February 5, 2013 at 4:48 am
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
February 5, 2013 at 5:41 am
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.
There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato
February 5, 2013 at 6:10 am
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
February 5, 2013 at 10:17 am
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.
February 6, 2013 at 9:04 am
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
February 6, 2013 at 10:27 am
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.
February 7, 2013 at 5:07 am
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.
February 7, 2013 at 11:00 am
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.
February 8, 2013 at 4:06 am
That makes sense. Thanks very much for your help with this.
February 8, 2013 at 8:40 am
You're welcome. Glad I was able to help.
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