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Performance

Blog posts relating to SQL Performance Tuning.

SQL Server Performance Tuning

Enhancing Database Efficiency

SQL Server tuning is the process of optimizing the database’s configuration, queries, and infrastructure to ensure the system runs as efficiently as possible. The goal is to reduce resource usage, increase query speed, and improve overall system responsiveness. Performance tuning involves analyzing server performance metrics, identifying bottlenecks, and implementing solutions that range from hardware upgrades to query rewriting. These optimizations are crucial for maintaining a smooth user experience and supporting business-critical applications.

A key aspect of performance tuning is query optimization. Poorly written SQL queries, missing indexes, or incorrect indexing strategies often lead to slow query execution. By using tools such as execution plans and SQL Profiler, DBAs can identify and correct inefficiencies in SQL code. Techniques like index tuning, query rewriting, and partitioning can drastically reduce query execution times. Regularly updating statistics and reorganizing or rebuilding indexes further enhances query performance and ensures accurate query plans.

Beyond queries, performance tuning also focuses on the SQL Server environment itself. Configuring server settings, adjusting memory allocation, and fine-tuning I/O operations play a significant role in optimizing database performance. Monitoring tools like Database Health Monitor (available at DatabaseHealth.com) provide insights into server health and potential issues. Stedman Solutions offers comprehensive SQL Server performance tuning services, combining years of experience with specialized tools to ensure your SQL Server environment performs at its peak. Learn more about our managed services at Stedman Solutions.

Instant File Initialization (IFI)

One way to improve performance on SQL Server is with IFI or Instant File Initialization.

Normally files are zeroed out on a database growth during an AUTOGROW, RESTORE, CREATE DATABASE or ALTER DATABASE. This is done by SQL Server when the file grows, it runs through that file and writes zeroes to the entire new allocation in the file. The zeroing process can take a great deal of time, the Instant file initialization process skips this zeroing, and just allocates the file. This works since SQL Server will just write each 8k page to disk as they are used, thus overwriting the uninitialized file.

Running some tests on a local virtual machine running SQL Server 2012 (similar results tested on SQL Server 2014 and SQL Server 2016), here is what I found.

  • Creating a new database (10GB file size) took 2 minutes and 6 seconds, with IFI enabled, it took just 4 seconds.
  • Autogrow of a data file witha 1GB growth size took 13 seconds without IFI, with IFI it took less than a second.

For IFI to work, the user account that SQL Server is running as needs the “Perform volume maintenance tasks” policy to be enabled.Read More »Instant File Initialization (IFI)

Duplicate Indexes, What a Waste

Today on the tuning minute on the SQL Data Partners Podcast, we discussed duplicate indexes, which lead me to think more about and and write this post.

You know there are many different ways of doing things in SQL Server, and often times you can argue that one way or the other is better, and given the right situation anything might be a good idea. However duplicate indexes are a different story.

When I talk about duplicate indexes, what I mean is 2 or more indexes on the same table that are exactly the same columns.  Something like this:


CREATE NONCLUSTERED INDEX [IX_LastName] ON [dbo].[Customer]
(
 [Lastname] ASC
);

CREATE NONCLUSTERED INDEX [dta123123123_LastName] ON [dbo].[Customer]
(
 [Lastname] ASC
);




Two indexes on exactly the same column. There is nothing to be gained here.


Read More »Duplicate Indexes, What a Waste

The Use of WITH NOLOCK Query Hint

TWITH NOLOCKhere is a great deal of confusion about the WITH NOLOCK query hint. The following is a transcription of a conversation that I had on the SQL Data Partners Podcast #57 about the WITH NOLOCK hint, and some of the misconceptions about it.

WITH NOLOCK Hint

Carlos: So the next on is the WITH NOLOCK hint and I think this is on the list, ultimately, because there’s a lot of misinformation out there.

Steve: Absolutely, yes. The NOLOCK hint is one of my peeves on SQL Server, actually. I see it used a lot and really 99 percent of the time I see it used, people think that it’s doing something different than what it does. So, I heard the statement, “But I want to run a query in the production system but not impact or block anyone else. Shouldn’t I just use NOLOCK?” And the answer there is NO. The NOLOCK hint tells SQL Server to ignore other people’s query locks. Basically, to read dirty or uncommitted data at that point, which can lead to missing rows or phantom rows or data showing up in results. But it doesn’t do anything to stop the locking or the blocking in any way on the query that’s calling it. So it’s the equivalent of saying read uncommitted on a specific table that it’s referencing.

Read More »The Use of WITH NOLOCK Query Hint

How Many VLFs is Too Many?

Here is an error that popped up in the SQL Server error log today:

5/6/2016 10:10:10 AM spid22s Database [DatabaseName] has more than 10000 virtual log files which is excessive. Too many virtual log files can cause long startup and backup times. Consider shrinking the log and using a different growth increment to reduce the number of virtual log files.

Virtual Log Files (VLFs) are part of the SQL Server log file. When space is allocated in the log due to growth, that new chunk of log is broken up into Virtual Log Files

After looking further into this server, the VLF count turned out to be around 163,000. That is certainly the highest VLF that I have ever seen.

My personal threshold for too many VLFs is usually around 200 with my preference to be less than 100 VLF files. Others may have their preferences, they will all be far less than 10,000, and certainly less than the 163,000 that I had the opportunity to see today.

Read More »How Many VLFs is Too Many?

Updating SQL Server Statistics

Updating SQL Server statistics may not be as obvious as it may sound.

IUpdating SQL Server Statisticsmagine this scenario. I invite you over for dinner and a game of Scrabble. After dinner, I start to do the dishes, while you set up the game. Rather than just doing the dishes that are dirty from our dinner, I decide to take all the dishes from the shelves, and drawers in my kitchen. I create a gigantic pile of all of the dishes, both clean and dirty on the kitchen counter. You get the game setup, and we start playing the game. Between turns, I jump back into the kitchen, and wash a few dishes, then on my turn, I jump back into the game and play my turn, then back to the kitchen again. During the game you are constantly waiting for me to jump back into the game to make my move. You could see how long this would draw out the game with me washing all the dishes, even the ones that were already clean. 4 hours later, I finally finish the dishes, and shortly thereafter we finish the game. It is unlikely, even if you love to play Scrabble that you would ever return to my house for dinner and a game again. Tomorrow night, when I have dinner with my family, I do the same thing, and every night after dinner I wash all the dishes in the kitchen, even the clean ones. You can see how wasteful this sounds, and perhaps some people would even label me as OCD or something worse in this scenario.

Although this is obviously the wrong way to do things, often times we end up doing the equivalent of this in our SQL Server maintenance tasks.

Read More »Updating SQL Server Statistics

Visualizing Log File VLF Sizing

One of my favorite queries this week is the following query that creates a text based bar chart to quickly help visualize the VLF files on any database log file. In the last month I have given the “TempDB Do This and Don’t Do That” presentation twice, once at the Bellingham SQL Server users group, and another time at SQL Saturday Redmond. One of the questions that came up after the users group meeting was around an easy way to visualize the Virtual Log Files. Its one thing to just get a count, but to see the sizing of each VLF is helpful to understand how VLFs work.

If you are not familiar with VLFs, these are Virtual Log Files, or the chunks that make up your entire log file. When your log file grows, VLFs are added to help manage the file space. For the log growth, between 4 and 16 VLFs are added for each growth of the log file (except on SQL Server 2014 and newer, which modifies the sizing algorithm a bit).

Read More »Visualizing Log File VLF Sizing