Shrinking a SQL Server Transaction Log

The transaction log for that database contains all transactions since the last transaction log backup, or the last time it was switched from simple recovery mode. Execute the following to get the definitive answer as to why SQL Server can not truncate the log and subsequently why the log is growing.

SELECT  d.Name
        ,d.log_reuse_wait_desc
FROM    sys.databases d
ORDER BY
        d.name

If you want point in time recovery then leave the DB in the full recovery model and take frequent log backups. Each log backup will contain all transactions since the last log backup. The log backup process is also responsible for clearing the log and marking the space for reuse i.e. the next transaction made in the DB will be written to the start of the truncated log in a circular fashion. This backup and reuse of the log is what prevents the log file from growing.

If you are not interested in point in time recovery and want to simplify the administration of the database. Then set the database to the simple recovery model and do not take t-log backups. SQL Server will automatically truncate the transaction log after each transaction is committed. Meaning that once the transaction has been committed to the log the record is overwritten by the next transaction etc.

Either way, once you've made one of these two decisions you can then shrink the log file down to a more reasonable size. Note ideally you want to make it large enough so it doesn't grow but not so large that you'll need to shrink it again. Also note that you can not shrink the active part of the log.

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You might also find the 'disk usage' report returned by right clicking the DB in Object Explorer > Reports > Standard reports > 'disk usage' useful for returning the free space in the t-log.

I also recommend that you Google why it's so important to keep the log chain intact from a DR point of view, and how switching from full to simple breaks the chain leaving you exposed to data loss.

Seeing as we take regular full db backups at midnight, would it be safe for me to temporarily put the database into Simple Recovery Mode (after one of these backups has run), shrink the log file to reclaim (virtually all of) the space and then put it back in to Full Recovery with the backup strategy mentioned above?

Yes, it would be safe provided that you interfere with no transactions when you do this, such as a late night load. In general, if a database should be in full recovery mode, you want regular T-Log backups. This will reduce the problem you're facing. I write "in general" because in some cases, I've seen people set a database to full without knowing why they did it. Let's assume this isn't that case.

You may want to consider why the log is this size relative to the database size, though. A 500MB database with a 116GB log seems very out of proportion for a one time event. I would suggest monitoring what's happening on the database to see how it arrived at that size in the first place.