IDE vs SCSI under Virtual Server

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

Cheers,
Ben