Glossary
Incremental backup
Incremental backup copies only the data that changed since the previous backup, whether that was a full backup or another incremental. Each run is small and quick, but a restore needs the last full backup plus every incremental taken after it, applied in order.
How an incremental chain builds and restores
A 5 TB database that changes by about 50 GB a day shows the pattern:
- Monday: a full backup copies all 5 TB.
- Tuesday to Thursday: each run copies only that day's 50 GB of changes.
- Restoring Thursday: the software applies Monday's full, then Tuesday's, Wednesday's and Thursday's incrementals, in that order.
Backup software finds changes by asking the operating system, hypervisor or database what moved, rather than reading everything again. It records which restore point depends on which in the backup catalog. If change tracking is lost, the next run reads everything, and the job takes as long as a full.
Incremental, differential and synthetic full compared
| Method | What each run copies | What a restore needs | Trade-off |
|---|---|---|---|
| Full | Everything | That one backup | Largest and slowest run |
| Incremental | Changes since the last backup of any kind | The full plus every incremental since | Smallest runs, longest chain |
| Differential | Changes since the last full | The full plus the latest differential | Runs grow through the cycle |
| Synthetic full | Nothing from production; built on backup storage from earlier backups | The new full | Shortens the chain without reading the source again |
Keeping the chain intact
One damaged or deleted link breaks every restore point after it, so retention applies to whole chains rather than single backups. Long chains carry more links that can fail and take longer to restore. Periodic fulls or synthetic fulls cap the chain, and a test restore of the newest point shows whether it still holds together. Because each run is small, incrementals allow shorter intervals, which narrows the data loss a recovery point objective has to allow for.
Frequently asked questions
What is the difference between incremental and differential backup?
An incremental copies changes since the last backup of any kind. A differential copies everything changed since the last full. Restoring from differentials needs two backups; restoring from incrementals needs the whole chain.
Is incremental-forever backup safe?
It is when the backup storage regularly builds synthetic fulls or merges old incrementals, and when restores of recent points are tested. Without either, the chain grows until one bad link affects months of restore points.