Simple definition
A differential backup captures changes made since a reference full backup. Each new differential therefore includes cumulative changes from that base rather than only changes since the previous backup run. The corresponding full backup must remain available for recovery.
Technical definition
In the classic model, restoring a differential point requires its full base and the chosen differential. Intermediate differentials are normally unnecessary. Change detection depends on the backup product or database engine. SQL Server has product-specific differential-base rules, and operations such as copy-only backups have distinct effects that should not be generalized to every backup system.
What is it used for?
Reduce recovery dependencies compared with a long incremental sequence, at the cost of a differential that usually grows between full backups. Choose the method according to data change rate, processing capacity and recovery objectives.
Practical example
A full backup runs on Sunday. Wednesday’s differential includes cumulative changes since Sunday, including data already captured on Monday and Tuesday. Recovering Wednesday requires the correct full and Wednesday’s differential, plus any additional elements required by the application recovery plan.
Common issues
- Differential growth exceeding the available backup window.
- A missing or incorrectly identified full base.
- Confusion with incrementals or transaction-log backups.
Operational checks
Identify the base, the selected recovery point and each backup’s actual type. Filenames and sizes alone do not establish dependencies: inspect the product’s catalog and metadata and confirm the intended application state.
- Identify the full backup associated with the differential.
- Check that both backup sets are accessible and valid.
- Review additional application requirements, such as logs where applicable.
- Restore in isolation and verify the resulting recovery point.
What to distinguish
| Element | Role |
|---|---|
| Full | Base containing the selected protected data. |
| Differential | Cumulative changes since its full base. |
| Incremental | Changes since a previous point according to the product. |
| Restore | Base and selected point, plus application-specific requirements. |
Limits and precautions
Fewer required files do not guarantee a faster restore in every environment. Preserve the base for as long as dependent recovery points are retained. The example describes a conventional differential scheme; use the product documentation to determine the exact recovery sequence and consistency requirements.
Official documentation
Related terms
Key takeaway: A differential backup contains cumulative changes since its full-backup base and differs from an incremental backup.