IDE vs SCSI under Virtual Server

IDE vs SCSI under Virtual Server

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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


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  • Cheers for the advice. I have been meaning to read up more about SCSI vs IDE in VS2005 but never found the time.

    On a different note what are Microsoft's plans now that VMware have released GSX Server for free as VMware Server? How does this affect your long term plans for VS and VPC (does it effect VPC? if not what about VMPlayer, does that affact VPC?).

    While I am a great fan of VPC and VS I sometimes find the lack of information for future vision (when compared to VMware) a little disappointing. Why are you not hyping up the next version of VPC already, one would assume you will be aiming to release a new version within 6 months of the release of Vista (I hope before Vista but something tells me I will need to do testing in VMware Workstation for Vista).

    Come on spill the beans ;)
  • Is there a similar performance benefit for Virtual PC?
  • Hi, Ben,
    This question refers to your post on kenel debugging Win XP on VM. How do you set up named pipe? How can you connect the named pipe with COM1?
    Thank you for your help
  • > If you are running Windows NT, 2000, XP, 2003 with Virtual Machine Additions installed; use SCSI.

    OK-- but *how*??

    Can you have a post describing how to switch a VPC image that uses the IDE controller to one that uses the SCSI controller?
  • Morgan -

    Sorry, I will never talk about future products before an official announcement has been made.

    Daren & Jeff -

    Virtual PC do not support SCSI.  This is a Virtual Server only post.

    Ying -

    Just go into the virtual machine settings UI, select COM1 and tell it you want to use a named pipe.

  • I'm trying to install SCO OpenServer on a machine with a SATA controller. How do I create a virtual hard drive that OpenServer will "detect" as an IDE drive?
  • And don't forget, initially creating your virtual disk as a SCSI drive on Virtual Server will use the slower emulation mode, causing your guest operating system installation to crawl compared to the IDE disk.  With R2, Microsoft adds the virtual floppy disk containing the SCSI Shunt Driver, which can be used by hitting "F6" during the Windows operating system installation to load the better performing SCSI emulation driver... and in return, speeding up your Windows guest installation.

  • Can I convert an image that was created on an IDE drive to a SCSI drive? Can I just add the SCSI driver to Windows 2003, then add the adapter, then change the properties of the VHD? How do I add the right SCSI driver to Windows 2003 after the initial install? Thanks!  
  • Neil -

    Many versions of SCO OpenServer will not install inside of VPC / VS

    Virtual Server User -

    If you have installed on IDE and want to move to SCSI what you need to do is:

    1) Add a SCSI controller to your virtual machine
    2) Boot the virtual machine
    3) Uninstall and reinstall the Virtual Machine Additions
    4) Shutdown the virtual machine
    5) Disconnect the drive from the IDE controller and add it to the SCSI controller

  • Hi Ben,

    I tried the steps you outlined in your post of 11 Feb. It worked for 5 out of 6 machines running on one of my virtual servers.
    On the 6th, the virtual machine never detects the SCSI card after it has been added to the configuration.
    I've tried adding the card several times, with and without the VM additions (13.552) installed, and no luck.

    Any ideas?
  • If I create a single-drive Win2003 VHD image as a SCSI drive, how do I easily change it to IDE?  I'd like the image to run fast on Virtual Server 2005 R2, but also be portable to Virtual PC when necessary.
  • Thomas -

    I have no idea - sorry.

    Victor -

    It should just work.

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