(Check out my Pluralsight online training course: SQL Server: Logging, Recovery, and the Transaction Log.)
Here’s an interesting situation that sometimes crops up: you’re performing log backups, your transaction log isn’t growing, and yet the value of log_reuse_wait_desc in sys.databases for your database says LOG_BACKUP.
What’s going on?
I was actually just teaching this stuff today here in our last class of the year in Chicago.
Firstly, what is log_reuse_wait_desc? It’s a field in sys.databases that you can use to determine why the transaction log isn’t clearing (a.k.a truncating) correctly. Usually you query it when a log file is growing and you want to know why, using code like:
SELECT [log_reuse_wait_desc] FROM [master].[sys].[databases] WHERE [name] = N'Company'; GO
You can read about all the possible values in the Books Online topic Factors That Can Delay Log Truncation.
One important thing to bear in mind is that the value returned is the reason that log clearing did not clear any VLFs (Virtual Log Files – see Understanding Logging and Recovery in SQL Server) that last time that log clearing was attempted. For instance, you may see a value of ACTIVE_BACKUP_OR_RESTORE but you know that your full backup has completed. This means that the last time log clearing was attempted, the backup was still running.
Back to the original question. If you have a transaction log that is not growing, and you’re taking regular log backups, but the log_reuse_wait_desc remains LOG_BACKUP, this is because zero VLFs were cleared when the previous log backup was performed.
How can that happen?
Imagine a database where there’s very little insert/update/delete/DDL activity, so in between your regular log backups there are only a few log records generated, and they’re all in the same VLF. The next log backup runs, backing up those few log records, but it can’t clear the current VLF, so can’t clear log_reuse_wait_desc. As soon as there are enough changes in the database that the current VLF fills up and the next VLF is activated, the next log backup should be able to clear the previous VLF and then the log_reuse_wait_desc will revert to NOTHING. Until the next log backup occurs and isn’t able to clear the current VLF, in which case it will go back to LOG_BACKUP again.
So LOG_BACKUP really means “either you need to take a log backup, or the log records that were backed up were all in the current VLF and so it could not be cleared.”
Hope this helps!