Migration to pvscsi from LSI for SQL Server on VMware; It *really* matters

I expect it to make a big difference.  Even so, I'm still pleasantly surprised how much of a difference it makes.

About the VM:
8 vcpu system.
19 logicaldisk/physicaldisks other than the C Windows install drive.
About the guests vdisks:
Each guest physicaldisk is its own datastore, each datastore is on a single ESXi host LUN.

On July 27th, the 19 SQL Server vdisks were distributed among 3 LSI vHBA (with 1 additional LSI vHBA reserved for the C install drive).

I finally caught back up with this system.  An LSI vHBA for the C install drive has been retained.  But the remaining 3 LSI vHBA have been switched out by pvscsi vHBA.

The nature of the workload is the same on both days, even though the amount of work done is different.  Its a concurrent ETL of many tables, with threads managed in a pool and the pool size is constant between the two days.

Quite a dramatic change at the system level :-)

Lets first look at read behavior before and after the change.  I start to cringe when read latency for this workload is over 150 ms.  100 ms I *might* be able to tolerate.  After changing to the pvscsi vHBA it looks very healthy at under 16 ms.

OK, what about write behavior?

Ouch!! The workload can tolerate up to 10ms average write latency for a bit.  5 ms is the performance target.  With several measures above 100 ms write latency on July 28th, the system is at risk of transaction log buffer waits, SQL Server free list stalls, and more painful than usual waits on tempdb.  But after the change to pvscsi, all averages are below 10 ms with the majority of time below 5 ms.  Whew!

Looking at queuing behavior is the most intriguing :-) Maximum device and adapter queue depth is one of the most significant differences between the pvscsi and LSI vHBA adapters. The pvscsi adapter allows increasing the maximum adapter queue depth from default 256 all the way to 1024 (by setting a Windows registry parameter for "ringpages"). Also allows increasing device queue depth from default 64 to 256 (although storport will pass no more than 254 at a time to the lower layer).  By contrast, LSI adapter and device queue depths are both lower and no increase is possible.

It may be counter-intuitive unless considering the nature of the measure (instantaneous) and the nature of what's being measured (outstanding disk IO operations at that instant).  But by using the vHBA with higher adapter and device queue depth (thus allowing higher queue length from the application side), the measured queue length was consistently lower.  A *lot* lower. :-)

3rd Try Charm: Why does pvscsi rather than LSI matter so much for SQL Server on VMware?

I've written two blog posts recently on the performance benefits of the VMware pvscsi vHBA over the LSI vHBA.

March 23, 2015
SQL Server on VMware: LSI vHBA vs pvscsi vHBA
http://sql-sasquatch.blogspot.com/2015/03/sql-server-on-vmware-lsi-vhba-vs-pvscsi.html

April 7, 2015
Another SQL Server VMware LSI vs pvscsi vHBA blog post
http://sql-sasquatch.blogspot.com/2015/04/another-sql-server-vmware-lsi-vs-pvscsi.html

These posts give details from different systems (although running a similar workload), and claim a benefit of roughly 10 fold in peak throughput and peak disk response times by switching from the default LSI vHBA to the pvscsi vHBA for the SQL Server LUNs.  That sounds a little fishy, doesn't it?  Especially if you are familiar with...

Achieving a Million I/O Operations per Second from a Single VMware vSphere® 5.0 Host
http://www.vmware.com/files/pdf/1M-iops-perf-vsphere5.pdf

Page 10 of the performance study above includes the following text.
"… a PVSCSI adapter provides 8% better throughput at 10% lower CPU cost."

That is a much more modest (and probably more believable) claim than 10x performance benefit.  What gives?

Here's a few details about the mechanics:
LSI vHBA has adapter queue depth 128, cannot be increased. LUN queue depth cannot be increased from default 32.
pvscsi vHBA has default adapter queue depth 256, and default LUN queue depth 64.  Adapter queue depth can be increased to 1024, LUN queue depth to 256 with Windows registry settings.
http://www.pearsonitcertification.com/articles/article.aspx?p=2240989&seqNum=3
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2053145

And here's a detail about the testing that I just happened to come across in the Longwhiteclouds blog.

"Maximum queue depth supported by LSI driver (in guest) cannot be changed. So to keep the aggregate outstanding I/Os per controller lower than the max queue depth we had to use 16 OIOs per vDisk. To have a fair comparison between LSI and pvSCSI, second test also had 16 OIOs per vDisk for pvSCSI as well. " Chethan Kumar, author of the VMware paper, as quoted on Longwhiteclouds.

http://longwhiteclouds.com/2012/03/22/1-million-iops-microsoft-vs-vmware-comparison/

So, comparison testing was done within the queue depth constraints of the LSI vHBA.  But, in the case of these Enterprise Edition SQL Server workloads, the number of outstanding IOs would often exceed 600 and microbursts as high as 1000 outstanding IOs occurred.  That's well outside the LSI adapter queue depth, and the queuing penalty can be high.  Even with 4 LSI adapters in a VM, the aggregate adapter queue depth would be only 512.

If a SQL Server workload doesn't burst more than 32 outstanding IOs per LUN or more than 128 outstanding IOs per vHBA adapter, the change to pvscsi would most likely bring rather modest performance benefits - along the lines of the 8% better throughput at 10% lower CPU utilization indicated in the whitepaper.  In fact, at that low level of outstanding IO... maybe there would be a slight performance decline.  That's because the LSI vHBA can allow an IO request up to 32mb in size.  SQL Server won't (yet) perform disk IO that large.  The largest disk IO I've seen from SQL Server has been 4 mb.*  The pvscsi vHBA adapter currently will allow a maximum disk IO size of 512kb.

However - really large disk IOs from SQL Server are in my experience fairly rare, and high aggregate queue length is more common.  For that reason, I heartily recommend using the pvscsi vHBA for SQL Server vms.  Retaining the LSI vHBA for the boot drive is common, even when pvscsi vHBAs are added for database LUNs.  I've got nothing against that approach. But its important to ensure that a SQL Server vm can handle the outstanding IO generated by its workload.  CPUs are hungry - feed them lots of data quickly :-).

*But Niko has shown that columnstore will do read IO up to 8 mb :-)
Clustered Columnstore Indexes – part 50 ("Columnstore IO")
http://www.nikoport.com/2015/04/04/clustered-columnstore-indexes-part-50-columnstore-io/