Schemes
First in, first out
A first in, first out (FIFO) backup scheme saves new or modified files onto the "oldest" media in the set, i.e. the media that contain the oldest and thus least useful previously backed up data. Performing a daily backup onto a set of 14 media, the backup depth would be 14 days. Each day, the oldest media would be inserted when performing the backup. This is the simplest rotation scheme and is usually the first to come to mind. This scheme has the advantage that it retains the longest possible tail of daily backups. It can be used when archived data is unimportant (or is retained separately from the short-term backup data) and data before the rotation period is irrelevant. However, this scheme suffers from the possibility of data loss: suppose, an error is introduced into the data, but the problem is not identified until several generations of backups and revisions have taken place. Thus when the error is detected, all the backup files contain the error. It would then be useful to have at least one older version of the data, as it would not have the error.Grandfather-father-son
Grandfather-father-son backup (GFS) is a common rotation scheme for backup media, in which there are three or more backup cycles, such as daily, weekly and monthly. The daily backups are rotated on a daily basis using a FIFO system as above. The weekly backups are similarly rotated on a weekly basis, and the monthly backup on a monthly basis. In addition, quarterly, half-yearly, and/or annual backups could also be separately retained. Often some of these backups are removed from the site for safekeeping and disaster recovery purposes.Tower of Hanoi
The Tower of Hanoi rotation method is more complex. It is based on the mathematics of theThree-tape Hanoi schedule
Four-tape Hanoi schedule
Five-tape Hanoi schedule
Extensions and example
Many variations are possible, and the concepts are readily extended to disc-based directories containing backups. Here are some options: * Save a base backup as set zero. * Save as many of the most recent backups as desired. * Save more than one of each set number, for greater coverage. Coverage automatically gets sparser the further back in time one goes, which approximates the likelihood of needing to do restores from past backups. And Tower of Hanoi has the huge advantage of freeing implementers from having to deal with managing hourly, daily, weekly, monthly, quarterly or annual management strategies. In general, backup set number ''set'' is used at ''seq'' = 2''set''−1 + j × 2''set'', ''j'' = 0, 1, 2, 3, 4, ..., where ''seq'' is the sequence or serial number of a backup (also the Tower of Hanoi move number). Here is an example showing coverage, including set 0, keeping at least the last 4 days, and recycling: * precious.20140515.seq.0 set 0 * precious.20150205.seq.256 set 9 * precious.20151026.seq.512 set 10 * precious.20160311.seq.640 set 8 * precious.20160516.seq.704 set 7 * precious.20160601.seq.720 set 5 * precious.20160609.seq.728 set 4 * precious.20160617.seq.736 set 6 * precious.20160618.seq.737.recycle set 1 * precious.20160619.seq.738 set 2 * precious.20160620.seq.739 set 1 * precious.20160621.seq.740 set 3 * precious.20160622.seq.741 set 1Weighted random distribution
An alternative arrangement is to keep generations distributed across all points in time is by deleting (or overwriting) past generations (except the oldest and the most-recent-''n'' generations) when necessary in a weighted-random fashion. For each deletion, the weight assigned to each deletable generation corresponds to the probability of it being deleted. One acceptableIncremented media method
This method has many variations and names. A set of numbered media is used until the end of the cycle. Then the cycle is repeated using media numbered the same as the previous cycle, but incremented by one. The lowest numbered tape from the previous cycle is retired and kept permanently. Thus one has access to every backup for one cycle and to one backup per cycle before that. This method has the advantage of ensuring even media wear, but requires a schedule to be precalculated.See also
* Off-site data protection * Legal holdReferences