[dekel_as]

what is the difference between differential and incremental backup the restore

[Ossian]

The difference is what happens to the Archive bit:

Incremental -- resets archive bit
Differential -- doesnt

To restore you need either:

Full + most recent differential (2 tapes) or
Full + All incrementals in order (>2 tapes)

[JeremyW]

To elaborate ('cause I got a few minute free time...)

The archive bit is set on a file when its created or modified. (or you can manually set it)

Differential will backup everything that has that archive bit set and during the process it leaves it set.

Incremental will backup everything that has that archive bit set (just like the differential) but will also clear the archive bit in the process of backing up the file. Having the archive bits cleared means that it won't backup the files next time the job runs unless they're modified.

And just for reference:
Normal will backup everything regardless of the archive bit and will clear the archive bit on any file that has it set.
Copy will backup everything regardless of the archive bit and will not change the archive bit's state.

[sorinso]

Originally Posted by Ossian

The difference is what happens to the Archive bit:

Incremental -- resets archive bit
Differential -- doesnt

To restore you need either:

Full + most recent differential (2 tapes) or
Full + All incrementals in order (>2 tapes)

And to further elaborate:

Full + Differentials method:
- longer backup window, because you back up every day more and more files, including those you backed up yesterday, and the day before, all since the last full backup;
- from the same reason, it takes more space too;
- fast restore, since you need only two tapes: the last full and the last differential;
- from the same reason, is more reliable (you are not dependent on anything else).
Full + Incremental method:
- smaller backup window, because you back up only the modified files since the last backup;
- more "economical" on the tape;
- to restore, you need the last full and all the incrementals. This means a long restore procedure;
- from the same reason, it is less reliable: if one of the incrementals is corrupt, none of the following will be relevant.

I am using Full+Differential, since I have no problem with the backup window, but need rapid restore procedures (thankfully, never had to restore a full server from the backup).

I hope this will help you decide what is best for you.
Good luck.