Incremental backups only back up changes since the last incremental backup. They are much more space efficient, but they are harder to restore from because you need multiple tape sets.
The simplest and most common form of tape rotation uses three sets (or generations) of tapes. The most common periods for the generations are a full backup monthly (the grandfather), a full backup weekly (the father) and a full or partial backup daily (the son). The father and grandfather tapes are stored offsite while the son tapes are used to do daily backups. This scheme will typically use daily differential backups, followed by a full backup every seventh day. The differential backups stay onsite while the weekly full backup is stored offsite for the week, and the monthly backup is stored offsite for as long as a permanent backup is needed. In addition to being simple, the three-generation system uses the fewest tapes and produces the least wear on the tape drives.
Tower of Hanoi
The Tower of Hanoi is the most complex of the common schemes and uses the largest number of tapes, but it's the most secure. The system starts with several full sets of backup tapes; for instance, five sets labeled A, B, C, D and E. Set A is used for every other backup. Set B is used for every fourth backup, Set C is used for every eighth backup and so on through the series. Basically, on the first non-A backup, you back up to the B media, on the first non-A, non-B media you back up on the C media and so on.
The six-tape system is especially suitable for enterprises that only have small amounts of data to backup. The canonical version doesn't back up over the weekends and alternates two tapes on Fridays, hence the name "six-tape." Of course, you can back up on the weekends as well, making it an eight-tape system.
In the six-tape version of the system, you make six backup sets, one each of Monday through Thursday and two of Friday (Friday1 and Friday 2) The Friday tapes are full backups and when the Friday backup is run, the tape is taken offsite and the previous Friday's backup is brought back. The Monday through Thursday backups can be either full or incremental backups.
Archiving where you can get at it
It's important that you have your offsite backups available 24/7. Data storage facilities can generally guarantee this, however, the common small-business alternative of storing offsite in a safety deposit box doesn't. If you need to get at your backups on evenings, weekends or holidays, you probably can't get into the vault until the bank opens.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.
This was first published in June 2008