"...There has been some confusion over when users should use SCSI or IDE virtual hard disk inside of their virtual machines. The first thing to note is that it does not matter what sort of physical hard disk you have in your computer when you are making this decision.

There are a number of factors to consider:

Our emulated IDE can only support 4 disks, while our emulated SCSI supports up to 28 disks
Our emulated IDE can only support up to 128GB disks, while our emulated SCSI can support up to 2TB disks
Our emulated IDE controller has a higher level of driver support for various operating systems than our emulated SCSI controller

The final thing to consider is performance - and this is a bit tricky. Contrary to common sense, the performance of our emulated SCSI controller is slower than that of our emulated IDE controller. The reason for this is that the SCSI controller is a lot more complicated to emulate than the IDE controller. Now - this changes once you have Virtual Machine Additions installed. As part of Virtual Machine Additions we install an accelerated SCSI driver. Once this driver is installed the performance of our emulated SCSI controller is significantly faster than our emulated IDE controller.

So with all of this in mind, my recommendation is this:

If you are running Windows NT, 2000, XP, 2003 with Virtual Machine Additions installed; use SCSI.
If you are running any other configuration; use IDE unless you specifically use more than 4 disks, or larger than 128gb disks..."

How to Convert Virtual IDE to Virtual SCSI:

If you have installed on IDE and want to move to SCSI what you need to do is:
Add a SCSI controller to your virtual machine
Boot the virtual machine (VM)
Uninstall and reinstall the Virtual Machine Additions
Shutdown the virtual machine
Disconnect the drive from the IDE controller and add it to the SCSI controller

Under Hyper-V we have two kinds of disk controller that you can add to a virtual machine – IDE disks and SCSI disks. A rough list of the differences between these controllers is as follows:

IDE:
Works on operating systems without integration services installed / available.
Can be used to boot a virtual machine

SCSI:
Supports hot add / remove of virtual hard disks.
(ed) Can NOT be used to boot a virtual machine.

The type of disk controller that you use in the virtual machine has nothing to do with the type of disk that you are using in your physical computer. SCSI VHDs can be stored on IDE disks, and IDE VHDs can be stored on SCSI disks.

You could not boot Hyper-V virtual machine from SCSI disk. Well, calling our SCSI controller "SCSI: is somewhat of a misnomer.

When we were working on Virtual Server we heard, loud and clear, that servers had SCSI disks – so virtual servers needed to have virtual SCSI disks. To this end we invested in the emulation of an Adaptec SCSI controller for Virtual Server. Unfortunately, this SCSI controller is a very advanced piece of hardware – and as a result was actually slower to emulate than the more simplistic IDE controller that we were already emulating.

In the end we had to extend our emulated Adaptec controller and write custom drivers for our supported operating systems in order to get good performance for SCSI in a virtual machine.

With Hyper-V we dropped the emulation of an Adaptec controller altogether. Instead we implemented our traditional emulated IDE controller and a new completely virtual, VMBUS based, storage controller - with no traces of emulation present.

It is this virtual storage controller that you are adding to a virtual machine when you choose to add a SCSI controller to a virtual machine.

The problem is that the BIOS that is used by our virtual machine has no knowledge of VMBUS and is only able to boot to emulated devices. This is why you can boot to an IDE controller and a legacy network adapter – but not to a SCSI controller or network adapter.

Generally, when talking to users about needing to boot off of SCSI disks in a virtual machine – there were two reasons that came up.

The first one was that SCSI could support larger virtual hard disks than IDE could. To address this we made the IDE controller in Hyper-V use 48-bit LBA. This allows you to attach virtual hard disks that are up to 2TB in size to an IDE controller.

The second reason was performance. But that too is not an issue.

In the child partition side of things is the “ (VSC)” is essentially the driver that gets loaded when you attach a SCSI controller to a virtual machine. It connects to VMBUS and allows us to perform Disk I/O without any emulation involved in a very high performance manner.

The next thing to notice is the “Fast Path Filter”. This is a filter driver that gets installed on all disk objects in the virtual machine – whether they are IDE or SCSI. It allows us to pass directly to the VMBUS based path for everything except low level disk operations (like partitioning a disk).

What this means is that once the integration services are installed the same code path is used for disk I/O whether you use an IDE disk or a SCSI disk (ed. so the speed is the same regardless of the IDE or SCSI selection.) There are two limitations that remain for IDE disks:

Bottom line, you could not boot off of SCSI disks in Hyper-V. Thanks.

Best regards,

Eric "G"
Partner Online Technical Community