A warning about Diagnostic Manager’s Query Monitor Feature in v9+

During a recent client engagement to look at the performance problems of a production SQL Server I ran into something I hoped to never see in real life. For a long time I’ve been a proponent of Extended Events in SQL Server, and I’ve really looked forward to seeing them used more and more inside of SQL Server for diagnostics data collection, especially by third party software vendors. I’ve done a lot of performance tests comparing Extended Events to SQL Trace and Extended Events generally has lower performance impact to SQL Server performance. However, I have also found and demonstrated how specific events like collecting the actual execution plan with Extended Events can severely impact a server’s performance. Any time Erin or I talk about Extended Events, whether it is at a conference or in our IEPT02 – Performance Tuning class, one of the key things we both repeat to people is that if you see an completely unexplainable drop in server performance and throughput, check for Extended Events sessions and Traces running on the server and look at what events are being collected. For Extended Events we always demo the impact of the query_post_execution_showplan event live to demonstrate why this event should not be used in production environments ever. Yesterday I was incredibly surprised to not only find the event in an Event Session on the affected server, but also to find that it came from Idera Diagnostic Manager’s Query Monitor feature.


If you have Diagnostic Manager 9.0 or higher, DO NOT enable the Query Monitor feature for your servers, without first disabling (uncheck) the box to Collect Query Plans (SQL Server 2008 and up only). If you have the Query Monitor feature enabled, check the properties for all SQL Server 2008+ servers and disable the Collect Query Plans (SQL Server 2008 and up only) then do a Refresh Alerts for the server,  right click the server in the console and click Refresh Server, to force it to modify the event session and remove the query_post_execution_showplan event.  I have already contacted Idera regarding the performance impacts of this event being used in production environments, and have provided images of CPU Usage with the default configuration and after removing execution plan collection, showing reductions of up to 20-25%. I have also been proactively working with clients I know have Diagnostic Manager installed to get their configurations changed to remove the performance impact from any server monitored by Query Monitor in their environments.

How Do I Disable this Event/Feature?

The idea behind the Query Monitor feature is to collect information about poor-performing queries based on configurable values for duration, logical_reads, CPU, or writes. This is not a bad thing and is something that most servers will want to have configured for monitoring. The problem is not with the functionality of the Query Monitor feature itself, but with just one extra event that gets added to the Extended Events session that gets created by default. To enable/disable the Query Monitor feature and change the behavior of it for an individual server, open the server’s Properties dialog from the right-click context menu in the console and click on the Query Monitor page as shown below.


If you simply accept the defaults for the Query Monitor feature, leaving the checkbox highlighted in red above checked, and turn it on for SQL Server 2012 or higher, an Extended Events session called SERVERNAMESQLdmQMEXevents will be created and started on your server that captures the query_post_execution_showplan event. You can examine the entire session for  yourself using the Script Session As… CREATE To… New Query Editor Window option in SSMS (or equivalent DMVs in earlier versions than 2012).

You can also use the Session Properties to see everything it’s doing (just as you can for all sessions created on your server, by any third-party tools):


Notice the highlighted event in the screenshot of the event session properties in Management Studio. This is bad! Even though the query_post_execution_showplan event has a duration filter greater than five seconds (5000000 microseconds) the performance impact of the event occurs for every single statement that gets executed inside of SQL Server because the event has to collect ALL of the default data elements, which include the showplan XML, before the predicate is evaluated. There is NO WAY to eliminate the performance overhead of this event because it collects the plan before filtering is applied. This event is one of a handful of events in Extended Events that have a warning about their performance impact in the event description:

Occurs after a SQL statement is executed. This event returns an XML representation of the actual query plan. Using this event can have a significant performance overhead so it should only be used when troubleshooting or monitoring specific problems for brief periods of time.

There is no warning about the performance impact anywhere in the Diagnostic Manager UI and based on the information in Idera’s online forums, Query Monitor gets enabled by default for all new instances unless you remove a checkbox on the Add Server wizard in the UI. For existing servers after an upgrade to Diagnostic Manager that were not previously using the Query Monitor feature, it remains disabled. However, when you go to the Query Details page for that server, a warning message is shown that Query Monitor is disabled, prompting you to click on it to configure the Query Monitor for the server.


Why am I writing this blog post?

The purpose of this post is to warn users of the performance impact that this feature has when the defaults are used to enable the Query Monitor against a server, and to at the same time show users how to fix the issue so that Query Monitor can be used to collect important diagnostic information without severely affecting the performance of the servers. I’ve already raised this issue with Idera separately, and I hope to see this event removed from a future update to their product entirely. The performance effects of this event outweigh the cost of having it enabled on production servers, which is why I warn anyone I talk to about Extended Events about this specific events usage in production servers. I think the usage of Extended Events by third party vendors instead of SQL Trace for this type of diagnostic data collection is a good thing overall, but I don’t believe a monitoring tool for performance should ever use an event that can severely degrade performance of a production server.

If you use Diagnostic Manager to monitor your SQL Servers, check the configuration of the Query Monitor feature on ALL SQL Server 2012+ servers and disable the execution plan collection or you WILL have performance degradation from the usage of the query_post_execution_showplan event being included in the event session monitoring your server!

Configuring a Multi-Subnet SQL Server Test Environment

Have you ever wanted to play with a multi-subnet SQL Server configuration for Availability Groups or Failover Clustering with SQL Server 2012 or higher? Getting an environment setup to support multiple subnets is not trivial, and a lot of times if you look online for information about how to configure routing for multiple subnets you will find a recommendation to use a Linux based VM separately as a router/appliance to handle the routing.  In this blog post I’m going to show you how to setup a virtual environment with a single Windows Server 2012 R2 server that performs the role of Active Directory Domain Controller, DNS server, DHCP host, and as a router for multiple subnets as the basis for a multi-subnet virtual environment for testing and playing with multi-subnet configurations for SQL Server 2012 and higher.  I am building this environment in VMware Workstation 10, but the same configuration can be performed under any hypervisor that supports multiple virtual network configurations, once the host configuration of the virtual networks has been appropriately configured.  VMware Workstation makes this configuration very easy through the Virtual Network editor that is included as a part of the VMware Workstation implementation.

Initial VM Configuration

For the initial VM setup, I created a VM with 2 vCPUs and 2GB RAM and then edited the settings to change the virtual network adapter settings to add two new network adapters to the VM.  The default setup includes one virtual network adapter for NAT, so I am going to add one network adapter on VMNet2, then add another adapter on VMNet3 as shown below.


The final hardware configuration for the VM should then look like this:


Now we can install Windows Server 2012 R2 and setup our basic server configuration.  In this case I’ve changed the name of the server to be DC, and set static IP’s to each of the network adapters, for VMNet2, and to VMNet3.


To properly identify each adapter, you can edit the VM settings and disconnect them one at a time so it shows Network cable unplugged for the network connection.  I usually set the NAT adapter to disconnected unless there is a need to have the VM connected to the internet through the host internet connection for some reason.  Once this has been done we can proceed to setup the server as our Active Directory Domain Controller, DNS server, DHCP host for the networks, and as a Router between the two subnets.

Configuring Active Directory Domain Services

To setup Active Directory Domain Services, open the Add Roles and Features Wizard, and then check Active Directory Domain Services as a role for the server.  It will open a popup window shown below to add additional features required by Active Directory Domain Services.  Click the Add Features button then click Next twice followed by the Install button.


When the installation finishes, don’t click the Close button.  Instead click on the link that says Promote this server to a domain controller to launch the Active Directory Domain Services Configuration Wizard.


Click Close


Click the Add a new forest radio button, then add the Domain Name and click Next


Set the domain functional level and then specify the recovery password and click next


Click Next


Click Next


Click Next and wait for the prerequisites checks to complete


Click Install and it will finalize the configuration of the Active Directory Domain Services on the server making it a new Domain Controller in the environment.  When it finishes Windows will prompt to sign you out as shown below, and then the VM will reboot.


Configuring DHCP

Configuring DHCP is not required, but it simplifies creating all the remaining VMs by automatically configuring their network settings based on the VMNet that they have been assigned on the virtual network adapters. Open the Add Roles and Features Wizard and add the DHCP Server role to the machine.


A popup will open with the additional features required and you can click Add Features.


Then click Next, followed by Install and when the installation finishes you can click the Complete DHCP Configuration.  If you mistakenly click Close, you can still complete the configuration by opening Server Manager and then clicking on Manageability under the DHCP box.


This will open up the Manageability Detail View shown below.


Click Go To DHCP.


Click the More link at the top in the yellow bar to the far right


Click the Complete DHCP configuration link.


Click Next


Click Commit


This authorizes the server to be a DHCP host, so now we need to customize the configuration of DHCP. Click the Start button then type in DHCP and open the DHCP console.


Right click on the IPv4 and choose New Scope then click Next.



I name the scopes by the Subnet they apply to so in this case 192.168.52.x and once we complete this scope a second will have to be made for the 192.168.53.x subnet by simply repeating these same steps and using that subnet for the gateway and DNS server addresses.


Set the starting and ending IP address range for the scope and click Next. Then click Next on the Add Exclusions and Delay and Lease Duration windows.


On the Configure DHCP Options page, make sure it says Yes, I want to configure these options now and click Next.


Add the local IP address for this server as the Router address, in this case and click Add, then click Next. (For the second scope you will use the other IP address, in this case the address)


If the IP Address for the DNS Server is not the local address for the subnet, remove any existing IP addresses, and add the local IP address for the subnet, in this case as the DNS server and then click Next. (Again, for the second scope you will use the other IP address, in this case the address)


Remove any WINS server IP Addresses and click Next.


Then click Next to activate the scope.  Now repeat these same steps for the 192.168.53.x subnet specifying the address for the router and DNS server and activate that scope.

Configuring Routing Between Subnets

To setup routing between the subnets, open the Add Roles and Features Wizard, and then check Remote Access as a role for the server.


Click Next three times and then check the Routing box.  It will open a popup window shown below to add additional features required for Routing.  Click the Add Features button.


This will also cause the DirectAccess and VPN (RAS) option to become checked as shown below.


Click Next twice and it will have the addition IIS role services window shown below with the required features checked.  Nothing needs to be changed here, just click Next again, followed by Install to install the Remote Access Role to the server.


When the installation completes, click Close.  Now click on Start and type in Routing and Remote Access and open the Routing and Remote Access management console.


Right click on the server and select the Configure and Enable Routing and Remote Access option.


Click Next on the first window then select the Custom Configuration option and click Next.


Check the box for LAN routing and click Next.  Then click Finish.


Now click the Start service button to start the Routing and Remote Access services.



EDIT: After initially publishing this blog post, Thomas Stringer (Blog | Twitter) pointed out that static routes don’t have to be configured and RIP can be used to simplify the configuration of routing.  So I am updating this post with the RIP configuration as an additional section. Either option will work but I have to admit that RIP is easier to configure. You only need to configure RIP or the Static Routes, don’t configure both options. 

Routing with Routing Information Protocol (RIP)

  1. Expand the IPv4 folder and right click on General and choose New Routing Protocol.
  2. Select the RIP Version 2 for Internet Protocol option and click OK.
  3. Now right click on RIP under IPv4 and choose New Interface.
  4. Select the interface and click OK.
  5. Then click OK again.
  6. Now right click on RIP under IPv4 and choose New Interface.
  7. Select the interface and click OK.
  8. Then click OK again.
  9. Now you are finished.

Routing with Static Routes

Expand the IPv4 folder and right click on Static Routes and choose the New Static Route option.


Select the interface and then add a Destination of, subnet mask of and gateway of to create a static route between the subnets.


Now create a New Static route again.


This time, change to the interface and set the Destination to, network mask to, and the Gateway to and click OK to add the static route between the subnets.


Restart the Routing and Remote Access Services and you’re all set.


To test the configuration, I created two new VMs named SQL1 and SQL2 with a single network adapter each.  SQL1 was configured on VMNet2 and SQL2 was configured on VMNet3, then joined both of the servers to the SQLskillsDomain.com domain.  The Windows Firewall needs to be configured to allow ICMP echo packets in the Inbound Rules on each of the servers by enabling the File and Printer Sharing (Echo Request –ICMPv4-In) rule that already exists in the Inbound Rules of the firewall. Once this is enabled on each of the servers, we can test with ping between the VMs as shown below:

SQL1 to SQL2


SQL2 to SQL1


These same steps can be performed in Windows Server 2008R2 and Windows Server 2012, with only minor differences in the configuration of the roles and features.  The Routing and Remote Access configuration to provide the routing between the subnets is nearly identical on all three versions of Windows.  Once the server is configured, the environment can be used to create a multi-subnet Windows Server Failover Cluster for a multi-subnet Availability Group or even a multi-subnet Failover Clustered instance.

Availability Groups and Columnstore Indexes

This week Paul, Kimberly, Erin, and Tim are at SQLIntersection in Arizona presenting, and one of the best things about presenting at conferences is the questions that get asked about SQL Server features.  Kimberly was asked a question about whether or not columnstore indexes could be used in a database that participates in an Availability Group or not and there is conflicting information available online. Since I have multiple 2012 and 2014 Availability Groups setup in my laptop for teaching our Immersion Events, I spent a little time testing and agreed to blog about the findings. 

Columnstore indexes can be created in any database participating in an Availability Group without a problem. If you create a nonclustered column store index, it can also be used on a readable secondary for queries. However, a clustered columnstore index can only be read on the primary database. If you try to query a clustered columnstore index on a readable secondary you will get error 35371,

Msg 35371, Level 16, State 1, Line 4
SNAPSHOT isolation level is not supported on a table which has a clustered columnstore index.

So as long as the columnstore index is nonclustered, it can still be read on the secondary, but keep in mind that the nonclustered columnstore index is not updateable through DML operations and has to either leverage partition switching or rebuilding the index to modify the data in the table. A good read on the differences can be found in the Books Online.

Update: SQL Server 2016 will support clustered columnstore indexes on readable secondaries as well.

New Performance Tuning using Wait Statistics Whitepaper

Last week a new whitepaper titled SQL Server Performance Tuning Using Wait Statistics: A Beginners Guide was made available through SimpleTalk. This paper is a combination of information from Chapter 1 of my Troubleshooting SQL Server: A Guide for the Accidental DBA book (Amazon|eBook download), also published by SimpleTalk, and a series of articles created by Erin on baseline data collection techniques.

Understanding the time a session spends waiting inside of SQL Server is an incredibly important part of performance tuning and diagnosing problems. As consultants we use the information contained in the wait statistics during health checks of servers for bottleneck identification, and as a part of root cause analysis while troubleshooting problems. This whitepaper will provide you an introduction into the world of performance tuning using wait statistics in SQL Server and will explain the common wait types and what they do and do not mean in the context of performance tuning and troubleshooting. You will also find code for collecting and aggregating the information available in SQL Server DMVs about the waits that have occurred in the past and what sessions are currently waiting for in the paper to simplify getting started with understanding the activity on you SQL Servers.

Table of Contents

  1. Introduction
  2. The SQLOS scheduler and thread scheduling
  3. Using wait statistics for performance tuning
  4. Investigating active-but-blocked requests using sys.dm_os_waiting_tasks
  5. Analyzing historical wait statistics using sys.dm_os_wait_stats
  6. Common wait types
  7. Wait Statistics baselines
  8. Summary
  9. Further reading
  10. About the authors
  11. About the technical editor

A big thanks goes to Tony Davis at Redgate for editing all of the content and merging the two sets of information into a single concise introduction to the world of Performance Tuning SQL Server using Wait Statistics. The whitepaper can be downloaded from the SimpleTalk website, or from the help section of our site through the link below.


Why I hate the ring_buffer target in Extended Events

I’ve had so many questions about the same problem with the ring_buffer target in Extended Events lately that I figured I would write a blog post that explains all the information I teach about the ring_buffer target and the problems associated with it.  Since the release of SQL Server 2012, and the new UI for Extended Events, I have never used the ring_buffer target once, in fact as the title of this post says I really hate the ring_buffer target. In this post I’ll go over the reasons I hate the ring_buffer target and hopefully convince you why you should use the file_target instead.

Missing Events

This is by far the most common problem I have to explain about the ring_buffer target by email.  Generally the question is phrased along the lines of:

“Hey Jonathan,

I got the code below from one of your articles on SQL Server central and it is not working. The problem I have is that when I run the code, it doesn’t show any deadlock graphs even though I know one just occurred in the application. It seems like I only see older deadlocks in the system_health session, but never the most recent one.  I’ve turned on Trace 1222 and get the information that way, so why doesn’t this work.”

The reality of the situation is that the events are actually there, you just can’t see them because of a limitation of the sys.dm_xe_session_targets DMV.  The target_data column of this DMV can only output roughly 4MB of XML data. The information about the 4MB formated XML limitation of the DMV was explained by Bob Ward on the CSS SQL Server Engineers blog post You may not see the data you expect in Extended Event Ring Buffer Targets…. back in 2009.  To demonstrate the effect of this limitation, lets look at the number of events contained in the ring_buffer target for the system_health event session on a SQL Server 2012 SP1+CU7 server that I have permission to share the information from using the following query.

    ring_buffer_event_count - event_node_count AS events_not_in_xml
(    SELECT target_data.value('(RingBufferTarget/@eventCount)[1]', 'int') AS ring_buffer_event_count,
        target_data.value('count(RingBufferTarget/event)', 'int') as event_node_count
    (    SELECT CAST(target_data AS XML) AS target_data  
        FROM sys.dm_xe_sessions as s
        INNER JOIN sys.dm_xe_session_targets AS st 
            ON s.address = st.event_session_address
        WHERE s.name = N'system_health'
            AND st.target_name = N'ring_buffer'    ) AS n    ) AS t;


The ring_buffer_event_count comes from the eventCount attribute maintained in the root RingBufferTarget element of the XML document returned, and the event_node_count is just a count of the RingBufferTarget/event nodes contained by the XML document returned from the DMV.  Here you can see that there are a total of 5000 events contained in the ring_buffer target, based on the new 2012 MAX_EVENTS_LIMIT option which is set at 5000 for the system_health session. Out of those, only 3574 are actually being output by the XML document from the DMV.  That leaves us with 1426 events that are not available, but still exist in memory. The XML document outputs the events oldest first, so the events that we can’t see are actually the most recently collected events and are the ones that we’d probably want to actually have access to.

We can look at the memory usage for the target using the query below to compare the binary data size maintained within the target, versus the size of the formatted XML being output by the DMV.

    target_data.value('(RingBufferTarget/@memoryUsed)[1]', 'int') AS buffer_memory_used_bytes,
    ROUND(target_data.value('(RingBufferTarget/@memoryUsed)[1]', 'int')/1024., 1) AS buffer_memory_used_kb,
    ROUND(target_data.value('(RingBufferTarget/@memoryUsed)[1]', 'int')/1024/1024., 1) AS buffer_memory_used_MB,
    DATALENGTH(target_data) AS xml_length_bytes,
    ROUND(DATALENGTH(target_data)/1024., 1) AS xml_length_kb,
    ROUND(DATALENGTH(target_data)/1024./1024,1) AS xml_length_MB
SELECT CAST(target_data AS XML) AS target_data  
FROM sys.dm_xe_sessions as s
INNER JOIN sys.dm_xe_session_targets AS st 
    ON s.address = st.event_session_address
WHERE s.name = N'system_health'
 AND st.target_name = N'ring_buffer') as tab(target_data)


Here we can see that the memory usage is roughly 1.7MB of binary data, but once serialized to formatted XML, the length of the document is roughly 4.7MB in size, which is larger than the published XML size, but only slightly.  This is the problem with the nature of the data that can be generated by Extended Events, it can be very compact in binary format, but serializing the formatted XML creates a lot of added overhead for the events that have been collected.  The system_health event session is specifically prone to having the maximum, 5000, number of events collected but the ring_buffer, but only a smaller subset of the events can actually be output by the DMV due to the sizes of the events that get collected by that event session.  The events most likely to have a fairly large size are the sp_server_diagnostics_component_output and the xml_deadlock_report events, since the sizes of the XML being returned by both of those events depends on the specifics of the conditions for when they fired.  I’ve seen sp_server_diagnostics_component_output events that are over 512KB in size on busy servers when problems occur, so that would significantly limit the available data that could be returned by the DMV for the ring_buffer target when one of those events is contained in the output XML.

No UI Support

This for me is probably the biggest reason that I don’t deal with the ring_buffer target anymore in my day to day work.  The UI for Extended Events doesn’t support breaking down the information that is contained in the ring_buffer target at all.  The only thing the UI does is show the XML that was output by the target_data column of the sys.dm_xe_session_targets DMV.


This means that to consume the data you have to either open the XML and scan down the events, or write XQuery to parse the XML into table form, which requires that you know the events, columns, and actions that were used in the event sessions definition to really access the data. If I am doing short term data collection and don’t want it persisted to a file in the file system on SQL Server 2012, I just opt for the Live View to stream the data back into the grid, where I don’t have to deal with the XML and can rapidly find the information I was interested.  For anything long term, or even looking at information from the system_health event session, I use the file_target, which the UI can read and process the events from, eliminating the need to do any XQuery manually.

Watch the Memory Usage

This is a new one to me, but it is something to be incredibly careful about with the way the ring_buffer target gets configured on a production server.  Two weeks ago, Andy Galbraith (Blog|Twitter) ran into an issue on a server where all connections were receiving 701 Insufficient System Memory errors. During his analysis Andy realized that the MEMORYCLERK_XE memory clerk in SQL Server was consuming close to 10GB of memory on a server with 16GB of total RAM with ‘max server memory’ configured to 11000MB.  The problem was that the event session that was created with a MAX_EVENTS_LIMIT of 1,000,000 events, but with no MAX_MEMORY configuration on the target.  Since the memory usage of the target is based on the events that have been captured this is, and no maximum was set, it can essentially use an unlimited amount of memory, leading to problems on the server if memory is limited like the example server Andy was troubleshooting.

New Article: Asynchronous Processing with Service Broker

My most recent article on SQLPerformance.com was published today, and begins a series on Service Broker use cases. While Service Broker doesn’t have the sexy UI support that other features of SQL Server have, it is a incredibly powerful tool.

An Introduction to Asynchronous Processing with Service Broker

Over the series I’ll be explaining Service Broker concepts and how you can leverage Service Broker to improve the end user experience where other performance tuning techniques might not.

SQL Saturday 273 Session Uploaded

Just over two weeks ago I presented a session on Memory Troubleshooting at SQL Saturday 237 here in Tampa. I promised to upload the demo scripts from this session for troubleshooting problems and I guess it is better late than never. They have been uploaded along with a PDF version of the slide deck to the session on the SQL Saturday site and can be downloaded from there.

Looking at read_microsec in sys.dm_os_buffer_descriptors in SQL Server 2012

Last week someone sent Kimberly an email asking what the read_microsec column in the sys.dm_os_buffer_descriptors DMV in SQL Server 2012 showed. The email was passed around our team and to be honest it was the first time I’d even heard of the column existing. The questioner also wanted to know if/how the read_microsec column be used. The Books Online topic for sys.dm_os_buffer_descriptors defines this column as, “The actual time (in microseconds) required to read the page into the buffer. This number is reset when the buffer is reused.”

This would make you think that the counter is per-page in the buffer pool, but I wanted to see what exactly it correlates to, and the results aren’t quite as straightforward as the Books Online description might seem. The read_microsec column isn’t the time required to read a single page in microseconds – it’s the time taken to complete reading into the buffer pool the original I/O block that the page was part of – whether that block was a single 8KB page or a much larger I/O. This is a completely different meaning when you think about the different sizes of I/O that SQL Server can potentially do.

Investigating this further

To look at this further, the first thing to do is identify the Page ID for a specific row in a database that we can use for tracking the I/O associated with bringing that page into the buffer pool. To do this, we can use the fn_PhysLocFormatter function and %%physloc%% as shown by Paul in his blog post, SQL Server 2008: New (undocumented) physical row locator function.

	sys.fn_PhysLocFormatter(%%physloc%%) AS [Physical_Loc],
FROM AdventureWorks2012_Slow.Production.Product
%%PhysLoc%% formatted results

%%PhysLoc%% formatted results

In my copy of AdventureWorks2012, this returns Page ID 791, as shown in the screenshot above. Using this Page ID we can clear the buffer pool with DBCC DROPCLEANBUFFERS and then rerun our query to read the page back into the buffer pool from disk. However, since we want to track what the meaning of the new column in sys.dm_os_buffer_descriptors is, first let’s create an Extended Events session to track the physical I/O operations and memory operations associated with the session_id that we are running our queries on.

    ADD EVENT sqlos.async_io_completed
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlos.async_io_requested
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlos.page_allocated
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlos.page_freed
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlserver.file_read
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlserver.file_read_completed
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlserver.physical_page_read
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlserver.sql_statement_completed
        (WHERE (sqlserver.session_id=55)),
    ADD EVENT sqlserver.sql_statement_starting
        (WHERE (sqlserver.session_id=55))
    ADD TARGET package0.event_file
        (SET filename=N'PageIOLatency.xel')

Now we can clear the buffer pool, start the event session, query the row from the Production.Product table, and then stop the event session and review the information contained in the event_file target. Specifically we want to look at the I/O operations for page_id 791 and what the duration was for reading the page.

Highlighted results for page_id 791

Highlighted results for page_id 791

Here I’ve highlighted the async_io_requested event and the last physical_page_read event associated with the I/O for page_id 791. If we look at the duration of the file_read_completed event, which is in milliseconds, we can see from the image above that it took 10 milliseconds to perform the read operation. The read_microsec column in sys.dm_os_buffer_descriptors is in microseconds, so this may only loosely correlate to the value showing for page_id 791 when we query dm_os_buffer_descriptors.

FROM sys.dm_os_buffer_descriptors AS obd
WHERE database_id = DB_ID('AdventureWorks2012_Slow')  AND
	obd.page_id = 791;
page_id 791 buffer_descriptor

page_id 791 buffer_descriptor

If we look at the timestamps for the async_io_requested and async_io_completed events in the event_file output, we can calculate out the delta in microseconds between the I/O request posting and the completing:

2014-02-05 08:37:40.0419342 — async_io_completion timestamp
2014-02-05 08:37:40.0309362 — async_io_requested timestamp
010998 — delta in microseconds

This is an exact match to the read_microsec in sys.dm_os_buffer_descriptors, which proves that the column provides the actual I/O time associated with that page being read into the buffer pool.

However, it is not actually that simple…

If we go back to the event_file results, we’ll notice that we didn’t just read one page into the buffer pool with our request, we read an entire extent, 8 pages, into the buffer pool with our I/O operation. So if we modify our query against sys.dm_os_buffer_descriptors to show all of the pages for the AdventureWorks2012_Slow database, we’ll see that all 8 of the pages that were read in a single read, have the same read_microsec value.

FROM sys.dm_os_buffer_descriptors AS obd
JOIN AdventureWorks2012_Slow.sys.allocation_units AS au
	ON obd.allocation_unit_id = au.allocation_unit_id
JOIN AdventureWorks2012_Slow.sys.partitions AS p
	ON au.container_id = p.partition_id
JOIN AdventureWorks2012_Slow.sys.objects AS o
	ON p.object_id = o.object_id
WHERE database_id = DB_ID('AdventureWorks2012_Slow')  AND
	o.is_ms_shipped = 0
ORDER BY obd.page_id, o.name, obd.read_microsec;
All pages read in buffer_descriptors

All pages read in buffer_descriptors

So for a single page, unless it is a single 8KB read that occurred, the read_microsec column doesn’t necessarily show an I/O related problem. We’d have to know the size of the original I/O block that read the page into the buffer pool to know if the amount of read_microsec is actually a problem. The reason for this is that with I/O latency, the size of the I/O operation performed matters. Larger block I/Os take longer to complete than smaller blocks. To demonstrate this, we can perform a query that would perform larger read-ahead reads from the data file and look at the impact to the read_microsec column for the pages that are read in the larger block I/O.

To do this, I used the sys.fn_PhysLocCracker() TVF in a CROSS APPLY against the Sales.SalesOrderDetail table to find the first set of contiguous rows in the table. Using the query below, this happened to be the TOP 2866 in my AdventureWorks2012_Slow database which covered 32 pages contiguously or 256KB starting at page_id 10672 and ending at page_id 10703.

FROM AdventureWorks2012_Slow.Sales.SalesOrderDetail
CROSS APPLY sys.fn_PhysLocCracker(%%physloc%%);

So if we start the event session again, clear the buffer pool, and read the TOP 2866 rows from the Sales.SalesOrderDetail table, the event data will show that all 32 pages were read with a single 256KB read-ahead read, and the duration for the file_read_completed event is 51ms.

256KB read-ahead read

256KB read-ahead read

If we look at the information in the read_microsec column for the 32 pages in sys.dm_os_buffer_descriptors after reading the table, we’ll see again that all of the pages have the same value.

FROM sys.dm_os_buffer_descriptors AS obd
WHERE database_id = DB_ID('AdventureWorks2012_Slow')  AND
	obd.page_id BETWEEN 10672 AND 10703
ORDER BY page_id
All pages have the same read_microsec value

All pages have the same read_microsec value


The read_microsec column of sys.dm_os_buffer_descriptors wouldn’t be my first go-to metric for looking at I/O latency or page-read issues in SQL Server. While the screenshots in this blog post showed that consecutive page_ids retrieved by a single I/O operation have the same read_microsec value, the testing was done on a SQL Server with no other load and doesn’t mimic production activity in any way. In a production server, you can’t rely on looking at consecutive pages in the output of sys.dm_os_buffer_descriptors to correlate whether or not the pages were all read at the same time in most cases. This is especially true if the server has experienced buffer pool memory pressure or has been active for a long period of time, since pages that aren’t actively being used may be removed to make space for new pages being read into the buffer pool, and a single page within a larger block of pages may remain in the buffer pool longer than the pages around it.

For troubleshooting I/O latency problems, I’d recommend reading Erin’s blog post on Monitoring Disk I/O from our Accidental DBA series, which covers other methods for looking at I/O latency including sys.dm_io_virtual_file_stats and the Physical Disk performance counters.

New Article: VMware CPU Hot Plug vNUMA Effects on SQL Server

My latest article on SQLPerformance.com was published today and highlights the potential for performance problems while using the hot-add vCPU feature in ESX 5+ with wide VMs for SQL Server.


This is a potentially hidden problem with serious performance implications that should be checked for if you are using VMware to run wide VMs with greater than 8 vCPUs on NUMA hardware.

Mapping wait types in dm_os_wait_stats to Extended Events

A few months back I received an email from a member of the community that was trying to filter the sqlos.wait_info event for some of the wait types that are filtered out by Glenn’s diagnostic queries, and to their dismay wasn’t able to find the specific wait types in the wait_types map in sys.dm_xe_map_values.  This scenario is something that I have long known about but never actually blogged about, though this blog post has been sitting in draft form since early 2012.  Now that things have started to slow down at the end of this year I took a little time and built a spreadsheet of the wait_type names in sys.dm_os_wait_stats that don’t match exactly to the map_value in sys.dm_xe_map_values.

I’d like to thank Bob Ward at Microsoft for taking the time to double check the spreadsheet I originally built for this blog post for accuracy and for the corrections on a few of the wait types he provided.  The list of wait types below was generated from SQL Server 2012 Service Pack 1 + Cumulative Update 6.

wait_type in sys.dm_os_wait_stats

map_value in sys.dm_xe_map_values