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Old 07-19-2011, 10:49 AM   #1
ddenton
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Disparity in IOPS between Sar and IOStat


Hello all...

I have a RHEL5 server that hosts an apache SSL proxy and about 20 tomcat instances. As of late we've had latency issues on the system that I can't pin down. In trying to diagnose whether the local HD is being over-utilized, I started gathering disk utilization stats using iostat and sar. For Sar, I'm using the "tps" metric, and for IOStat I'm combining reads and writes per second for the raw disk device, sda. When I put the stats into excel, the profile of the graphed data points match up for the most part, but sar is reporting the values for the same data points as being many magnitudes higher.

Can anybody give me a hint as to why one tool would report the same data differently when (as far as I know) both of them pull their disk I/O stats from the same place?

Thanks!

Dan
 
Old 07-20-2011, 03:05 PM   #2
ddenton
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Little help?

Thanks...
 
Old 07-20-2011, 06:55 PM   #3
syg00
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Maybe look at Q 2.7 in the FAQ. Might be relevant.
 
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Old 07-21-2011, 09:48 AM   #4
ddenton
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Telling me to read the site FAQ doesn't help. I've googled, read the man pages both both commands, and searched LQ for an explanation as to why both commands are reporting wildly different values (but nonetheless graphed they have the same rough profile).

Both commands supposedly report iops for physical devices, and both utilities have cron jobs set to fire every minute, so the interval is the same.

The system is RHEL5 with the 2.6.18-194.11.4.el5 kernel. Sysstat version is 7.0.2-3.

Any more help would be appreciated.
 
Old 07-21-2011, 05:01 PM   #5
ddenton
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I think I've found the difference between the two. I have another server identical to this that when using KSAR to graph the SAR output that shows block device statistics, and one of those stats is a tp/s stat that matches up to IOStat's tp/s figure.

Now all I have to do is find out why one of these servers isn't showing block device stats in SAR.
 
  


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