Virtual Machine I/O – Fairness vs. Performance

Cormac Hogan is a senior technical marketing architect within the Cloud Infrastructure Product Marketing group at VMware. He’s responsible for storage in general, with a focus on core VMware vSphere storage technologies and virtual storage, including the VMware vSphere® Storage Appliance.

Hogan had a recent post in his blog that caught our attention. Titled Virtual Machine I/O — Fairness versus Performance, the post focuses on I/O queue settings internal to the VMkernel and how they are used to balance fairness and performance for virtual machine I/O.

The post delves into queue depths, including LUN/device queue depths, and how these can be tuned to provide different performance for your I/O. It provides detail on how the VMkernel uses  these settings and how performance can be sacrificed when you need to balance I/O resources across several guest machines.

Raxco Software President Bob Nolan added, via a comment on the blog, the following insight, among others:

“If fairness means throttling the I/O requests, then getting the most out of the available I/O requests makes sense. Defragmenting Windows guests systems generates larger I/O and increases sequential I/O. Both would help performance when the numbers of outstanding I/O requests are constrained.”

The full blog post and Bob’s complete comment are here.

Category: PerfectDiskVirtualization