Community hub
Recent from talks
Knowledge base stats:
Talk channels stats:
Members stats:
Backup rotation scheme
A backup rotation scheme is a system of backing up data to computer media (such as tapes) that minimizes, by re-use, the number of media used. It determines how and when each piece of removable storage is used for a backup job and how long it is retained after backup data is stored on it. Over time, different techniques have evolved to balance data retention and restoration needs with the cost of additional data storage media. Such a scheme can become quite complex if it incorporates incremental backups, multiple retention periods, and off-site storage.
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 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 3-months basis using a FIFO system as above. The weekly backups are similarly rotated on a bi-yearly basis, and the monthly backup on a yearly 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.
The Tower of Hanoi rotation method is more complex. It is based on the mathematics of the Tower of Hanoi puzzle, using a recursive method to optimize the back-up cycle. Every tape corresponds to a disk in the puzzle, and every disk movement to a different peg corresponds with a backup to that tape. So the first tape is used every other day (1, 3, 5, 7, 9, ...), the second tape is used every fourth day (2, 6, 10, ...), the third tape is used every eighth day (4, 12, 20, ...).
A set of n tapes (or other media) will allow backups for 2n−1 days before the last set is recycled. So, 3 tapes will give 4 days' worth of backups, and on the 4th day Set C will be overwritten; 4 tapes will give 8 days, and Set D is overwritten on the 9th day; 5 tapes will give 16 days, etc. Files can be restored from 1, 2, 4, 8, 16, ..., 2n−1 days ago.
The following tables show which tapes are used on which days of various cycles. A disadvantage of the method is that half the backups are overwritten after only two days.
Hub AI
Backup rotation scheme AI simulator
(@Backup rotation scheme_simulator)
Backup rotation scheme
A backup rotation scheme is a system of backing up data to computer media (such as tapes) that minimizes, by re-use, the number of media used. It determines how and when each piece of removable storage is used for a backup job and how long it is retained after backup data is stored on it. Over time, different techniques have evolved to balance data retention and restoration needs with the cost of additional data storage media. Such a scheme can become quite complex if it incorporates incremental backups, multiple retention periods, and off-site storage.
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 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 3-months basis using a FIFO system as above. The weekly backups are similarly rotated on a bi-yearly basis, and the monthly backup on a yearly 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.
The Tower of Hanoi rotation method is more complex. It is based on the mathematics of the Tower of Hanoi puzzle, using a recursive method to optimize the back-up cycle. Every tape corresponds to a disk in the puzzle, and every disk movement to a different peg corresponds with a backup to that tape. So the first tape is used every other day (1, 3, 5, 7, 9, ...), the second tape is used every fourth day (2, 6, 10, ...), the third tape is used every eighth day (4, 12, 20, ...).
A set of n tapes (or other media) will allow backups for 2n−1 days before the last set is recycled. So, 3 tapes will give 4 days' worth of backups, and on the 4th day Set C will be overwritten; 4 tapes will give 8 days, and Set D is overwritten on the 9th day; 5 tapes will give 16 days, etc. Files can be restored from 1, 2, 4, 8, 16, ..., 2n−1 days ago.
The following tables show which tapes are used on which days of various cycles. A disadvantage of the method is that half the backups are overwritten after only two days.