Prevent SQL Server Outages by Monitoring Transaction Log Growth

I’ve lost count of the times I’ve been called after hours due to a drive filling up. The usual culprit? Transaction file log growth.

Monitoring the growth of your SQL Server transaction log files is crucial for maintaining database performance and ensuring system reliability. Unchecked transaction log growth can lead to disk space issues and system outages. Let’s take a look at some of the causes of transaction log file growth, why it’s important to monitor, and how you can use the sp_whoisactive procedure to keep an eye on it.

What Causes the Transaction Log File to Grow

The transaction log file records all transactions and database modifications made by each transaction. Several factors can contribute to its growth:

• Long-Running Transactions: Transactions that take a long time to complete prevent the log from truncating because SQL Server needs to maintain the log records until the transaction is committed.
• Recovery Models:
  • Full Recovery Model: In this model, the transaction log will not truncate until a log backup occurs. If log backups are not taken regularly, the log file will grow indefinitely.
  • Bulk-Logged Recovery Model: Similar to the full recovery model, it is used for bulk operations. Log growth can still be an issue if backups aren’t scheduled appropriately.
  • Simple Recovery Model: Although SQL Server automatically truncates the log in this recovery model, you must still watch for transactions that are making large changes all at once. For instance, if you update millions of rows in one transaction, this could be enough to cause the log file to grow.
• Lack of Log Backups: Without regular log backups in the full or bulk-logged recovery models, the log cannot truncate, leading to continuous growth.
• High Transaction Volume: A sudden spike in transaction activity can cause the log file to grow rapidly to accommodate the increased volume.
• Replication and Availability Groups: Features like availability groups and replication can delay log truncation until the data is sent to the replicas or replicated databases. Seeing the log file continue to grow even after taking a log backup? Check the log_reuse_wait_desc column in sys.databases. It’ll indicate why.
• Open Transactions: Uncommitted transactions hold on to log space because SQL Server needs the log records to roll backup if necessary. Who hasn’t started their session with “begin transaction” and gone home for the weekend (forgetting the commit)?

Why it Matters

The impact can be massive, from crashing applications to filling up disk space and affecting all other databases on the same drive.

• Disk Space Consumption: Uncontrolled log growth can consume all available disk space, leading to application failures and downtime.
• Performance Degradation: Large log files can slow down database operations, including backups and recovery processes, affecting overall system performance.
• Recovery Time Objectives (RTOs): In disaster recovery scenarios, large transaction logs can increase the time it takes to restore databases, impacting business continuity.
• Maintenance Challenges: Managing and maintaining oversized log files can be cumbersome, requiring more time for routine operations like backups.
• Risk of Data Loss: If the disk runs out of space due to log growth, new transactions cannot be logged, leading to potential data loss or corruption.

How to monitor it using sp_WhoIsActive

sp_whoisactive is a powerful stored procedure developed by Adam Machanic that provides a comprehensive view of the current activity on your SQL Server. It can be used to monitor transaction log growth.

Here’s how you can use it:

Installation

First, download the script from https://github.com/amachanic/sp_whoisactive and run it on your instance. I usually place it in a database named DBA vs. master.

Monitoring Active Sessions

Run the procedure to see all active sessions:

USE DBA
GO
EXEC sp_whoisactive

Identifying Long-Running Transactions

To focus on transactions that might be causing log growth, look for sessions with high values in the tran_log_writes column, which shows the number of log records written by the session. You’ll need to set the @get_transaction_info to 1 to include transaction details.

EXEC sp_whoisactive @get_transaction_info = 1

This will add columns related to transaction duration and log usage.

Ordering by log writes

If you’re interested in sessions consuming the most log space, order the results accordingly:

EXEC sp_whoisactive @get_transaction_info = 1, @sort_order = '[tran_log_writes] DESC'

Automating Monitoring

Schedule sp_whoisactive to run at regular intervals and log the output to a table for historical analysis or to send an alert if certain thresholds are met. This can help you identify patterns over time (i.e., Mike usually starts a new session every Friday evening and leaves for the weekend.). The following code will create a table, dbo.WhoIsActiveLog, with the appropriate schema. The second statement will then execute sp_whoisactve and store the results within this table.

--create the dbo.WhoIsActiveLog table
DECLARE @schemaDefinition VARCHAR(MAX)
EXEC sp_WhoIsActive 
    @find_block_leaders = 1, 
	@get_transaction_info = 1,
	@get_locks = 1,
	@return_schema = 1,
	@schema = @schemaDefinition OUTPUT

SELECT @schemaDefinition =  REPLACE(@schemaDefinition,'<table_name>','dbo.WhoIsActiveLog')
EXEC (@schemaDefinition)
GO
EXEC sp_WhoIsActive 
    @find_block_leaders = 1, 
	@get_transaction_info = 1,
	@get_locks = 1,
	@destination_table = 'dbo.WhoIsActiveLog'

Once you identify sessions causing excessive log growth, you can investigate the underlying queries or processes:

• Optimize Queries: Rewrite inefficient queries to reduce transaction time.
• Adjust Recovery Models: If appropriate, and point-in-time recovery isn’t required, consider switching to the simple recovery model. However, this won’t help if it’s a long running transaction making many changes simultaneously.
• Schedule Regular Backups: Ensure that log backups are taken frequently to prevent log file bloat.
• Kill Problematic Sessions: As a last resort, terminate sessions that are causing issues, but be cautious of potential data loss.

Want to work with The SERO Group?

Want to learn more about how The SERO Group helps organizations take the guesswork out of managing their SQL Servers? Schedule a no-obligation discovery call with us to get started.