How to demonstrate the fill ratio of separate tempdb files of equal size in SQL Server

This topic reminds of me this little quiz of which jug will fill first.

Pretty much all of the documentation and recommendations out there say to keep the tempdb data files the same size so that the round-robin data flow works properly, i.e. the tempdb data files fill up evenly. This means that the data for a large temp table is actually split across the files and does not reside in one file.

Below is the code necessary to prove this scenario.

I tested this process on Microsoft SQL Server 2012 – Service Pack 1.

If working with a default installation of SQL Server Express The below script should print out the code to generate four equally sized (500 Mb) tempdb data files with no auto growth.

Run the script against the instance, review the print out and then copy/paste and run it against the instance

SET NOCOUNT ON

IF OBJECT_ID('tempdb..#sfs') IS NOT NULL
	DROP TABLE #sfs;
	
DECLARE @TempDbDirectory VARCHAR(MAX)
DECLARE @Sql VARCHAR(MAX)

CREATE TABLE #sfs (
	fileid TINYINT
	,filegroupid TINYINT
	,totalextents INT
	,usedextents INT
	,dbfilename SYSNAME
	,physfile VARCHAR(255)
	);

INSERT INTO #sfs
EXEC ('USE  tempdb; DBCC showfilestats;');

SET @TempDbDirectory = (
		SELECT REPLACE(physfile, 'tempdb.ndf', '')
		FROM #sfs
		WHERE dbfilename = 'tempdev'
		)

SET @Sql = '
USE [tempdb]
GO

DBCC SHRINKFILE (
		N''tempdev''
		,100
		)
GO

USE [tempdb]
GO

ALTER DATABASE [tempdb] MODIFY FILE (
	NAME = N''tempdev''
	,NEWNAME = N''tempdev1''
	)
GO

USE [master]
GO

ALTER DATABASE [tempdb] MODIFY FILE (
	NAME = N''tempdev1''
	,FILENAME = N''' + @TempDbDirectory + 'tempdb.ndf''
	,SIZE = 512000 KB
	,FILEGROWTH = 0
	)
GO

ALTER DATABASE [tempdb] ADD FILE (
	NAME = N''tempdev2''
	,FILENAME = N''' + @TempDbDirectory + 'tempdb2.ndf''
	,SIZE = 512000 KB
		,FILEGROWTH = 0
	)
GO

ALTER DATABASE [tempdb] ADD FILE (
	NAME = N''tempdev3''
	,FILENAME = N''' + @TempDbDirectory + 'tempdb3.ndf''
	,SIZE = 512000 KB
		,FILEGROWTH = 0
	)
GO

ALTER DATABASE [tempdb] ADD FILE (
	NAME = N''tempdev4''
	,FILENAME = N''' + @TempDbDirectory + 'tempdb4.ndf''
	,SIZE = 512000 KB
		,FILEGROWTH = 0

	)
GO

ALTER DATABASE [tempdb] MODIFY FILE (
	NAME = N''templog''
	,FILEGROWTH = 512000 KB
	)
GO
'

PRINT @Sql
Once the script has run restart the server so the changes can take effect.
Once restarted when you look in the SSMS object explorer for the properties of the tempdb you should see a window like below showing the 4 tempdb data files.
tempdbFiles
Because SQL Server has just restarted nothing should be in these files.
You can test this by running the script below. Which return results like this. As you can see GB_Used is 0 for the 4 tempdb data files.
empty Temp Db

/*
Credit for this script goes to:
DAVE TURPIN
http://www.daveturpin.com/2011/07/how-to-drop-a-tempdb-database-file/
*/

-- Is there data in the second file of tempdb?
--drop table #sfs
--drop table #fixed_drives
--drop table #output_table
--drop table #databases
--drop table #dbf
--drop table #fg


IF OBJECT_ID('tempdb..#sfs') IS NOT NULL
	DROP TABLE #sfs;

IF OBJECT_ID('tempdb..#fixed_drives') IS NOT NULL
	DROP TABLE #fixed_drives;

IF OBJECT_ID('tempdb..#output_table') IS NOT NULL
	DROP TABLE #output_table;

IF OBJECT_ID('tempdb..#databases') IS NOT NULL
	DROP TABLE #databases;

IF OBJECT_ID('tempdb..#dbf') IS NOT NULL
	DROP TABLE #dbf;

IF OBJECT_ID('tempdb..#fg') IS NOT NULL
	DROP TABLE #fg;

--------------------------
-- Save result set from showfilestats
--------------------------
CREATE TABLE #sfs (
	fileid TINYINT
	,filegroupid TINYINT
	,totalextents INT
	,usedextents INT
	,dbfilename SYSNAME
	,physfile VARCHAR(255)
	);

------------------------------
-- Save result set from sys.database_files
------------------------------
CREATE TABLE #dbf (
	[file_id] INT
	,file_guid UNIQUEIDENTIFIER
	,[type] TINYINT
	,type_desc NVARCHAR(60)
	,data_space_id INT
	,[name] SYSNAME
	,physical_name NVARCHAR(260)
	,[state] TINYINT
	,state_desc NVARCHAR(60)
	,size INT
	,max_size INT
	,growth INT
	,is_media_ro BIT
	,is_ro BIT
	,is_sparse BIT
	,is_percent_growth BIT
	,is_name_reserved BIT
	,create_lsn NUMERIC(25, 0)
	,drop_lsn NUMERIC(25, 0)
	,read_only_lsn NUMERIC(25, 0)
	,read_write_lsn NUMERIC(25, 0)
	,diff_base_lsn NUMERIC(25, 0)
	,diff_base_guid UNIQUEIDENTIFIER
	,diff_base_time DATETIME
	,redo_start_lsn NUMERIC(25, 0)
	,redo_start_fork_guid UNIQUEIDENTIFIER
	,redo_target_lsn NUMERIC(25, 0)
	,redo_target_fork_guid UNIQUEIDENTIFIER
	,back_lsn NUMERIC(25, 0)
	);

------------------------------
-- Save result set from sys.filegroups select * from sys.filegroups
------------------------------
CREATE TABLE #fg (
	[name] SYSNAME
	,data_space_id INT
	,[type] CHAR(2)
	,type_desc NVARCHAR(60)
	,is_default BIT
	,is_system BIT
	,[filegroup_id] UNIQUEIDENTIFIER
	,log_filegroup_id INT
	,is_read_only BIT
	);

-- Populate #disk_free_space with data 
CREATE TABLE #fixed_drives (
	DriveLetter CHAR(1) NOT NULL
	,FreeMB INT NOT NULL
	);

INSERT INTO #fixed_drives
EXEC master..xp_fixeddrives;

CREATE TABLE #output_table (
	DatabaseName SYSNAME
	,FG_Name SYSNAME
	,GB_Allocated NUMERIC(8, 2)
	,GB_Used NUMERIC(8, 2)
	,GB_Available NUMERIC(8, 2)
	,DBFilename SYSNAME
	,PhysicalFile SYSNAME
	,Free_GB_on_Drive NUMERIC(8, 2)
	);

SELECT NAME AS DBName
INTO #databases
FROM sys.databases
WHERE database_id <= 4
	AND state_desc = 'ONLINE';

DECLARE @dbname SYSNAME;

SELECT @dbname = (
		SELECT TOP (1) DBName
		FROM #databases
		);

DELETE
FROM #databases
WHERE DBName = @dbname;

WHILE @dbname IS NOT NULL
BEGIN
	-- Get the file group data
	INSERT INTO #sfs
	EXEC ('USE ' + @dbname + '; DBCC showfilestats;');

	INSERT INTO #dbf
	EXEC ('USE ' + @dbname + '; SELECT * FROM sys.database_files;');

	INSERT INTO #fg
	EXEC ('USE ' + @dbname + '; SELECT * FROM sys.filegroups;');

	-- Wrap it up!
	INSERT INTO #output_table (
		DatabaseName
		,FG_Name
		,GB_Allocated
		,GB_Used
		,GB_Available
		,DBFilename
		,PhysicalFile
		,Free_GB_on_Drive
		)
	SELECT @dbname AS DATABASE_NAME
		,fg.NAME AS [File Group Name]
		,CAST(((sfs.totalextents * 64.0) / 1024000.0) AS NUMERIC(8, 2)) AS GB_Allocated
		,CAST(((sfs.usedextents * 64.0) / 1024000.0) AS NUMERIC(8, 2)) AS GB_Used
		,CAST((((sfs.totalextents - sfs.usedextents) * 64.0) / 1024000.0) AS NUMERIC(8, 2)) AS GB_Available
		,sfs.dbfilename
		,sfs.physfile
		,CAST((fd.FreeMB / 1000.0) AS NUMERIC(8, 2)) AS Free_GB_on_Drive
	FROM #sfs sfs
	INNER JOIN #dbf dbf ON dbf.[file_id] = sfs.fileid
	INNER JOIN #fg fg ON fg.data_space_id = sfs.filegroupid
	INNER JOIN #fixed_drives fd ON fd.DriveLetter = SUBSTRING(sfs.physfile, 1, 1);

	SELECT @dbname = (
			SELECT TOP (1) DBName
			FROM #databases
			);

	IF @dbname IS NOT NULL
		DELETE
		FROM #databases
		WHERE DBName = @dbname;

	TRUNCATE TABLE #sfs;

	TRUNCATE TABLE #dbf;

	TRUNCATE TABLE #fg;
END

SELECT CONVERT(INT, CONVERT(CHAR, current_timestamp, 112)) AS CaptureDate
	,DatabaseName
	,FG_Name
	,GB_Allocated
	,GB_Used
	,GB_Available
	,DBFilename
	,PhysicalFile
	,Free_GB_on_Drive
FROM #output_table
ORDER BY DatabaseName
	,FG_Name
To test how the files fill you can run the script below which will create a temp table and write 6,553,599 rows of 1 into the table which should reserve over 100 Mb worth of space.

SET NOCOUNT ON; 

DECLARE @x INT

SET @x = 1

CREATE TABLE #MyTempTable (id BIGINT)

WHILE @x < 6553600
BEGIN
		;

	INSERT INTO #MyTempTable (id)
	VALUES (1)

	SET @x = @x + 1
END;
Once it has complete if you run the script above to test the tempdb fill usage again you should see the files have been filled evenly.
Image of evenly filled temp db files

 

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