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Let's say I have a 'MyDB' SQL Server 2005 database (simple recovery) in which I do a Full backup on Sunday, and Differentials every other night

BACKUP DATABASE [MyDB] TO DISK = N'c:\Database Backups\MyDB\MyDB_Full.bak'


BACKUP DATABASE [MyDB] TO DISK = N'c:\Database Backups\MyDB\MyDB_Diff.bak'

What does the differential backup process use to decide what data gets backed up on the differential nights? Does it need the mydb_full.bak file to do its business?

If I wanted to save disk space, could I zip up the mydb_full.bak file to a .zip file after it's created without adversely affecting the differential backups, and if I needed to restore, just unzip the full backup before starting?

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up vote 7 down vote accepted

No - differential backups don't use the full backup file itself as a reference. You can (and should!) safely move your full backup dump to another machine or whatever you like.

SQL Server stores internally a bitmap of dirty extents (parts of the database which have changed since the last full backup), and when you run a differential backup, it consults the bitmap and only writes those changed parts of the database to the backup.

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Just curious, is it possible and are there any advantages to turning this feature off? – Sean Howat Apr 6 '10 at 21:40
Not possible as far as I know... – James Apr 6 '10 at 22:11
You can do a backup that doesn't reset the tracking of changed extents (useful for an ad-hoc backup on top of the normal scheme). – Richard Apr 7 '10 at 9:52
Yep - "copy only" backup – James Apr 7 '10 at 15:28
cool didnt know that – Nai Apr 21 '10 at 18:35

I just did a quick test on this by first creating a full backup of one of my test dbs, deleting it, and then running a differential backup. To my surprise, the differential backup ran fine so I don't think it's going off the last full backup file itself.

So it does seem like you'd be able to zip your full backups without a problem. I am curious to hear exactly where it determines it's start point for differential backups now so I'm hoping someone can enlighten us.

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