Disk Latency on Redhat 5 VIrtual Machine

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We have a Virtual Machines running with Redhat 5.5 and Oracle database on it. From the operating system point of view the Disk shows more latency, but in the background(SAN) and the latency for those LUN's are less than 5ms. Like to get some advise for this issue?

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Ya Community Member 55 points
9 December 2015 6:35 PM

What is the hypervisor used? This could very well be due to qdepth limitations.

ra Community Member 20 points
9 December 2015 6:42 PM

Hypervisor is Vmware ESX 5.5 u2. Can you be more specific on the qdepth limitation as the queue depth shows in the screen show is less than 1.

Ya Community Member 55 points
9 December 2015 9:32 PM

qdepths are at many different levels. I would increase the per host qdepth on the ESXi host. So many factors could be causing this. Maybe another guest is competing for IO on the host. This would cause latency on a guest that would otherwise appear to have inactive IO.

ra Community Member 20 points
10 December 2015 6:56 AM

Thank you for the help Yates.

Jamie Bainbridge's picture Red Hat Guru 7252 points
10 December 2015 7:24 AM

You've pointed out that you're using the cfq I/O scheduler, but the Oracle Tuning suggestion is to use the deadline I/O scheduler.

Is this block device using the paravirtualized vmw_pvscsi storage controller, or is it using the emulated mpt2sas or similar? The paravirt controller should be faster than the emulated controller. We didn't ship the paravirt drivers in RHEL5, so you'll need to build these during the install of VMWare Tools.

ra Community Member 20 points
10 December 2015 7:43 AM

Thank you for the reply Jamie, we are not aware of that someone limited the disk IOPS on VM level which made this latency. Thanks a lot for the info.

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