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