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. The scheme determines how and when each piece of removable storage is used for a backup job and how long it is retained once it has backup data stored on it. Different techniques have evolved over time to balance data retention and restoration needs with the cost of extra data storage media. Such a scheme can be quite complicated if it takes incremental backups, multiple retention periods, and off-site storage into consideration. 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 the