When DBCC DROPCLEANBUFFERS doesn’t work…

Here’s an interesting question that came up in our IEPTO1 class in Chicago this week (paraphrasing):

I was doing a demo recently where I was demonstrating physical I/Os occurring. I used DBCC DROPCLEANBUFFERS, then enabled SET STATISTICS IO ON and performed a SELECT operation. I was expecting to see physical reads occurring but I only saw logical reads. What’s going on? Why weren’t there any physical reads after I’d flushed the buffer pool with DBCC DROPCLEANBUFFERS?

It’s a very interesting question. If you ask most people what DBCC DROPCLEANBUFFERS does, you’ll get the response that it clears out the buffer pool (as my student expected).

But it doesn’t.

It drops *clean* pages from the buffer pool only.

A clean page is one that has not been changed since it was read into memory or last written to disk. A dirty page is one that has not been written to disk since it was last changed. Dirty pages are not dropped by DBCC DROPCLEANBUFFERS, they are only made clean by writing them to disk (either through one of the various kinds of checkpoints or by the lazy writer – or one of the per-NUMA node lazy writers if you have NUMA configured).

The demo in question had updated the table being selected, and so when DBCC DROPCLEANBUFFERS was executed, the pages from that table remained in memory – hence no physical reads were required for the subsequent SELECT.

If you want to ensure that all pages from a database are flushed from memory, you need to first perform a manual CHECKPOINT of that database and then run DBCC DROPCLEANBUFFERS.

You can then verify that there are no pages in memory for the database in question using the following code:

SELECT *,
	[DirtyPageCount] * 8 / 1024 AS [DirtyPageMB],
	[CleanPageCount] * 8 / 1024 AS [CleanPageMB]
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 

Hope this helps some of you out there!