Transaction Log Backups for the Accidental DBA

The Next Quest

In this installment of SQLCoOp, we are writing about backups. I have chosen to focus on Transaction Log Backups. Why? Well, at one of my former jobs, I found myself in the role of Accidental DBA. The company did not have automated backups. They were manually and infrequently performed. I knew this needed to be changed. And this is where my post begins.


Manual and Frequent Backups

Since I wasn’t a DBA, I started small. I performed manual full backups of our databases often.  I made sure that our backups were stored on a different LUN than our data files. It was working great…until it wasn’t. One day everything stopped.

Lessons Learned

I learned several things that day.

1. The difference between the Full Recovery Model and the Simple Recovery Model is the Transaction Log being generated. We were using a Full Recovery Model because I wanted transactions to be logged.

2. Creating a full backup does not affect the transaction log at all. You have to also create a backup of the transaction log. If you don’t, it will continue to grow until it fills the entire drive. And then your database will stop.

3. If you’ve created a dedicated drive for your log files and they use all the space, then your database will stop. You can’t grow the drive without emptying the drive first.

4. After (finally) getting a backup of the transaction log, the Log file will not change in size, even though it is “empty”. You will need to shrink it.

You see, log files can grow as more space is needed. When a backup of the log file occurs, the log file will stay the same size, but the data inside will be truncated. Once the log file uses all the space again, it will need to grow again. This is normal. The log file only needs to be shrunk when it is taking up the entire drive. Read this article for the technical explanation of how log files work.

5. If you get stuck and no one in the office can help, then go to twitter to ask for help. Use the hashtag #SQLHelp with a short synopsis of why you need help. I spent three hours fixing the situation with the aid of a SQL Community member. Why three hours? Well the drive was full and there was no way to get more space.

Tools For Next Time

Now I am armed with some tools that I can use to help prevent finding my log file drive full and tools to fix the problem when it does happen again.

1. The drive location of the log files needs to be monitored and an alert needs to be setup to email/text you when the drive is almost full. Make sure to test the alert. It won’t do you any good if it can’t get to you.

2. Set the maximum log file size to a specific value and the value needs to be smaller than the drive the log file is residing in. Paul Randal (b|t) has a great article called Choosing Default Sizes for Your Data and Log Files that talks about how to choose log file sizes.

The database will still stop if that limit is reached, but providing the drive isn’t full, you’ll be able to increase the size of log file so that the database can function again. Then you can fix the problem that caused the log file to fill up.

3. Create a large unimportant file. Name it something like ‘DeleteMeInAnEmergency.txt’ and put it on the drive where your log files are stored. This will guarantee that you will have extra room on your drive when a log file fills the drive. You can then move this unimportant file to another drive which will free up space instantly. You can get your database running again and then fix the problem that caused the log file to fill up.

4. Bookmark, memorize, or get a tattoo of this link from Glenn Berry (b|t) called How to Diagnose and Correct a “Runaway” Transaction Log. It has the steps to fix the log file and make it happy again. While you are there, check out his diagnostic queries to learn more about your database servers.

5.  Setup an automated backup plan. This can easily be done with Ola Hallengren’s (b) Backup Maintenance Solution (and it’s free). It is really easy to setup and configure to meet your needs. While you are there, you can also check out his Index Maintenance Scripts. They are wonderful.

NOTE: Always test code from the internet in development and QA before deploying to production.
Don’t Stop Yet

Don’t forget to check out these blog posts by the rest of the SQL CoOp team on the subject of Backups:

To follow our quest for SQL knowledge through this collaborative project, follow the #SQLCoOp tag on Twitter.

See you next time!!

On a SQL Collaboration Quest

Four SQL professionals gathered from the four corners of the world to share their SQL knowledge with each other and with their readers: Mickey Stuewe from California, USA, Chris Yates from Kentucky, USA, Julie Koesmarno from Canberra, Australia, and Jeffrey Verheul from Rotterdam, The Netherlands. They invite you to join them on their quest as they ask each other questions and seek out the answers in this collaborative blog series. Along the way, they will also include other SQL professionals to join in the collaboration.

Original Post: On a SQL Collaboration Quest

One comment

  1. SQL Server says:

    Backups – They Are Needed, Who Knew?

    Backups are essential for a successful business model. That statement may or may spark some topics for

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