Re: [Xen-devel] PAE support revisited

[Karen White <kawhite@xxxxxxxxxx>]

There are at least two separate efforts looking at enabling PAE in
Xen that I know of.  A couple of us are looking at it and we have
started correspondance with Gerd Knorr, who is also looking at it.

We have no firm idea on when a working implementation will
be available, but are hoping sooner rather than later.

Karen


On Mon, 2005-03-21 at 21:21 +0200, Nuutti Kotivuori wrote:
> I will represent here a commercial look into Xen and PAE support, as
> seen by my company Stinghorn (www.stinghorn.com). The points raised
> are probably not too interesting from a technical standpoint.
> 
> A little background. We are currently offering virtualized products
> and services on top of our own virtualization platform, which is based
> on UML. However, due to the innate limitations in UML, we are
> investigating Xen as an alternative.
> 
> But, the lack of PAE support, that is Xen supporting more than 4G of
> memory, looks like to be an almost showstopper for us. Let me explain
> why.
> 
> We need to be able to have our products working in setups of various
> sizes. In the small end a setup consists of just one or few virtual
> machines running on a rather low end PC, providing services. A medium
> sized setup would consist of running several somewhat memory hungry
> services on a single machine, such as several antivirus/antispam
> gateways. And a large setup would be an operator which takes it for
> granted that they will install atleast 6 gigabytes of memory into the
> machine to run many virtual machines. And these are still x86 servers
> we are talking about, not x86_64.
> 
> While the x86_64 support is progressing nicely, and will support more
> than 4G memory, that does not really help us in this case. Even if
> x86_64 support in Xen was available today, we cannot move all our
> products to be x86_64 only. This is because it is still a young
> platform. Companies do not have x86_64 hardware lying around for
> testing. Distributions' support for x86_64 is still at times a bit
> lacking - we would have to settle only for a few select Linux
> distributions. There are still bugs and undiscovered problems in
> running programs in 64-bit mode. And even the availability of
> Intel-based x86_64 hardware is a bit of a problem. In a year, the
> situation could be entirely different, but as of today, switching
> everything over just is not viable.
> 
> So we would have to support x86 *and* x86_64 - x86 for all the low-end
> cases and x86_64 for the cases requiring over 4G of memory. But, that
> would mean that we would have to have two versions of all our products
> - one for x86 and one for x86_64. Even if we would manage to have just
> two different kernels and the same 32-bit userland (which is unlikely
> to be without problems since some things do communicate with the
> kernel), we would still have double testing effort - once for x86 and
> once for x86_64. And that is a high price to pay.
> 
> We could possibly make due if Xen on x86_64 would support 32-bit
> guests in a way close enough to native 32-bit so that we would only
> have to test the host on x86 and x86_64 and not every different guest
> we provide. But as it stands now, that doesn't seem likely.
> 
> As you can see, we are left with very little options. But if PAE
> support would be in Xen, we could stay x86-only for quite some time
> still. 32-bit only Xen would be fine for us until x86_64 is the norm,
> instead of the exception, and there are services that really require
> it.
> 
> So, we are looking into ways to make this happen. We tried to contact
> XenSource, to ask if it was possible to contract the work for PAE from
> them, but got no reply to our inquiries. I've seen work estimates from
> two or three months to less than a month on the list - but obviously
> it is hard to say without a closer look. Although personally I would
> be very willing to try and make it happen, unfortunately my workload
> is required elsewhere.
> 
> In any case, this is all still only under evaluation and now
> commitments have been made. We do have a working platform at the
> moment, even if it is not perfect, and there would have to be clear
> and definite advantages within our business cases to make the
> switch. But, unless this whole 4G mess gets solved somehow, it is very
> likely we will reconsider Xen again in half a year or in a year, when
> the world looks different again.
> 
> -- Naked
> 
> 
> 
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