What is a retention period?
A retention period is the length of time a backup, or a single restore point, is kept before it becomes eligible for deletion. It is set by policy, applied by the backup software and, when immutability is in use, enforced by the storage as well.
How is a retention period defined?
Backup software expresses retention either as a number of restore points, such as the last 14, or as a length of time, such as 30 days. Longer-term tiers are added on top with GFS retention, which keeps selected weekly, monthly and yearly points for extended periods. When a restore point passes its retention, the software deletes it, provided no later point still depends on it.
How does retention differ from immutability?
The two are related and often confused. Retention is a policy held by the backup software. An immutability period is a lock held by the storage.
| Aspect | Retention period | Immutability period |
|---|---|---|
| Where it is set | Backup job or policy | Repository or bucket, applied per object |
| What it does | Schedules deletion | Blocks deletion and change |
| Who can shorten it | Backup administrators | Nobody in compliance mode |
| If the backup server is compromised | Can be altered | Still enforced by the storage |
What decides how long retention should be?
Three inputs set the length. Recovery needs come first: how far back a restore might have to reach, including for an intrusion discovered weeks after it began. Regulation and contracts may set a minimum for some data and a maximum for personal data. And cost sets a practical limit, because every extension holds more data. Matching retention to recovery needs gives a method, and retention policy design covers the tiers.
How does the retention period affect storage?
Stored capacity is roughly the size of a full backup plus the daily change multiplied by the number of days retained, adjusted for data reduction. Doubling retention does not double capacity, but it adds a predictable amount. Two effects push actual usage above the nominal figure: restore points that cannot be deleted because later ones in the backup chain depend on them, and immutability locks that extend beyond the retention date. The cost of another year of retention shows the arithmetic.
Frequently asked questions
Is a longer retention period always safer?
It gives more history to restore from, but it costs capacity and can conflict with rules that limit how long personal data is kept.
What happens when a retention period ends?
The restore point becomes eligible for deletion and is removed by the backup software's next retention run, unless a lock or dependency still holds it.
Can different systems have different retention periods?
Yes. Most organizations define a few tiers and assign systems to them by importance and regulatory need.