April 04, 2011

Performance Counters for SQL server DBA

Available Mbytes

Definition

Available Mbytes stands for free unallocated RAM and displays the amount of physical memory, in MB, available to processes running on the computer.

Interpretation

This counter only displays the last value and is not an average.
If the value is less than 20/25 percent of installed RAM it is an indication of insufficient memory.
Less than 100 MB is an indication that the system is very starved for memory and paging out.
Fluctuations of 100 MB or more can indicate that someone is logged in remotely into the server.
Pages/Sec

Definition

Pages/sec is the number of pages read from the disk or written to the disk to resolve memory references to pages that were not in memory at the time of the reference.

Interpretation

1 This is the sum of two counters - Pages Input/sec and Pages Output/sec.

2 The threshold is normally 20 pages/sec, although one has to investigate activity on the server before concluding paging is the problem.

3 Spikes in pages/sec are normal and possible due to backups, big files/data being written to disk and after reboot.

4 SQL Server has to be configured to dynamically manage to the "Dynamically configure SQL Server memory" option, and the "Maximum Memory" setting should be set to the maximum RAM possible with allowing room for OS. SQL Server should also ideally be the only application on the server.

5 High Available mbytes and low paging file % usage with high pages/sec may not indicate a problem, may merely be indicating that the system is reading a memory mapped file sequentially.

6 Also investigate Page Faults per second, which is the cumulative sum of hard and soft page faults since when the system rebooted. It may be hard to interpret this counter since it is a cumulative value and may be very large but if you have multiple programs sharing the computer with SQL Server you may be able to see which program is causing the paging by looking at each program’s page faults per second.

No comments: