Transaction Log Growth During Index Rebuilds

Of all the problems that can cause a database transaction log to fill up, perhaps one lesser-known cause is a large clustered index rebuild. Let’s look at a scenario where this issue could cause a headache.

Too Much of a Good Thing

Here’s our scenario: you’re woken up by an alert in the middle of the night that a database transaction log is full. You wonder if transaction log backups have been running. You check job history and confirm that yes, transaction log backups are still running and completing. No out-of-the-ordinary transactions or jobs are running either, so what gives? If you have had a large influx of data, SQL Server might be maxing out the log file to keep up with rebuilding a clustered index.

Let’s say you have an auditing table in place. Someone notices that it’s not auditing enough information. Some tweaks are made, and now the audit table is flooded with data tracking every mouse click and login. That’s great…but that type of change and data increase comes at a cost.

If you’re rebuilding the clustered index, you’re going to need the available disk space for the log file to support a copy of that entire table. You might think backing up the transaction log every minute or every few minutes will keep the log size down, but that won’t help in this case. Once the rebuild starts, it’s going to be one transaction, so you (and your log file) are in it for the long haul.

What’s the Fix?

Make sure you’re not rebuilding the index if you don’t need to. If index fragmentation is over 80%, then it’s worth rebuilding. If it’s in the 50%-80% range, then reorganizing is at least worth considering.

Let’s assume you’re running in FULL recovery model. Do you need to be FULL recovery model 24/7, including the time when index maintenance executes? Is point-in-time recovery essential? If not, it may be beneficial to switch to SIMPLE or BULK LOGGED recovery model during index maintenance so the log doesn’t fill up next time.

That leads us to the last “fix” I’ll mention, and it’s a bit obvious: add disk space. Add enough space to handle the increased amount of data and adjust for what may be a new normal.

More Data, More Problems

Changes in the amount of data writing to a database can lead to unexpected issues. If you’re aware of recent data changes and your transaction log is running low, review your indexing strategy.

Thanks for reading!