Do you know how fast your disks are?

You really should!  For a while now I have been talking to people about how disk throughput is the most common bottleneck in private cloud deployments.  But yet most people do not know how fast their disks actually are.  To give an idea of the sort of impact disk speed can have on virtual machine performance - let me share this story:

I frequently do performance bench-marking on different builds and configurations of Hyper-V.  One of the benchmarks I do is "Windows installation time".  I have a special installation CD image that I created - which installs Windows Server 2012 R2 in a completely unattended fashion, and shuts down the virtual machine when it is completed.  Recently, I was worried that we might have introduced a significant performance regression in a daily build of Hyper-V - only to realize that I had performed my Windows installation test on a different hard disk than I usually do.  This lead to me bench marking the speeds of the different disks that I had - and performing my installation test on each configuration.

Here are the summary notes:

  • Brand new SATA3 SSD
    • Speed: ~60,000 8K IOPs
    • Windows Installation time: 3 minutes, 45 seconds
  • Two year old SATA3 SSD
    • Speed: ~15,000 8K IOPs
    • Windows installation time: 9 minutes, 50 seconds
  • RAID 10 SATA2 7,200 RPM HDD
    • Speed: ~700 8K IOPs
    • Windows installation time: 32 minutes, 25 seconds

This is quite a significant difference.  It is important to note that for these tests - all other factors (CPU, Memory, etc...) were identical.  If you have never tested the speed of your storage - Jose Barreto has a great blog post on how to do this here: