SQL Server Backup and Restore - Simple Talk

Chapter 7: Differential Backup and Restore
Will differential backups always be smaller than full backups?
For the most part, if you refresh your base backup on a regular schedule, you will find that a differential
backup should be smaller in size than a full database backup. However, there are situations where a
differential backup could become even larger than its corresponding base backup, for example if the base
backup is not refreshed for a long period and during that time a large amount of data has been changed
or added. We'll discuss this further shortly, in the Possible issues with differential backups section.
Let's assume that the same moderately-sized production database, mentioned above,
has a backup strategy consisting of a weekly full backup, hourly transaction log backups
and daily differential backups. I need to retain on disk, locally, the backups required to
restore the database to any point in the last three days, so that means storing locally the
last three differential backups, the last three days' transaction log backups, plus the base
full backup. The full backup size is about 22 GB, the log backups are, on average, about 3
GB each, and 3 days' worth of differential backups takes up another 3 GB, giving a total of
about 28 GB. If I simply took full backups and log backups, I'd need almost 70 GB of space
at any time for one database.
Deciding exactly the right backup strategy for a database is a complex process. We want
to strive as far as possible for simplicity, short backup times, and smaller disk space
requirements, but at the same time we should never allow such goals to compromise the
overall quality and reliability of our backup regime.
Differential backup strategies
In what situations can the addition of differential backups really benefit the overall
database recovery scheme? Are there any other cases where they might be useful?
208