What is GFS retention (grandfather-father-son)?
GFS retention, short for grandfather-father-son, is a backup retention scheme that keeps restore points at several intervals for different lengths of time. Recent points are kept densely, usually every day, and older ones are thinned out to one per week, one per month and one per year, so that history reaches back a long way without storing every daily backup.
How do the three generations work?
The names come from tape rotation. The son is the daily backup, kept for a short period such as two weeks. The father is one backup per week, kept for a month or two. The grandfather is one per month, kept for a year or more. Many policies add a fourth tier of yearly points kept for several years.
When a daily point reaches the end of its retention, it is deleted unless it has been flagged as that week's, month's or year's point, in which case it moves up a tier and stays.
| Tier | Typical frequency | Typical retention |
|---|---|---|
| Son | Daily | 7 to 30 days |
| Father | Weekly | 4 to 8 weeks |
| Grandfather | Monthly | 12 months |
| Yearly (optional) | Once a year | 3 to 10 years |
Why use GFS retention instead of keeping every backup?
Keeping every daily point for seven years would mean more than 2,500 restore points per system. GFS keeps a few dozen and still offers a point from last night, from last month and from three years ago. Most restore requests concern the last days, and the rare request for older data seldom needs a specific day. The scheme matches how restores are actually asked for, and it is the basis of most retention policy design.
How does GFS retention affect storage capacity?
Each long-term point has to be restorable alone, so backup software stores it as a full backup or a synthetic full. On repositories that share unchanged blocks between fulls, each extra point costs only the data that changed since the previous one. On repositories that cannot, each weekly, monthly and yearly point costs close to a full copy. The number of GFS points multiplied by how different they are from each other is what drives long-term capacity.
How does GFS retention interact with immutability?
GFS points are often the ones an organization most wants protected, because they cover the longest history. When the repository supports it, the immutability period on a GFS point can be set to match its retention, so that a yearly point is locked for its whole life. That protects it from deletion and also means it cannot be removed early if capacity runs short, so long locks should be set deliberately. S3 Object Lock is the usual mechanism on object storage.
Frequently asked questions
Is GFS retention only for tape?
No. The scheme started with tape rotation, but backup software applies the same tiers to disk and object storage repositories.
How many restore points does a typical GFS policy keep?
A common policy of 14 daily, 4 weekly, 12 monthly and 3 yearly points keeps about 33 restore points per system.
Does a GFS point have to be a full backup?
It has to be restorable without relying on points that will be deleted before it. In practice that means a full or synthetic full, or a block-sharing layout that gives the same independence.