Andre Leibovici has posted the second part of his linked clone IO study and it makes interesting reading. The first part is here and the second here they are well worth the read.
One of the things Andre calls out is that having Windows cache the application files saves a lot of IO on the replica and that with sufficient memory in the VM windows doesn’t touch it’s page file. This highlights a trend, giving VMs lots of RAM reduces disk IO load.
This is true for server workloads as well as desktop, this post on the VROOM! blog calls out Exchange 2010 as benefiting from more RAM and I’ve had recent discussions about database applications on Linux that benefit from large amounts of OS RAM as disk cache. For the Linux VM halving the configured RAM on the VM multiplied the IO rate by three, a change that was quickly backed out. The message was repeated in the podcast I recorded with Michael Webster where we talked about virtualising Tier One applications.
When I started “doing” virtualisation it was all about how much you could do with how little, but that doesn’t always answer as a way to work. Some workloads need a lot of RAM to do a lot of work. The RAM is often cheaper than buying a huge amount of spindles of disk or expensive SSD and cache for the SAN.
© 2011, Alastair. All rights reserved.
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