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I've been trying to get my mind around this one for the last hour, if anyone has a pointer to who it's done I would appreciate it:

Background: Our VMWare ESXi guests use scsi-based virtual disks all over. This lets us expand our virtual disks in real-time, a feature we use a LET, both on the OS drive (requires windows 2008 and later) and any data drives.

Now, evaluating Hyper-V I need to emulate this. The doc states that a hyper-v guest can only use IDE as boot device, and only SCSI-disk have the option to expand on the fly. So, I want to install Windows 2008R2 with two drives: Disk 0: IDE: boot files only Disk 1: SCSi: Windows (regular C drive)

If possible, I want to avoid giving the IDE-disk any drive letter at all, so that for a regular server admin it will look just like a regular (vmware) guest, with the windows folder on C: and all that.

Have anyone done this? Before I started researching it I thought this would be the way everyone did it, but I can't find much good info out there...

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Thanks, will do. It's because I'm asking the difficult ones ;-) –  Trondh Jul 21 '11 at 17:43
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3 Answers

up vote 2 down vote accepted

As user48838 alludes, you can't boot Hyper-V VMs from the "SCSI Adapter" (because it's not a SCSI adapter at all, they just used that name because people were familiar with it. Related, QEmu/KVM calls it's driver virtio as most people understand virtualization much better than when MS picked the names of it's drivers).

The "SCSI Adapter" is a complicated storage IO bus that Hyper-V's BIOS doesn't support; it's BIOS doesn't support the VMBUS at all. The Windows Boot Manager can only pull winload.exe from storage the BIOS can communicate with (IDE disk, IDE CD, Floppy, and PXE).

In short, you can't do what you want to do. Though you can put the system drive on a dynamic disk and make it 2TB to begin with. This also allows you to shrink (recover unused space) the VHD occasionally.

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Here's some notes on the IDE vs. SCSI for Hyper-V which may offer information to consider:

http://blogs.msdn.com/b/virtual_pc_guy/archive/2009/12/01/why-hyper-v-cannot-boot-off-of-scsi-disks-and-why-you-should-not-care.aspx

The following should have what you need to accomplish what you described:

http://en.wikipedia.org/wiki/NTLDR

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There's no NTLDR in Server 2008 or newer OSes. It was replaced by the Windows Boot Manager and Winload.exe –  Chris S Jul 21 '11 at 17:54
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There's no need to use SCSI on Hyper-V. IDE Emulation address 48-bit with a limit of 2TB and the performance is exactly the same as IDE (you can test yourself).

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He never mentioned anything about IDE being faster or slower. He wants to be able to dynamically resize the VHDs live; which IDE disks can not do. The synthetic driver is faster in certain situations as well. The emulated IDE driver has two extra layers of complexity than the synthetic (SCSI). –  Chris S Jul 21 '11 at 20:30
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