SQL Inside the Storage Engine: What’s in the buffer pool?
The answer is that pages are stored in buffers in the buffer pool (aka buffer cache or data cache), and the buffers are indeed hashed so they can easily be found by database. You can see what pages are currently in the buffer pool, and their status using thesys.dm_os_buffer_descriptors DMV in 2005:
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| SELECT * FROM sys.dm_os_buffer_descriptors; GO |
database_id file_id page_id page_level allocation_unit_id page_type row_count free_space_in_bytes is_modified ----------- -------- -------- ----------- -------------------- -------------- ----------- ------------------- ----------- 1 1 9 0 6488064 BOOT_PAGE 1 7362 0 1 1 6 0 6488064 DIFF_MAP_PAGE 2 6 0 1 1 7 0 6488064 ML_MAP_PAGE 2 6 0 1 1 104 0 262144 DATA_PAGE 100 4196 0 1 1 105 0 851968 DATA_PAGE 65 5041 0 1 1 106 0 262144 DATA_PAGE 197 413 0 1 1 107 0 262144 DATA_PAGE 207 23 0 1 1 108 1 262144 INDEX_PAGE 7 7949 0 . . |
I cut off the output rather than list all 3258 pages in the buffer pool on my laptop. The DMV gives you back some info from the pages themselves as well as you can see(remember all this is in memory so it’s quick to find).
I played around with the DMV a little bit and came up with a neat script that will tell you may many clean and dirty pages there are in the buffer pool per-database.
I played around with the DMV a little bit and came up with a neat script that will tell you may many clean and dirty pages there are in the buffer pool per-database.
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| SELECT ( CASE WHEN ([is_modified] = 1) THEN N 'Dirty' ELSE N 'Clean' END ) AS N 'Page State' , ( CASE WHEN ([database_id] = 32767) THEN N 'Resource Database' ELSE DB_NAME ([database_id]) END ) AS N 'Database Name' , COUNT (*) AS N 'Page Count' FROM sys.dm_os_buffer_descriptors GROUP BY [database_id], [is_modified] ORDER BY [database_id], [is_modified]; GO |
Page State Database Name Page Count ---------- ----------------------- ---------- Clean master 302 Dirty master 1 Clean tempdb 88 Dirty tempdb 52 Clean model 56 Clean msdb 622 Dirty msdb 5 Clean adventureworks 110 Clean DemoRestoreOrRepair 64 Clean DBMaint2008 88 Clean DemoFatalCorruption1 64 Clean DemoFatalCorruption2 64 Clean broken 64 Clean DemoFatalCorruption3 64 Clean DemoCorruptMetadata 111 Clean DemoDataPurity 88 Clean SalesDB 123 Clean DemoNCIndex 88 Clean shrinktest 88 Clean DemoRestoreOrRepairCopy 64 Clean DemoSuspect 64 Clean FileHeaderTest 96 Clean MultiFileDB 96 Clean HA2008 88 Clean SalesDB_Snapshot 21 Clean BootPageTest 88 Clean Resource Database 599 |
Later this week I’ll try to blog a script that can tell you how much of a particular table is in memory ([Edit 12/8/11: that script is here]). Enjoy!