What the VLF
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This document summarizes a presentation about Very Large Filesystems (VLFs) in SQL Server. It provides background on the presenter and defines a VLF as a virtual log file that breaks transaction log files into smaller chunks. It explains that too many or too few VLFs can cause performance issues and provides steps on how to check the number of VLFs and correct any issues through manual resizing of the transaction log file. The key points are that the optimal number of VLFs depends on factors like log throughput and backup frequency and finding the right balance is important for performance.
What the VLF
- 1. What the VLF? Mike Hillwig Boston, MA
- 2. Mike Hillwig SQL Server DBA Working with SQL Server since SQL 7 Senior DBA at hosting division of a financial software company owned by a large financial institution Resume includes Acme Packet, Shawmut Design and Construction, Equitable Resources
- 3. Mike Hillwig Owned by Two Pugs Avid Cook Lover of Blue Cheese And Bacon Geek at Heart
- 4. Obligatory Social Networking Slide mikehillwig.com sqlserverpedia.com twitter.com/mikehillwig anysocialnetworkingsite/mikehillwig
- 5. Housekeeping Blog post at mikehillwig.com/go/vlf Please complete your session evaluations.
- 6. Beware of the Blogs There is some amazing advice out there. But… Anybody can put bad advice on the internet Trust people you know Assume I’m an idiot I don’t trust people who say “ALWAYS” or “NEVER” Test everything in your own test environment first.
- 7. What is a VLF? Very Large Filesystem A Virtual Log File Construct within a transaction log file Breaks your transaction log files into smaller chunks
- 8. I am not THE Expert Kimberly Tripp from SQLSkills wrote THE definitive blog post on VLFs. Linked from my post at mikehillwig.com/go/vlf
- 9. Why do we care? Too many VLFs can cause performance problems with DML Too few VLFs can cause a performance problems with transaction log backups
- 10. How did this happen? Most frequent cause is transaction log files that grow in small increments. Are you growing your transaction log files by 10%? If so, you probably have a high number of VLFs.
- 11. How will I know? Cue the Whitney Houston reference DBCC LOGINFO WITH TABLERESULTS
- 12. Demo
- 13. The Goldilocks Problem This one has too many This one has too few This one is just right. And there is a just right for your environment.
- 14. But how do we know what’s just right? You have to baseline and test Know that thousands of VLFs may be too many Know that a handful may be too few Number of VLFs will vary based on the size of your transaction log file
- 15. Does Size Matter? Yes. But size isn’t everything. Look at throughput And look at the size of your VLFs, not just the number And the frequency of your transaction log backups is a factor as well.
- 16. This is a Balancing Act Amount of throughput Size of the VLFs Frequency of transaction log backups
- 17. How do we fix it? Note how big our transaction log file is today Shrink it down Manually grow it. Be careful not to grow it to more than 8 GB at a time, though. This could lead to HUGE VLFs.
Editor's Notes
- #7: Good advice out there, but…Lots of outdated information. Do DB servers really need 2x RAM for PF?