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When To Use VMware PVSCSI (And When Not To)

VMware PVSCSI is an optimized SCSI controller driver that is optimized to handle heavy I/O loads generated by a virtual machine. I am often asked, "When should one use VMware PVSCSI?" The answer is simple: Use it when virtualizing high I/O applications, such as Microsoft SQL Server or Exchange Server.

VMware recommends that you use the PVSCSI driver when virtual machines are generating over 2,000 IOPS. That is a very high number of IOPS. To put things in perspective for you, a 15K SAS drive generates about 170 randomized IOPS.

That being said, the LSI SCSI driver that is available and used in most cases by default is optimized to handle both low and high I/O and can adjust accordingly. In most cases, the use of the LSI SCSI driver will yield excellent performance and will consume acceptable CPU cycles on the ESX host.

Where VMware PVSCSI shines is when you get into the larger IOPS requirements. In those cases, the PVSCSI driver will consume significantly fewer CPU cycles than the LSI driver.

Now with the release of vSphere 4.1, VMware tweaked the PVSCSI driver for better performance with workloads generating less than 2,000 IOPS. However, the accepted best practice still dictates the use of the LSI driver for all low to medium workloads.

VMware spent a lot of time tweaking the LSI driver for random VM workloads and, as such, it's a victim of its own success. Now, I am sure that with every release of vSphere, the PVSCSI driver will be further enhanced and will get to a point where it can handle all types of workloads without any penalties.

Posted by Elias Khnaser on 03/22/2011 at 12:49 PM


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