On Wed, Sep 15, 2010 at 11:19:38AM +0200, Rudi Ahlers wrote:
Hi all,
I'm just curios and would like some input from the community on this one. We're busy budgeting for a couple of new servers and I thought it would be good to try out the Core i7 CPU's, but see the majority of them don't offer VT-d, but just VT-x. Looking at the LGA1366 range, only the "Intel lga1366 i7 980XE" (from the list of what our suppliers stock) have VT-d, and it costs 4x more than "Intel lga1366 i7 930" or 2x more than "Intel lga1366 i7 960". From a budget perspecitve I could purchase 4 more CPU's, which could translate to 40x - 80x more VM's being hosted for the same capital outlay. Experience has shown that we under-utilize CPU's by a great margin and memory / HDD IO is our biggest bottleneck on any server.
So, if VT-d really necessary? We mainly host XEN virtual machine for the hosting industry, i.e. we don't need / use graphics rendering inside VM's, or need DAS on the VM's, etc.
VT-d is marketing term for Intel's IOMMU (IO MMU) implementation, and it's used *only* for PCI passthru, aka giving guest VM direct PCI access to some physical PCI device (nic, hba, etc) on the host hardware.
Xen can actually do PCI passthru *without* VT-d for PV guests, but for Xen HVM guests you *need* VT-d (if you want to use PCI passthru).
VT-d is NOT required for running HVM/Windows guests.
VT-x is the CPU feature that makes it possible to run unmodified guests. VT-d is the chipset IOMMU feature for PCI passthru.
See: http://wiki.xensource.com/xenwiki/XenPCIpassthrough
-- Pasi