Thursday, November 5, 2015

High DiskSpd Activity in VMware VMs - Part 2: More Questions

In my most recent previoust blog post I showed some perfmon graphs, with 1 second collection interval, from some tests I was running with DiskSpd in an 8 vcpu VMware VM, against a guest LUN managed by the pvscsi vHBA.

High DiskSpd Activity in VMware VMs - Part 1: Start of an investigation 

In that particular test run, the main concern was the high privileged time on vCPU 0, which is also the location of the SQL Server logwriter.

Although most of my tests showed high CPU utilization on vcpu 0, there was some variability.  Sometimes it would be a different single vcpu bearing the cost of handling the vHBA.  Sometimes the work would be shared among a few vcpus.

Here are some results of a separate test, mere minutes away from the last test I shared, on the same system.  These results show another concern I have about high SQL Server disk IO within VMware VMs.

In this case, cost of managing disk IO has been balanced across vCPUs 0, 1, 2, and 3.  The remaining 4 vcpus have little involvement in managing the disk IO.

That presents an interesting question - why is the cost dispersed in this case?  It may be extremely preferable to disperse the cost over 4 vcpus, rather than keep vcpu 0 (*especially* vcpu 0) nearly fully consumed with privileged time.
But there's another important question - and one which may be familiar to lots of other folks running SQL Server in VMware VMs by the time I'm done.  What's going on in the nearly 6 second valley?

If I look at IOPs (reads/sec since this was a read-only test) the valley is evident, too.

Was the test running?  Oh, yeah.  In fact, this test had a target queue length (total outstanding IO against the tested Windows volume) of 16 - and the valley was the only sustained time that the target was achieved constantly. (As an aside: I still see folks - performance experts even!! - proclaim 'current disk queue length' as a metric without value.  They're wrong.)

But... if there was a constant queue length of 16 reads while IOPs dropped really low...

Yep.  Really high latency. So that valley in activity and utilization is really a performance divot.  (That's one of my favorite terms.  You might end up seeing it a lot if you read my blog regularly.)

Sometimes a chart of the values over time put performance divots with extreme outlier values into better perspective.

Disk Reads/sec Current Disk Queue Length  Avg. Disk sec/Read
14:50:18 65389.65 12 0.00022
14:50:19 58861.78 16 0.00024
14:50:20 42.96 16 0.31733
14:50:21 18.98 16 0.71181
14:50:22 8.96 16 0.50111
14:50:23 272.43 16 0.09647
14:50:24 0.00 16 0.00000
14:50:25 25.35 16 1.24708
14:50:26 54419.92 11 0.00039
14:50:27 65439.81 12 0.00022

So - average read latency of under a millisecond at roughly 60,000 read IOPs with a divot to under 300 read IOPs at average read latency from 96 milleseconds to over 1 full second.  And one second ending at 14:50:24 with NO reads completed at all.

That raises some serious questions.  I've seen this behavior numerous times on numerous systems.  Hopefully in the next couple of weeks I'll be able to get to the bottom of it, so I can share some answers rather than just more questions.  :-)

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