Differential backups in SQL Server 2008 R2 I created a maintenance plan to backup all user databases (full) and set them to expire after three days.

This has proved to be cost inefficient in regards to disk space so I want to switch to differential backups.

I then created a maintenance plan to backup all user databases (differential) and set them to expire after seven days.

I had thought the first time this job ran that the first backup would have to be full but it's not, it's significantly less than what a full backup should be.

I thought it would work like this:

Day 1: Full

Day 2-7: Differential

Then on day one it would do another full backup and start all over.

Am I just misunderstanding how differential backups work?

I mean what did the plan back up differentially if it didn't do a full backup first?

Thanks!
Differential backups include data that has changed since the last full backup (ignoring any full backups taken with the COPY_ONLY option).

If you take a differential backup immediately after a full backup, it will be very small, as little (or no) data will have changed. As time goes on, the differential backups will become larger and larger, until you do another full backup, which resets the differential change map.

Note that you must still have the last full backup taken prior to the differential backup, or the differential backup will be useless.