotation schemes and the pros and cons of each.
An effective data backup system using tape requires more than one tape set. A tape set is the number of tapes needed to do the appropriate backup. If a full backup takes two tapes, then two tapes constitute a tape set. If you do incremental backups on weekdays that only take one tape per backup, then your tape set for those days is one tape.
One-tape set backup
The least sophisticated -- and riskiest -- scheme is not to rotate your backup tapes at all. Simply back up to the same tape every day, occasionally replacing the tape as it wears. This is probably the most common method and is used by many small- to midsized businesses (SMBs).
One-tape backup is better than not backing up at all, but not by much. If something goes wrong with the backup or restore, or if the tape fails, you're left with no backup at all. Because the tape is so heavily used, the chances that something will go wrong multiply. An adequate tape backup scheme requires multiple tape sets.
One nasty characteristic of backups, especially tape backups, is that it is entirely possible to make a bad backup and not know it. Backup software typically performs on-the-fly error checking and most software will also do verification -- if you're willing to take the time to let it. But if the data itself has been corrupted, say by a virus, the backup will happily save the corrupted -- and unusable -- data.
The first lesson from this is to do test restores on a regular basis. The second lesson is to keep multiple backups stretching back over time. The further back you have to go when you restore, the more data you will lose, but you shouldn't lo...
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se everything.
Round Robin tape rotation scheme
One simple scheme is to have five backups tapes (one for each day of the work week) and to use each one in succession.. This way, you use the same tape every day of the week. For extra protection, you can use more than one tape for one day of the week, say Friday, and rotate the Friday tape offsite every week.
This system pretty much requires that you do a full backup every day. This may be fine for a small business without a lot of data to preserve, but it can mean a good-sized backup window every day.
Round Robin also uses one tape per day, and the number of tapes mounts up quickly if you want to keep backups for longer periods. The Round Robin scheme is popular with small businesses because of its simplicity.
Grandfather-Father-Son
The next most common tape rotation scheme is Grandfather-Father-Son, or GFS. It is straightforward and provides pretty good protection with a reasonable number of tape sets.
There are many variations of GFS with different requirements for the number of tapes. They take anywhere from eight to 22 tape sets for a five-day-a-week backup if you keep the backed up data anywhere from a month to a year (or infinitely, if the grandfather tape is rotated out of the lineup instead of being recycled).
The most common version of GFS involves taking a daily (usually incremental) backup Monday through Thursday (the son) with a full backup every Friday (the father). At the end of the month, another full backup is taken and stored off site (the grandfather).
The variations come in how long the backups are preserved. Recycling the grandfather tape with each use only requires eight tapes, but the oldest backup in the active set is only a month old. At the end of the month, last month's grandfather tape is either rotated back into the pool or it is taken out of the rotation and archived.
Tower of Hanoi tape rotation scheme
The Tower of Hanoi rotation is the most complex tape rotation strategy that is commonly used. It has the characteristic that adding a tape set to the rotation beyond the basics doubles the length of the backup period. Its other main characteristic is that it is complicated and hard to track. It is useful when you need to keep backups stretching over a long period of time on a reasonable number of tapes.
The system gets its name from a nineteenth-century puzzle that demonstrates the effects of a combinatorial explosion. The Tower of Hanoi rotation harnesses that combinatorial explosion to provide data protection. With daily backups it provides protection for 2^(N-1) days, with N being the number of tape sets.
The basic tape set, call it A, is used every other day for backups. With two tape sets the second set, B, is used on alternate days. Add a third set, C, and it alternates with B.
Thus, a four-set rotation is:
ABACABADABACABAD
A five-set rotation is:
ABACABADABACABAEABACABADABACABA
In other words with a five-tape set, you have backups that are one (A), 2 (B), 4(C) 8 (D) and 16 (E) days old. Add a sixth tape and you've got 32 days of backup. By the time you get to a 10 tape set you're maintaining backups more than three years back.
The Tower of Hanoi is uses tapes economically, especially if you want to maintain backups for a considerable time. It also reflects the reality that the further back you go the less likely you are to need to recover from the tape, hence the progressively longer backup periods.
Replace your tapes regularly
Finally, remember that even the best cared for tapes will eventually fail. Plan on replacing your tapes on a regular schedule depending on how much each tape is used. In a GFS scheme the daily backup tapes should probably be replaced each year or so and the weekly and monthly tapes (assuming the monthly tapes aren't archived) accordingly.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.
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