Backup Concepts
Appendix C: Backup and Recovery Best Practices 119
Differential Backups
A differential backup records only the data that has changed since the last full
backup of the same part of the data/same data files/same database subset.
Usually, these backups are smaller and faster than full backups, but larger and
longer than Transaction log backups. To restore a database, a differential backup
requires only the last full backup as a prerequisite, and does not need any of the
other differential or Transaction log backups performed between it and the last
full backup. A differential backup is also faster to restore than a Transaction log
backup because transactions do not need to be reprocessed.
Note:
If a database is very active or if a long time has passed since the last full
backup, a differential backup could take as long as a full backup.
For more information, see the topic,
Differential Backup Timing
(see page 119).
Differential Backup Timing
Perform differential backups as a supplement to full backups. Because they are
usually faster and smaller, you can perform them more often than full database
backups. They are also typically more efficient because they require less space
on media and have a briefer impact on database performance than frequent full
backups. Additionally, you can use them to minimize the number of Transaction
logs you need to recover during a restore, because you need to restore only the
Transaction log backups performed since the differential backup.
Differential backups are most beneficial under the following circumstances:
■
A relatively small portion of the data in the database has changed since the
last database backup. Differential database backups are most efficient if the
same data is modified frequently.
■
You are using the Simple Recovery model, which does not permit
Transaction log backups, and you want to perform backups more frequently
than is practical for full database backups.
■
You are using the Full or Bulk-Logged Recovery model and want to minimize
the time it takes to replay Transaction log backups when restoring a
database.
Note:
After you have modified the structure or configuration of the database
(for example, by adding more data or log files or changing the recovery model),
you must perform a full database backup before performing a differential or
Transaction log backup. For more information, refer to
Required Full Backups
(see page 116).
