Dot Net Rules

Looking into Temp database usage in SQL Server

I have been delivering a certified course in MS SQL Server 2012 recently and I was asked several questions about Tempdb usage and temporary objects.

In this post I will try to shed light on this particular issue.

Temporary tables and table variables make use of the system tempdb database.

There is only one tempdb system database per SQL Server instance so if there is a huge usage of temporary objects in this database it can be the point of contention.

When you create an entry in database it needs to allocate space.This is also valid for the tempdb database.There are three types of pages involved in the allocation process in the tempdb data file:

• Page Free Space (PFS)
• Shared Global Allocation Map (SGAM)
• Global Allocation Map (GAM).

When there is a great page allocation contention in tempdb, the whole allocation process can suffer and we can experience with PAGELATCH waits.

In order to address the issue above, you can have a number of tempdb data files that are equal to the number of cores.For example if you have a system with less than 8 cores e.g 6 you should add/set up 6 data files for the tempdb.If you have a system with more than 8 cores you should add 8 data files for the tempdb and then if the contention is still big you can add 4 more data files.

By saying cores in this post I mean logical cores and not physical cores.So if you have 8 physical cores, then you have 16 logical cores and 32 logical cores if hyper-threading is enabled.

I will  provide some demos in order to show you what tempdb contention might look like and what are the main wait latches that occur.

I have installed SQL Server 2012 Enterprise edition in my machine but you can use the SQL Server 2012/2008 Express edition as well.

I am connecting to my local instance through Windows Authentication and in a new query window I type (you can copy paste)

In this snippet, I create a new database and then create a new table with some constraints

CREATE DATABASE mytempdbcontention
GO

USE mytempdbcontention;
GO
 CREATE TABLE dbo.footballer
 (
 [FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
 [lastname] [varchar](15) NOT NULL,
 [firstname] [varchar](15) NOT NULL,
 [shirt_no] [tinyint] NOT NULL,
 [position_played] [varchar](30) NOT NULL,

);

GO

ALTER TABLE dbo.footballer
ADD CONSTRAINT CK_Footballer_Shirt_No
CHECK (shirt_no >= 1 AND shirt_no <= 11)

GO

ALTER TABLE dbo.footballer
ADD CONSTRAINT CK_Footballer_Position
CHECK (position_played IN ('goalkeeper','defender','midfielder','striker'))
GO

Now I need to populate the table with 50.000 rows. This is the script you need to execute in order to make this happen.

You can download it here. Rename the insert-footballer.doc to insert-footballer.sql and execute the script in a new query window.

Now I need to create a script that will create tempdb contention. This is the script that creates a temporary object- #footballer, populates the #footballer from the footballer table and then selects from it. Finally it drops the temporary object.

USE mytempdbcontention;
GO

SET NOCOUNT ON;
GO

WHILE 1 = 1
 BEGIN

IF OBJECT_ID('tempdb..#footballer') IS NOT NULL

 DROP TABLE #footballer;

CREATE TABLE #footballer
 (
 [FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
 [lastname] [varchar](15) NOT NULL,
 [firstname] [varchar](15) NOT NULL,
 [shirt_no] [tinyint] NOT NULL,
 [position_played] [varchar](30) NOT NULL,

);
 INSERT INTO #footballer
 (lastname,
 firstname,
 shirt_no,
 position_played)
 SELECT TOP 4000
 lastname,
 firstname,
 shirt_no,
 position_played

 FROM dbo.footballer;

SELECT lastname
 FROM #footballer;

DROP TABLE #footballer;
 END
 GO

Now I am going to create a .cmd file where I will create contention to the tempdb.

You can download it here. Rename the temp-sql.cmd.doc to temp-sql.cmd. Make sure you execute it (by double-clicking it).

This will create lots of contention to the tempdb. We need to see exactly what this contention is and the wait latches that have occurred.

Execute the script below.

USE tempdb
go
SELECT session_id, wait_duration_ms,wait_type, resource_description
from sys.dm_os_waiting_tasks
where wait_type like 'PAGE%LATCH_%'

and

resource_description like '2:%'

As you can see from the picture below, I have PAGEIOLATCH_SH wait types.This wait type occurs when a task is waiting on a latch for a buffer that is in an I/O request. The latch request is in Shared mode.

I have one tempdb data file, the default configurations

I have 8 cores in this machine so I will add 7 mores tempdb data files of equal size (MS recommendation).

Execute the script below.

USE [master]
GO

ALTER DATABASE [tempdb]
MODIFY FILE ( NAME = N'tempdev', SIZE = 500 MB ,
FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev2', FILENAME = N'd:\DATA\tempdb2.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev3', FILENAME = N'd:\DATA\tempdb3.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev4', FILENAME = N'd:\DATA\tempdb4.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev5', FILENAME = N'd:\DATA\tempdb5.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev6', FILENAME = N'd:\DATA\tempdb6.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev7', FILENAME = N'd:\DATA\tempdb7.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

Now run the query again and observe the results.

USE tempdb
go
SELECT session_id, wait_duration_ms,wait_type, resource_description
from sys.dm_os_waiting_tasks
where wait_type like 'PAGE%LATCH_%'

and

resource_description like '2:%'

You will see that there are no wait types hence no tempdb contention.

Stop now the temp-sql.cmd file so the contention of the tempdb stops.

Hope it helps!!!