BE 8.6--Differential backups shrinking?

[slzusd]

Why?

I am running Veritas Backup Exec v8.6 on a Windows 2000 Server with Service Pack 3. I am running two backup jobs--a full backup and a differential backup (not an incremental). The full backup is running weekly and is generally around 35-GB in size (growing steadily, of course). The differential backup job usually starts around 1.2-GB and grows as expected. However, on several occasions, the diffential job actually decreases in size.

Does anyone know why a differential job would actually decrease in size between full backups?

Has this happened to anyone else?

 

[SQLBill]

Yes, if I understand your question correctly.

If this is what you mean:

Full Backup - 35GB
Differential - 1.2 GB
Full - 36 GB
Differential - .7 GB
Full - 37GB
Differential - 1.2 GB

A differential backup is a backup of the changes to the file(s) since the last full backup. So, there may not have been as many changes as before.

-SQLBill

 

[slzusd]

SQLBill,

Thanks for the reply. I understand that incrementals backup files that have changed and reset the archive bit, but differentials do not reset the archive bit. Thus, incrementals can vary widely in size. However, differentials should always be increasing in size, since they back up all files since the last full backup (when the archive bit was reset).

Here's an example of my recent backups:

Friday -- Full backup = 34,572,008,802
Monday -- Diff backup = 1,202,166,243
Tuesday -- Diff backup = 3,920,277,311
Wednesday -- Diff backup = 3,578,922,315
Thursday -- Diff backup = 3,751,718,693

I also had the same problem on a different server:

Friday -- Full backup = 39,142,584,187
Monday -- Diff backup = 2,795,106,476
Tuesday -- Diff backup = 3,449,207,005
Wednesday -- Diff backup = 3,268,755,611
Thursday -- Diff backup = 3,544,525,036

Maybe this is not a problem, but it does concern me. I've increased logging to include everything, so I guess I'll have to compare file by file...