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ggjjbb1983

Posted Tuesday, February 05, 2013 4:48 AM

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Hi,

I'm currently using Idera SQL DM to pull back the following metric:

- Disk Reads / Sec

Is there any reason why this result is different than what I get from PerfMon for the same metric / same time / same disk?

Thanks

Post #1415736

opc.three

Posted Tuesday, February 05, 2013 5:41 AM

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It's possible SQLdm is getting it's information from DMVs (if that counter is even available through the DMVs) which are not guaranteed to be exactly the same reading as what you would get from perfmon. I would actually be surprised if two separate perfmon traces running concurrently retrieving that same counter on the same interval would be guaranteed to line up 100% of the time.

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Post #1415770

ggjjbb1983

Posted Tuesday, February 05, 2013 6:10 AM

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Ok thanks, thats what I was thinking too.

Im looking into collecting performance data over a period of time, and just wondering if there are any pro's cons' of using a tool such as Idera SQL DM vs Perfmon. I realise that a con of using Perfmon will be more difficult to aggregate the data, and its not guranteed to be correct if my caluclations are out.

Thanks

Post #1415782

Robert Davis

Posted Tuesday, February 05, 2013 10:17 AM

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Hi, I work for Idera and have collaborated with the dev team on quite a bit of work on SQL diagnostic manager so perhaps I can help. Can you clarify which counters you are looking at in Performance Monitor? Is it logical disk or physical disk? There is not always a mapping of 1 to 1 for logical to physical drives.

Another thing you to consider is that they are taking the counters at different sampling rates.

SQL diagnostic manager is pulling the data from the OS via WMI. This particular set of data is not from DMVs.

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Post #1415980

ggjjbb1983

Posted Wednesday, February 06, 2013 9:04 AM

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Hi Robert, thanks for your reply.

I'm taking the Disk Reads from the following performance counter:

PhysicalDisk(D:)\Disk Reads/sec

In Idera Im selecting:

Reports > Analyze > Resources > Disk Details.

Im selecting the same drive as I did in Perfmon (D) and runing the report.

I then compared the exact time (e.g. 09:00) between Idera and Perfmon for the same counter but the values are different. I see that Idera captures the data every 5 minutes, so I'm assuming that the value shown for 09:00 is an average between 08:55 and 09:00; whereas Perfmon shows the value for exactly 09:00.

Thanks

Post #1416572

Robert Davis

Posted Wednesday, February 06, 2013 10:27 AM

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What were the values given by each for 9:00? Also, does your server need this patch applied to fix a WMI bug: http://support.microsoft.com/kb/961435. WMI may be reporting incorrect numbers.

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Post #1416636

ggjjbb1983

Posted Thursday, February 07, 2013 5:07 AM

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Im seeing the following value in Perfmon:

2754.68731587591

Im seeing the following value in Idera:

983.00

These are both for the same Drive and same Time.

I dont believe I have the patch applied; however I'm running Windows 7, so it may not be applicable.

Post #1416978

Robert Davis

Posted Thursday, February 07, 2013 11:00 AM

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So the difference you're seeing is definitely the difference in collection intervals. You can adjust the collection interval if you need to for troubleshooting a particular issue, but I would not recommend leaving it at a more frequent collection interval for regular operations. Everything comes with an overhead, and the more often you collect metrics, the bigger the effect it has on the server.

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Post #1417254

ggjjbb1983

Posted Friday, February 08, 2013 4:06 AM

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That makes sense. Thanks very much for your help with this.

Post #1417582

Robert Davis

Posted Friday, February 08, 2013 8:40 AM

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You're welcome. Glad I was able to help.

My blog: SQL Soldier
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Sr. Product Consultant and Chief SQL Server Evangelist @ Idera
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Post #1417747

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