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SQL Sentry, Inc. - SQL Sentry Performance Advisor
SQL Sentry Performance Advisor for SQL Server delivers an advanced performance dashboard with relevant Windows and SQL Server metrics in a single view along with detailed insight of heavy SQL, blocking, deadlocks, and disk bottlenecks. Performance Advisor is packed with ground-breaking features that are not found in any other performance monitoring software, all designed with the singular goal of simplifying the process of optimizing your SQL Server performance.Learn more!
Determine SQL Server memory use by database and object
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By: Aaron Bertrand | Read Comments (15) | Related Tips: More > Dynamic Management Views and Functions |
Problem
For many people, the way that SQL Server uses memory can be a bit of an enigma. A large percentage of the memory your SQL Server instance utilizes is consumed by buffer pool (essentially, data). Without a lot of digging, it can be hard to tell which of your databases consume the most buffer pool memory, and even more so, which objects within those databases. This information can be quite useful, for example, if you are considering an application change to split your database across multiple servers, or trying to identify databases that are candidates for consolidation.
Solution
A Dynamic Management View (DMV) introduced in SQL Server 2005, called sys.dm_os_buffer_descriptors, contains a row for every page that has been cached in the buffer pool. Using this DMV, you can quickly determine which database(s) are utilizing the majority of your buffer pool memory. Once you have identified the databases that are occupying much of the buffer pool, you can drill into them individually. In the following query, I first find out exactly how big the buffer pool currently is (from the DMV sys.dm_os_performance_counters), allowing me to calculate the percentage of the buffer pool being used by each database:
-- Note: querying sys.dm_os_buffer_descriptors
-- requires the VIEW_SERVER_STATE permission.
DECLARE @total_buffer INT;
SELECT @total_buffer = cntr_value
FROM sys.dm_os_performance_counters
WHERE RTRIM([object_name]) LIKE '%Buffer Manager'
AND counter_name = 'Total Pages';
;WITH src AS
(
SELECT
database_id, db_buffer_pages = COUNT_BIG(*)
FROM sys.dm_os_buffer_descriptors
--WHERE database_id BETWEEN 5 AND 32766
GROUP BY database_id
)
SELECT
[db_name] = CASE [database_id] WHEN 32767
THEN 'Resource DB'
ELSE DB_NAME([database_id]) END,
db_buffer_pages,
db_buffer_MB = db_buffer_pages / 128,
db_buffer_percent = CONVERT(DECIMAL(6,3),
db_buffer_pages * 100.0 / @total_buffer)
FROM src
ORDER BY db_buffer_MB DESC; In the above query, I've included the system databases, but you can exclude them by uncommenting the WHERE clause within the CTE. Note that the actual filter may need to change with future versions of SQL Server; for example, in SQL Server 2012, there is a new database for Integration Services called SSISDB. You may want to keep an eye on system databases just to have a complete picture, seeing as there isn't much you can do about their buffer pool usage anyway - unless you are using master or msdb for your own custom objects.
That all said, here are partial results from an instance on my local virtual machine:
Clearly, the SQLSentry database - while only representing 258 MB - occupies about 70% of my buffer pool for this instance. So now I know that I can drill into that database specifically if I want to track down the objects that are taking up most of that memory. You can once again use the sys.dm_os_buffer_descriptors only this time, instead of aggregating the page counts at the database level, we can utilize a set of catalog views to determine the number of pages (and therefore amount of memory) dedicated to each object.
USE SQLSentry;
GO
;WITH src AS
(
SELECT
[Object] = o.name,
[Type] = o.type_desc,
[Index] = COALESCE(i.name, ''),
[Index_Type] = i.type_desc,
p.[object_id],
p.index_id,
au.allocation_unit_id
FROM
sys.partitions AS p
INNER JOIN
sys.allocation_units AS au
ON p.hobt_id = au.container_id
INNER JOIN
sys.objects AS o
ON p.[object_id] = o.[object_id]
INNER JOIN
sys.indexes AS i
ON o.[object_id] = i.[object_id]
AND p.index_id = i.index_id
WHERE
au.[type] IN (1,2,3)
AND o.is_ms_shipped = 0
)
SELECT
src.[Object],
src.[Type],
src.[Index],
src.Index_Type,
buffer_pages = COUNT_BIG(b.page_id),
buffer_mb = COUNT_BIG(b.page_id) / 128
FROM
src
INNER JOIN
sys.dm_os_buffer_descriptors AS b
ON src.allocation_unit_id = b.allocation_unit_id
WHERE
b.database_id = DB_ID()
GROUP BY
src.[Object],
src.[Type],
src.[Index],
src.Index_Type
ORDER BY
buffer_pages DESC;
Here are the results from this database. Notice that I've captured both clustered and non-clustered indexes, for clustered tables and heaps, and for illustrative purposes I have also created an indexed view.
Please keep in mind that the buffer pool is in constant flux, and that this latter query has explicitly filtered out system objects, so the numbers won't always add up nicely. Still, this should give you a fairly good idea of which objects are using your buffer pool the most.
When investigating the performance of your servers, buffer pool data is only a part of the picture, but it's one that is often overlooked. Including this data will help you to make better and more informed decisions about direction and scale.
Next Steps
- Take an inventory of the highest buffer pool consumers on your servers, and see if the results surprise you.
- Make a plan to periodically review this information and watch for significant changes.
- Review the following tips and other resources:
Last Update: 5/19/2011
About the author
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Comments and Feedback:
| Thursday, May 19, 2011 - 4:31:22 AM - Moinu | Read The Tip |
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This is a great article. Is there any way by which we could find out the details of memory used in other places like in proc cache etc? |
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| Thursday, May 19, 2011 - 7:21:41 AM - Ahmad | Read The Tip |
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Goodish !!!
somewhat related to buffers i am in need to effectively use DBCC FREESYSTEMCACHE if possible to replace hectic DBCC DropCleanBuffers for during testing performance of queries?? :(
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| Thursday, May 19, 2011 - 11:42:16 AM - Aaron Bertrand | Read The Tip |
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Moinu, have a look at these articles from Kimberly Tripp: http://www.sqlskills.com/BLOGS/KIMBERLY/post/Procedure-cache-and-optimizing-for-adhoc-workloads.aspx |
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| Sunday, May 22, 2011 - 6:20:30 PM - Daniel | Read The Tip |
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Excellent article!!! Congrats!!! |
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| Wednesday, June 01, 2011 - 4:42:13 PM - TheSmilingDBA | Read The Tip |
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Excellecent article!!! Need to explain how to use resutls to tune queries or request more RAM. Thanks, Thomas |
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| Friday, June 03, 2011 - 12:34:18 PM - Bruce Samuelson | Read The Tip |
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Here's a slight correction to these excellent tools. The last line of the first query should be ordered by buffer pages rather than buffer MB because the latter rounds to the same value for some DBs.
ORDER BY db_buffer_pages DESC, db_name ASC; --db_buffer_MB DESC; |
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| Thursday, June 30, 2011 - 4:05:49 AM - Muratos | Read The Tip |
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Excellent. Can you develop it further to include sum(row_counts) and sum(real_table_row_count)? I think we can understand whether all table rows in memory or not using that way. |
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| Thursday, May 24, 2012 - 11:57:50 PM - Momfei | Read The Tip |
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GODLIKE! Excellent article! Thanks you so much to share this VERY x999 useful technique. |
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| Tuesday, November 20, 2012 - 11:15:22 AM - Krishna | Read The Tip |
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Very good article for beginner DBA's.. Thanks alot Sir |
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| Monday, January 21, 2013 - 12:00:24 PM - Ammad | Read The Tip |
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Fantastic article! Is there a way to include which user or process was accessing the tables? |
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| Monday, January 21, 2013 - 2:08:38 PM - Aaron Bertrand | Read The Tip |
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Ammad not really an easy way through the DMVs directly unless you parse the plan cache for any currently running queries and determine it that way - however that may actually cause more performance problems than you could possibly hope to solve. If you want to know who is querying specific tables, that might be a better job for trace, audit, or 3rd party monitoring like SQL Sentry Performance Advisor. http://www.sqlsentry.net/performance-advisor/sql-server-performance.asp *Disclaimer: I work for SQL Sentry. |
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| Thursday, January 24, 2013 - 8:40:07 AM - John Henderson | Read The Tip |
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It appears that for SQL 2012 in order to get the total buffer value you need to alter the where clause to be "counter_name = 'Database pages';" instead of "counter_name = 'Total Pages';" Great article though... |
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| Thursday, January 24, 2013 - 10:03:32 AM - Aaron Bertrand | Read The Tip |
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Thanks for the note John; this was written well before SQL Server 2012 was public. :-) |
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| Tuesday, March 19, 2013 - 10:14:39 AM - Kevin Di Sotto | Read The Tip |
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Hi Aaron Love this query ! Why would indexes that are not being used as much as other indexes be take up a bigger percentage of buffer memory? Does it all depend on the size of the Index?
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| Tuesday, March 19, 2013 - 4:56:33 PM - Aaron Bertrand | Read The Tip |
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Kevin, Yes, an index (or a range) is either in memory or not, if it is referenced 15 times or 20 times it is still only represented once. |
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