Hyper-V: SCSI vs IDE - Do you really need an IDE and SCSI drive for best performance?

Wow... blogging is fun and can really help people understand a topic. Other times it can be a chore when the whole picture is not presented up front. This is just what I did in a previous post http://blogs.msdn.com/tvoellm/archive/2007/12/12/which-is-better-ide-or-scsi-windows-server-virtualization-08-code-name-viridian-controller-performance.aspx. In this post I talked about emulated IDE vs Sythetic SCSI which is really not a fair comparison. I also left out the IDE Filter driver that should have the performance of synthetic SCSI devices.


Here are some blog postings that help clarify a few points;

·  Integration Components and Enlightenments

· Boot from SCSI in Virtual Server vs. Boot from IDE in Windows Server virtualization by John Howard


So on the question – Do you really need an IDE and SCSI drive for best performance?

The question / statement was posted in the following blog for which I really appreciate the point of view of the author http://www.virtualization.info/2007/12/hyper-v-will-not-boot-virtual-scsi.html.


Just like all things in performance - it depends. 


First it is generally good practice to separate the “OS partition / disk” from “Data & Workload partitions / disks”. So in that respect yes two disks is a good idea. Once you have committed to having two disks my choice would be to use SCSI for the second drive. This is mostly following the premise of using synthetic devices whenever available.


If you are running workloads where you don’t mind sharing the drive the operating system a single drive will be just fine since the IDE filter driver (you get this by installing the Integration Components) is designed to give good performance. You might also want to use a single drive for portability.


If you decide to use two drives you can try both the IDE filter driver and Synthetic SCSI device to see which works best for your workloads. They should be virtually the same (no pun intended). Switching is easy since it just a matter of reconfiguring the Virtual Machine. The same VHD can be used (just not at the same time – besides it would be weird to dual port of a drive into a VM on IDE and SCSI).


Performance rule of thumb: Don’t start tweaking knobs until you know your goals and find there is a problem.


Keep in mind there are only 2 IDE controllers with 2 devices each. One device will be your boot drive and one might be your CDROM. If you follow the standard configuration (1 boot disk + 1 CDROM) and want more than two additional drives you will have to use SCSI.


[[ Since Hyper-V is not RTM'ed all numbers are subject to change as are the behaviors. ]]