I was investigating why my differential backups were taking almost as long as my full backups and I noticed that the size of the first differential backup after the daily full backup was 2/3 the size of the full backup. However the subsequent differential backups for the remainder of the day only grew in size by about 1%.
Nothing has significantly changed in the usage pattern on the database and historically the size of the first differential backup after the daily full backup was about .5% of the size of the full backup. I've looked at the day over day size of the full backups and they only grow by about 2%.
The daily backup schedule is defined as one full backup at 1AM followed by 5 differential backups between the hours of 7AM and 11:59PM. Typically the full backup takes about an hour and a half to complete and that includes the verify step. The differentials were taking about 20 mins to complete including the verify step, however now they are taking about an hour and ten mins to complete.
I've confirmed the database usage pattern hasn't changed, there are no new jobs that are writing/changing a significant amount of data between the window of the full backup and the first differential backup and I don't seen any file subsystem errors.
With all of what I've confirmed above my question is what would cause the first differential backup of the day to have become so large when the subsequent differentials on the same day only grow by 1%?
- SQL Server 2008r2 Enterprise
- LiteSpeed for SQL Server
- Database is in Simple Recovery mode
- Backup target is a file share on the network