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differential backup

can someone give me an idea of how big a 30gb database differential backup can be.
It’s dependant on how much data is added/changed in the database between your backups. You have to test it out for your specific environment. /Argyle
data changes quite often in the database.
It depends how is it changing. Basically the diff backup is marking each extent in which a modification occured since the last full backup, and copying all those marked extents. If you have a lot of modifications on the same data (meaning, you are updating more or less the same lines many times) than it should not be too big. If you have modifications spread on many extents than backup will be bigger. It also depends on the indexes. If you have, for example, a table with a clustered index on surname, it is possible that not too many inserts will casue a copy of all the table. A clustered index on an inc. field will only append to the table therefore in general will copy less extents. Bambola.

If you take full backup and say after 30 mins if you take using DIFFERENTIAL clause or do transaction log backup, you will know the size of the file easily. HTH _________
Satya SKJ

thanks all

Also it needs to be observed if there has been some index creation in during the duration. This may affect the differential backu size as well. Gaurav