So I wrote a post a little while ago about filesystem alignment, and why I think it’s important. You can read it here. Obviously, the issue of what to do with guest OS file systems comes up from time to time too. When I asked a colleague to build some VMs for me in our lab environment with the system disks aligned he dismissed the request out of hand and called it an unnecessary overhead. I’m kind of at that point in my life where the only people who dismiss my ideas so quickly are my kids, so I called him on it. He promptly reached for a tattered copy of EMC’s Techbook entitled “Using EMC CLARiiON Storage with VMware vSphere and VMware Infrastructure” (EMC P/N h2197.5 – get it on Powerlink). He then pointed me to this nugget from the book.
I couldn’t let it go, so I reached for my copy (version 4 versus his version 3.1), and found this:
We both thought this wasn’t terribly convincing one way or another, so we decided to test it out. The testing wasn’t super scientific, nor was it particularly rigorous, but I think we got the results that we needed to move forward. We used Passmark‘s PerformanceTest 7.0 to perform some basic disk benchmarks on 2 VMs – one aligned and one not. These are the settings we used for Passmark:
As you can see it’s a fairly simple setup that we’re running with. Now here’s the results of the unaligned VM benchmark.
And here’s the results of the aligned VM.
We ran the tests a few more times and got similar results. So, yeah, there’s a marginal difference in performance. And you may not find it worthwhile pursuing. But I would think, in a large environment like ours where we have 800+ VMs in Production, surely any opportunity to reduce the workload on the array should be taken? Of course, this all changes with Windows 2008. So maybe you should just sit tight until then?