In this post, I am sharing a script to check dirty pages or clean pages in SQL Server Memory area. Most of the DBAs know about DBCC DROPCLEANBUFFERS which clear the data or pages from the memory area.

We need to execute DBCC DROPCLEANBUFFERS while testing the performance of queries.

First time when you execute your query, it requires physical reads but for the second execution it stores the information in memory, and it will perform fast compare to previous execution.

But few people said that DBCC DROPCLEANBUFFERS is not working all the times. I don’t know why but found below script to check dirty pages and clean pages in the Memory area.

Dirty pages mean has not written to disk and Clean pages say the page is brought in to memory by the buffer manager.

SQL Server: DBCC to Clean Cache and Clean Buffer for Stored Procedure

SELECT 
    DatabaseName
	,DirtyPageCount
	,CleanPageCount
	,[DirtyPageCount] * 8 / 1024 AS DirtyPageInMB
    ,[CleanPageCount] * 8 / 1024 AS CleanPageInMB
FROM
    (SELECT
        (CASE WHEN ([database_id] = 32767)
            THEN N'Resource Database'
            ELSE DB_NAME ([database_id]) END) AS [DatabaseName],
        SUM (CASE WHEN ([is_modified] = 1)
            THEN 1 ELSE 0 END) AS [DirtyPageCount],
        SUM (CASE WHEN ([is_modified] = 1)
            THEN 0 ELSE 1 END) AS [CleanPageCount]
FROM sys.dm_os_buffer_descriptors
GROUP BY [database_id]) AS [buffers]
ORDER BY [DatabaseName]
GO

Reference is taken from sqlskills.com:

Why do we require to clear the cache and buffer before executing a stored procedure in SQL Server?
Sometimes it needs to clear buffer and cache for measuring the exact time duration between the executions.
SQL Server provides two DBCC commands to clear the cache and buffer.

DBCC FREEPROCCACHE :

You can use above DBCC command to clear the cache of the Stored Procedure. When you use this command, it clears all old query plans and recompiles all your statements.

It removes all the elements from the plan cache and forces SQL Server for a recompile. It does not clear any execution statistics for the compiled stored procedure.

You should use this command carefully because every time it creates a new plan for the same query so it may decrease your query performance. But it is required during performance testing because you cannot measure query execution time without removing plan data from the cache.

DBCC DROPCLEANBUFFERS :

Using the above DBCC command, you can remove all clean buffers from the buffer pool. During performance testing of your stored procedure, this DBCC command is handy to clear all buffer data from the buffer pool.

It also makes sure that all data are not coming from the buffer and fetching from the physical data pages so that you can measure the real performance between the executions.

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS