Tempdb monitoring - Free space alert

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SQL Server uses tempdb for many features, including row versioning, service broker and query execution amongst others.  During the execution of queries, data used for sorts and hash joins can be “spilled” to tempdb if the memory granted to the query isn't enough to store the data.  If this happens for a very big query or lots of small queries or for queries on instance that makes extensive use read committed snapshot or any combination of many reasons, an error resembling the one below can be received...

Msg 1105, Level 17, State 2, Line 3
Could not allocate space for object 'dbo.SORT temporary run storage: 140737494384640' in database 'tempdb' because the 'PRIMARY' filegroup is full. Create disk space by deleting unneeded files, dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup.

Due to the way I configure tempdb (a single drive dedicated to the datafiles with multiple equal sized datafiles all pre-grown to their maximum size) the advice offered in the error message wont work. Instead, I create a SQL Agent alert to warn me before the space is all used up.

Recreating the problem

Running the following query in Adventure Works will create a plan with sorts and hash joins, but with the correct statistics and enough memory available, will not need to use tempdb...

SELECT * 
FROM Sales.SalesOrderDetail s1
INNER HASH JOIN Sales.SalesOrderDetail s2
ON s1.rowguid = s2.rowguid
ORDER BY s1.rowguid

PerfMon confirms that the free space in tempdb doesn't change...

By fooling the optimiser into thinking there's only 1 row in the table...

UPDATE STATISTICS Sales.SalesOrderDetail WITH ROWCOUNT = 1

...the query plan generated will include a spill to tempdb because the estimated number of rows is very wrong and the subsequent memory grant is too low.

...and tempdb will have to store the data, as can be seen in PerfMon.

This isn't a problem if the space in tempdb is available but if the there isn't enough space, errors will occur.
 

Solutions
What can be done when tempdb becomes full? Creating more space is the obvious answer. Enabling autogrowth on the datafile(s) can help but its best practise to have multiple, equal sized tempdb datafiles. Turning on trace flag 1117, which expands all datafiles in a filegroup at the same rate is a good idea although this is a server wide setting so will affect other databases on the instance.
My solution, creating an alert, doesn't really solve the problem but at least gives an early warning so evasive action can be taken.
Tempdb is getting full” alert
Although I call it an alert, its actually made up of the following 3 things...
1 – A SQL Agent alert
2 – A SQL Agent job
3 – A stored procedure
Starting from the bottom, the stored procedure is used to find what's using tempdb and then send an email with this info. I check current running queries, the size of the row store and the size of permanent tables created in tempdb although other checks can be added. To check the current running queries, I use Adam Mechanics sp_whoisactive to look for queries with a tempdb_allocation above zero and insert the 5 with the highest values into a temporary table (yes, I realise what I'm doing here 🙂 ). I use sys.dm_os_performance_stats for the row store size and sys.allocation_units for the permanent table size.
Once all the tempdb checks are collected, a dynamic HTML block of code is built to be used to email the DBA team using sp_send_dbmail.

USE[master]
GO
CREATEPROC[dbo].[spTempDBSizeInfo]
AS
DECLARE@subjectvarchar(500)
DECLARE@bodyNVARCHAR(MAX)
DECLARE@body_formatvarchar(20)='HTML'
SELECT@subject='Free TempDB space on '+@@ServerName+' - '+CAST(100 -cast((A.TotalSpaceInMB-cntr_value/1024)*100/A.TotalSpaceInMBasint)ASVARCHAR(3))+'% left of '+cast(A.TotalSpaceInMBasvarchar(12))+'MB'
FROMsys.dm_os_performance_counters
CROSSAPPLY(selectsum(size)*8/1024 asTotalSpaceInMBfromtempdb.sys.database_fileswheretype=0) ASA
WHEREcounter_name='Free Space in tempdb (KB)'
CREATETABLE[dbo].[#tblWhoIsActive](
[dd hh:mm:ss.mss][varchar](8000)NULL,
[session_id][smallint]NOTNULL,
[sql_text][xml]NULL,
[login_name][nvarchar](128)NOTNULL,
[wait_info][nvarchar](4000)NULL,
[tran_log_writes][nvarchar](4000)NULL,
[CPU][varchar](30)NULL,
[tempdb_allocations][varchar](30)NULL,
[tempdb_current][varchar](30)NULL,
[blocking_session_id][smallint]NULL,
[reads][varchar](30)NULL,
[writes][varchar](30)NULL,
[physical_reads][varchar](30)NULL,
[query_plan][xml]NULL,
[used_memory][varchar](30)NULL,
[status][varchar](30)NOTNULL,
[tran_start_time][datetime]NULL,
[open_tran_count][varchar](30)NULL,
[percent_complete][varchar](30)NULL,
[host_name][nvarchar](128)NULL,
[database_name][nvarchar](128)NULL,
[program_name][nvarchar](128)NULL,
[start_time][datetime]NOTNULL,
[login_time][datetime]NULL,
[request_id][int]NULL,
[collection_time][datetime]NOTNULL)ON[PRIMARY]TEXTIMAGE_ON[PRIMARY]
EXECsp_WhoIsActive
@get_transaction_info=2,
@get_plans=1,
@destination_table='#tblWhoIsActive'
SELECTTOP5
[dd hh:mm:ss.mss],
session_id,
CAST(sql_textasvarchar(MAX))ASsql_text,
login_name,
(REPLACE(tempdb_current,',','')*8)/1024 astempdb_current_MB,
(REPLACE(tempdb_allocations,',','')*8)/1024 astempdb_allocations_MB,
CPU,
reads,
writes
INTO#tblWhoIsActive_temp
FROM#tblWhoIsActive
WHEREREPLACE(tempdb_allocations,',','')>0
ORDERBYtempdb_allocationsDESC
IF@@ROWCOUNT=0
BEGIN
SET@body=N'<head>'+ N'<style type="text/css">h2, body {font-family: Arial, verdana;} table{font-size:11px; border-collapse:collapse;} td{background-color:#F1F1F1; border:1px solid black; padding:3px;} th{background-color:#99CCFF;}</style>'+
N'<h2><font color="#0000ff" size="4">Free TempDB space</font></h2>'+ N'</head>'+ N'<p>'+' '+'</p>'+N'<body>'+N' <hr> '+N'<h1><font color="#0000ff" size="2">The top 5 TempDB intensive queries currently running are:</font></h1>'+
N' '+N'<font color="#0000ff" size="2">No queries using TempDB currently running</font>'+N' '+ N' <br></br>'+ N'<p>'+' '+'</p>'+ N' <hr> '+N'<h1><font color="#0000ff" size="2">TempDB components size MB:</font></h1>'+
N' '+ N'<table border="1">'+ N'<tr><th>PersistedTableSizeMB</th><th>VersionStoreSizeMB</th></tr>' +
CAST((
SELECTtd=SUM(au.total_pages)*8/1024,'',
td=MAX(A.VersionStoreSizeMB),''
FROMtempdb.sys.partitionsp
INNERJOINtempdb.sys.allocation_unitsau
ONp.hobt_id=au.container_id
CROSSAPPLY(SELECTcntr_value/1024 ASVersionStoreSizeMBFROMsys.dm_os_performance_countersWHEREcounter_name='Version Store Size (KB)')ASA
WHEREOBJECT_NAME(p.object_id)notlike'#%'
FORXMLPATH('tr'),TYPE )
ASNVARCHAR(MAX))+
N'</table>'+ N'</body>';
END
ELSE
BEGIN
SET@body= N'<head>'+ N'<style type="text/css">h2, body {font-family: Arial, verdana;} table{font-size:11px; border-collapse:collapse;} td{background-color:#F1F1F1; border:1px solid black; padding:3px;} th{background-color:#99CCFF;}</style>'+
N'<h2><font color="#0000ff" size="4">Free TempDB space</font></h2>'+ N'</head>'+ N'<p>'+' '+'</p>'+N'<body>'+N' <hr> '+N'<h1><font color="#0000ff" size="2">The top 5 tempdb intensive queries currently running are:</font></h1>'+
N' '+ N'<table border="1">'+ N'<tr><th>[dd hh:mm:ss.mss]</th><th>session_id</th><th>sql_text</th><th>login_name</th><th>tempdb_allocations_MB</th><th>tempdb_current_MB</th><th>CPU</th><th>reads</th><th>writes</th></tr>'+
CAST((
SELECTtd=[dd hh:mm:ss.mss],'',
td=session_id,'',
td=CAST(sql_textASVARCHAR(256)),'',
'',
td=login_name,'',
td=tempdb_allocations_MB,'',
td=tempdb_current_MB,'',
td=CPU,'',
td=reads,'',
td=writes,''
FROM#tblWhoIsActive_temp
ORDERBYtempdb_allocations_MBDESC
FORXMLPATH('tr'),TYPE )
ASNVARCHAR(MAX))+
N'</table>'+ N' <br></br>'+ N'<p>'+' '+'</p>'+ N' <hr> '+ N'<h1><font color="#0000ff" size="2">TempDB components size MB:</font></h1>'+N' '+ N'<table border="1">'+
N'<tr><th>PersistedTableSizeMB</th><th>VersionStoreSizeMB</th></tr>' +
CAST((
SELECT
td=SUM(au.total_pages)*8/1024,'',
td=MAX(A.VersionStoreSizeMB),''
fromtempdb.sys.partitionsp
INNERJOINtempdb.sys.allocation_unitsau
ONp.hobt_id=au.container_id
CROSSAPPLY
(SELECTcntr_value/1024 ASVersionStoreSizeMBFROMsys.dm_os_performance_countersWHEREcounter_name='Version Store Size (KB)')ASA
WHEREobject_name(p.object_id)notlike'#%'
FORXMLPATH('tr'),TYPE )ASNVARCHAR(MAX))+
N'</table>'+ N'</body>'
END
EXECMSDB.dbo.sp_send_dbmail
@profile_name='ProfileName'
,@recipients='DBATeam'
,@subject=@subject
,@body=@body
,@body_format=@body_format

The stored procedure is called by an unscheduled SQL Agent Job.

The SQL Agent job is executed by a performance condition alert being triggered within SQL Agent alerts, with the option set to only raise the alert every 5 minutes. This value maybe too high for some people but can be altered on the options page.

The SQL Agent alert is configured to be triggered when the Free Space falls below a certain value, which I normally set at 50% of the size of tempdb.

As tempdb size can differ for different instances, i've done a little bit of TSQL trickery to make the script runnable on any instance.

USE [msdb]
GO
DECLARE @PercentToWarn DECIMAL(12,2) = 50
SET @PercentToWarn = @PercentToWarn/100
DECLARE @InstanceName VARCHAR(128) =
(SELECT CASE WHEN SERVERPROPERTY ('InstanceName') IS NULL THEN 'SQLSERVER'
ELSE 'MSSQL$' + CAST(SERVERPROPERTY ('InstanceName') AS VARCHAR(32)) END)
DECLARE @TempWarnSize INT = 
 (SELECT SUM(size)*8 FROM tempdb.sys.database_files WHERE type = 0) * @PercentToWarn
 
DECLARE @Job_id UNIQUEIDENTIFIER= (
 select job_id FROM sysjobs WHERE name = 'zzTempDB_Size_Info')
 
DECLARE @PerfCond VARCHAR(1024) = 
@InstanceName + ':Transactions|Free Space in tempdb (KB)||<|' + CAST(@TempWarnSize AS VARCHAR(132)) + ''
BEGIN
TRY
EXEC msdb.dbo.sp_help_alert 'FreeSpaceInTempdb'
END TRY
BEGIN
CATCH
EXEC msdb.dbo.sp_add_alert
@name=N'FreeSpaceInTempdb',       
@enabled=1,       
@delay_between_responses=300,       
@include_event_description_in=0,      
@performance_condition=@PerfCond,       
@job_id=@Job_id
END CATCH                 
GO

Results 

When tempdb free space drops below the configured value (50% by default although on this instance its configured to 80%), an email will be sent the DBAs.

Further Reading 

Although the tempdb 2005 whitepaper is starting to get a bit old, the details contained within are still largely relevant to subsequent SQL Server versions and is an informative read.

https://technet.microsoft.com/en-gb/library/cc966545.aspx

Also worth watching is a PASS lecture called Inside Tempdb by Bob Ward.

https://www.youtube.com/watch?v=SvseGMobe2w&feature=share&list=PLF80A8A233EE9F22F

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