Generation principle
The generation principle , also known as the grandfather-father-son principle , is a data backup strategy in IT . It ensures that there are always several backups in different time increments (grandfather, father, son) to create different versions for one possible restoration available. The generation principle is one of the most common rotation schemes used in tape rotation backup, in which backup tapes are reused according to a specific scheme.
Fuse type
The generation principle is also not tied to a specific type of backup (complete, differential, incremental).
Detailed example
In the scenario described below, the grandfather-father-son backup is used as part of a five-day week. As above, 20 storage media are used: four son media (S1 – S4), four father media (V1 – V4) and twelve grandfather media (G1 – G12). It is assumed here that a backup fits on such a medium.
Procedure : The scenario begins on January 1st, which - for the sake of simplicity - is a Monday. A calendar set to the year 2007 is suitable for comprehension.
- 1 week
- On Monday, January 1st, the first daily backup (son) takes place on medium S1. The same is done with media S2 – S4 for the days Tuesday to Thursday. On Friday, January 5th, the first weekly backup (father) takes place on medium V1.
- 2 weeks
- On Monday, January 8th, the daily backup will take place again on the S1 medium, which means that it will be overwritten for the first time after one week. As in the first week, the backups for the days Tuesday to Thursday are made on media S2 – S4. On Friday, January 12th, the second weekly backup will take place on medium V2.
- 3rd and 4th week
- As in the two weeks before, the daily backups are made on media S1-S4. The third and fourth weekly backups are made on media V3 and V4 on Fridays.
- 5th week
- The week starts on Monday, January 29th, as usual. Since Wednesday, January 31st, is the last day of the month, the first monthly backup (grandfather) is created on medium G1 (in addition to the daily backup on medium S3). As usual, the S4 medium will be backed up on Thursday. The backup on Friday takes place again on the first father medium, V1, which is overwritten for the first time after one month.
- 2 months
- February is the same as January. An old weekly backup is overwritten every Friday. On Wednesday, February 28th, a monthly backup will take place again, this time on the medium G2.
- 3 month
- The first four weekly backups in March are carried out in the same way as in February, so they overwrite the media V1 – V4. Since this March has five Fridays, the weekly and monthly backups are made on the G3 medium on March 30th (see above).
- 4th - 12th month
- The backups are made here in the same way as in the previous months. On Monday, December 31st, the last monthly backup is made on the as yet unwritten medium G12.
- 2 years
- Here, at the end of January, the monthly backup is saved again on medium G1, which means that it is overwritten for the first time after a year. So data from up to a year can be retrieved.
