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I’m looking for some clarity on the SQL Server log file. I have a largish database (2GB) which lately wasn’t backed up for whatever reason. The log file for the database grew to around 11GB which from my understanding is all the transactions and statements that occurred in the database.
My questions:
What causes the database log file to be flushed? What does ‘flush’ actually mean? What are the consequences of doing a file shrink or database shrink on a large log file?
Once you backup the transaction log, those transactions are truncated from the log but the space used by the operation is not automatically recovered. If you are doing regular transaction log backups, this can be a good thing. The assumption being that the space being used for transactions and will be needed again in the future. Continuously shrinking the transaction log can be bad for performance since when needed again the database will need to expand the log.
So to fix your issue, first do a full backup and a transaction log backup. You don’t need to go to Simple mode or single-user. Then shrink the database and set up a transaction log backup every few hours. On my servers I do it every 10 minutes, but it entirely depends how often one needs for their environment. Monitor how the log size changes, be sure to leave enough room such that it does not have to expand regularly, plus a little extra for fun.
When you do shrink the database use DBCC SHRINKFILE rather than DBCC SHRINKDATABASE since the latter will shrink the entire database, not just the log file. Also you don’t have as much control over how much space is being recovered.