Inconsistent information from different DMVs

Question

I've a server with 12 GB of physical RAM. I just lowered its 'Max Server Memory (MB)' setting from 10 GB to 8 GB.

If I look at the processes' committed memory in Perfmon, I see:

select *
from sys.dm_os_performance_counters
where [object_name] like 'SQLServer:Memory Manager%';
go

Target Server Memory (KB):  8388608
Total Server Memory (KB):   8388608

Showing that SQL Server has already relased the additional 2 GB of memory from the Buffer Pool.

However, if I look at the sys.dm_os_sys_info:

select [bpool_committed]
     , [bpool_commit_target]
     , [bpool_visible]
from sys.dm_os_sys_info;
go

I get inconsistent results:

bpool_committed     = 1048576
bpool_commit_target = 1048576
bpool_visible       = 1048576

Shouldn't sys.dm_os_sys_info reflect the information on the performance counters?

I'm running Microsoft SQL Server 2005 SP1 on Windows Server 2003.

Answer 1

The values were indeed inconsistent and went back to normal after an instance restart.

I recommend not using the sys.dm_os_sys_info for this kind of information since it's not reliable and use the performance counters instead.

Answer 2

There is no inconsistency:

1048576 * 8 = 8388608 KB => 8 GB;

Both Perfmon and sys.dm_os_sys_info show 8 GB;

[bpool_visible], [bpool_commit_target] and [bpool_committed] - in SQL 2005 all these are defined as "Number of 8-KB buffers ..." (http://msdn.microsoft.com/en-us/library/ms175048(v=sql.90).aspx), so you have to multiply the result by 8 to get KB; Starting with SQL 2008 R2 you no longer have to do the multiplication.

If you want to use a multiplication within the query try:

SELECT CAST((bpool_committed * 8) / (1024.0 * 1024.0) AS DECIMAL(20, 2)) AS buffer_pool_committed_memory_in_GB
    , CAST((bpool_commit_target * 8) / (1024.0 * 1024.0) AS DECIMAL(20, 2)) AS buffer_pool_target_memory_in_GB
    , CAST((bpool_visible * 8) / (1024.0 * 1024.0) AS DECIMAL(20, 2)) AS buffer_pool_visible_memory_in_GB 
FROM sys.dm_os_sys_info;