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Re: [Xen-users] is Intel VT-d "really" necessary?

To: "xen-users" <xen-users@xxxxxxxxxxxxxxxxxxx>, <rudi@xxxxxxxxxxx>
Subject: Re: [Xen-users] is Intel VT-d "really" necessary?
From: "Nick Couchman" <Nick.Couchman@xxxxxxxxx>
Date: Wed, 15 Sep 2010 08:38:44 -0600
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>>> On 2010/09/15 at 03:20, Rudi Ahlers <Rudi@xxxxxxxxxxx> 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.

That's interesting...is it the motherboards you're seeing this lack of support 
on, or the chips?  I'm pretty sure the i7 processors themselves support VTd, 
you just have to have the BIOS and MB support for it.

> 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.

In an environment where you're planning on running many VMs on a single host, I 
don't imagine VTd is going to be very helpful to you.  I use it here where I 
work, but I use it on desktop systems where folks need access to graphics 
cards, serial cards, etc., from Windows, and not on my server systems.  It 
really depends on what you're doing as to whether you think you'll need it or 
not - if you cannot think of a situation where *any* type of PCI card needs to 
be forwarded through to an HVM domU, then you're probably okay without it.  The 
few situations I can think of are:
- Serial port cards, but this is really more common on desktops
- Phone/Voicemail systems - If you're using any sort of telephony card with an 
HVM domU you'll need direct PCI access, which requires VTd.
- Other, vendor-specific add-in PCI cards, for applications like industrial 
automation, etc.  But you won't see much of this in a data center.

Also, keep in mind that, IIRC, PV domUs can access PCI devices without VTd, so 
if you're going to be running PV kernels on these systems, or the PV kernels 
you're running are the ones that need access to the PCI devices, then you're 
probably fine without it.  I'm sure it offers some performance enhancements 
over software-based IOMMU, but I don't know what those are.


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