VMware PVSCSI Adapter performance and low I/O Workloads

I’ve recently been implementing a vSphere deployment and have been looking at the new features introduced as part of Virtual Machine Hardware 7.   Obviously one of the major new components is the new Para Virtualised SCSI (PVSCSI) adapter which I wrote about way back in May 2009.   When it first came out there were a number of posts regarding the much improved I/O Performance and latency reduction this new adapter delivered, such as Chad Sakac’s I/O vSphere performance test post.

So the other day I stumbled across a tweet from Scott Drummond who works in the VMware Performance Engineering team. Following a little reading and a bit of digging around it appears that the use of PVSCSI comes with a small caveat.   It would appear that if you use the PVSCSI adapter with low I/O workloads you can actually get higher latency than you get with the LSI Logic SCSI adapter (see the quote below)

The test results show that PVSCSI is better than LSI Logic, except under one condition—the virtual machine is performing less than 2,000 IOPS and issuing greater than 4 outstanding I/Os.

This particular caveat has come to light following some more in-depth testing of the PVSCSI adapter performance.   The full whitepaper can be found at the following link.

PVSCSI whitepaper

For those who don’t want to read the technical whitepaper, a summary of the issue can be found in the following VMware KB article.

VMware KB 1017652

So basically, as opposed to just using the PVSCSI adapter as default with VMs running version 7 of the virtual hardware have a think about it’s I/O profile and whether the PVSCSI or LSI logic adapter would be best.

About the author

Craig Stewart

I’m an IT professional with over 12+ years working in both the public and private sector within the UK. I currently work within the UK financial industry, specialising in infrastructure delivery and integration using products from a host of vendors including Microsoft, Citrix, VMware and EMC to name but a few.

I’ve had an unhealthy interest in all aspects of virtualisation since working on projects deploying VMware Virtual Infrastructure 3 and Citrix XenApp. I have followed this up by achieving certifications in a number of these technologies and trying to find the time to continue learning about and spreading the good word of virtualisation.

Hope you find something interesting here at

1 Comment

Leave a Comment