Questions about certain Xen limits and features

Questions about certain Xen limits and features

[Vessey, Bruce A]

(I previously posted this on the Xen-users list, but received no
responses.  Hopefully this team can help me.)

I've been away from the Xen community for a while and, unfortunately,
haven't kept up with its current state.  I'm assembling a comparison of
virtualization solutions (Xen, VMware, Hyper-V, ...) and I have a few
questions about Xen's current functionality.  I'd appreciate it if
somebody would be willing to take the time to answer.  I'm interested in
the current *released* version (i.e. Xen 3.3 -- not unstable) for
x86_64.  Of course, if you know about on-going work that will change any
of these answers, I'd appreciate knowing that too.

Thanks,
Bruce


(1)  What's the largest number of "logical processors" that Xen supports
on the host system?  For me, this translates to cores.  I prefer to keep
hyperthreading out of the picture.  (I believe that this was 126 in 3.3,
but is changing to "unlimited" based on Jan Beulich's recent work.  Is
that correct?)

(2a)  What's the largest host memory that anybody has tested?
(2b)  What's the expected host memory limit (if somebody was able to
configure a system that large)?

(3a)  What's the largest guest memory that anybody has tested?
(3b)  What's the expected guest memory limit (if somebody was able to
configure a system that large)?

(4a)  Does Xen support hot-add of physical hardware to the host system?
If so, which devices?
(4b)  Hot remove?

(5a)  What's the maximum # of virtual NICs per guest?
(5b)  What's the maximum # of virtual HBAs per guest?
(5c)  What's the maximum # of LUNs per guest?

(6)  What's the maximum # of powered-on guests per system?

Re: Questions about certain Xen limits and features

[Keir Fraser]

On 7/11/08 14:01, "Vessey, Bruce A" <Bruce.Vessey@unisys.com> wrote:

> (1)  What's the largest number of "logical processors" that Xen supports
> on the host system?  For me, this translates to cores.  I prefer to keep
> hyperthreading out of the picture.  (I believe that this was 126 in 3.3,
> but is changing to "unlimited" based on Jan Beulich's recent work.  Is
> that correct?)

The default build configuration is for 64 CPUs. You have to re-build Xen to
configure support for more CPUs than that. Yes, 126 is the limit for 3.3.

> (2a)  What's the largest host memory that anybody has tested?
> (2b)  What's the expected host memory limit (if somebody was able to
> configure a system that large)?

I think 256GB has been tested. Theoretically we're good up to 1TB, and we
could expand beyond even that without too much trouble.

> (3a)  What's the largest guest memory that anybody has tested?
> (3b)  What's the expected guest memory limit (if somebody was able to
> configure a system that large)?

I think 64GB guests are tested fairly often. There's no particular reason
that a guest couldn't use all available memory, whatever that is (except for
32-bit PV guests which are limited for arcane technical reasons).

> (4a)  Does Xen support hot-add of physical hardware to the host system?
> If so, which devices?
> (4b)  Hot remove?

Depends what the usage scenario of those devices is. One answer would be
that Xen supports device hotplug as far as dom0 supports it.

> (5a)  What's the maximum # of virtual NICs per guest?
> (5b)  What's the maximum # of virtual HBAs per guest?
> (5c)  What's the maximum # of LUNs per guest?
> (6)  What's the maximum # of powered-on guests per system?

Theoretically only limited by available resources. I don't know what's
actually been tested.

 -- Keir

Re: Questions about certain Xen limits and features

[Jan Beulich]

>>> Keir Fraser <keir.fraser@eu.citrix.com> 07.11.08 15:22 >>>
>> (2a)  What's the largest host memory that anybody has tested?
>> (2b)  What's the expected host memory limit (if somebody was able to
>> configure a system that large)?
>
>I think 256GB has been tested. Theoretically we're good up to 1TB, and we
>could expand beyond even that without too much trouble.

1Tb has been tested recently by IBM. However, giving anything beyond
512Gb to Dom0 (or any other domain) is expected have issues, unless
the kernel used is one that has fixes to deal with that (the patch I had
submitted for that apparently caused some trouble in the 2.6.18 tree).
The tools also aren't prepared to deal with DomU-s larger than 512Gb,
I believe.

Jan

RE: Questions about certain Xen limits and features

[Vessey, Bruce A]

Thanks. 




-----Original Message-----
From: xen-devel-bounces@lists.xensource.com
[mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of Jan Beulich
Sent: Friday, November 07, 2008 9:31 AM
To: Keir Fraser; Vessey, Bruce A
Cc: xen-devel@lists.xensource.com
Subject: Re: [Xen-devel] Questions about certain Xen limits and features

>>> Keir Fraser <keir.fraser@eu.citrix.com> 07.11.08 15:22 >>>
>> (2a)  What's the largest host memory that anybody has tested?
>> (2b)  What's the expected host memory limit (if somebody was able to 
>> configure a system that large)?
>
>I think 256GB has been tested. Theoretically we're good up to 1TB, and 
>we could expand beyond even that without too much trouble.

1Tb has been tested recently by IBM. However, giving anything beyond
512Gb to Dom0 (or any other domain) is expected have issues, unless the
kernel used is one that has fixes to deal with that (the patch I had
submitted for that apparently caused some trouble in the 2.6.18 tree).
The tools also aren't prepared to deal with DomU-s larger than 512Gb, I
believe.

Jan


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Re: Questions about certain Xen limits and features

[Stefan de Konink]

Keir Fraser wrote:
>> (2a)  What's the largest host memory that anybody has tested?
>> (2b)  What's the expected host memory limit (if somebody was able to
>> configure a system that large)?
> 
> I think 256GB has been tested. Theoretically we're good up to 1TB, and we
> could expand beyond even that without too much trouble.

I have seen *very* weird bugs related to kernel error reporting when I 
made the switch from 16GB to 32GB of memory. I can do this 'semi' 
dynamically using a Dell 1950.

To be able to actually boot system with 32GB I have to append; i8042.noaux


Might be good reference for other people.



Stefan