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Old 04-04-2011, 08:40 AM
Soren Hansen
 
Default Revisit Xen support

2011/4/3 Clint Byrum <clint@ubuntu.com>:
> Excerpts from Clint Byrum's message of Fri Apr 01 16:51:04 -0700 2011:
>> Other than people already having familiarity with Xen, what is a
>> compelling reason to support it in favor of, or in addition to, KVM?
> Not one person has stood up and said that KVM blows Xen away, or is even
> "better".

Um, no... because you didn't ask.

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Old 04-04-2011, 09:40 AM
Clint Byrum
 
Default Revisit Xen support

Excerpts from Soren Hansen's message of Mon Apr 04 01:40:39 -0700 2011:
> 2011/4/3 Clint Byrum <clint@ubuntu.com>:
> > Excerpts from Clint Byrum's message of Fri Apr 01 16:51:04 -0700 2011:
> >> Other than people already having familiarity with Xen, what is a
> >> compelling reason to support it in favor of, or in addition to, KVM?
> > Not one person has stood up and said that KVM blows Xen away, or is even
> > "better".
>
> Um, no... because you didn't ask.
>

Fair enough.

Maybe we should ask though. Adding Xen back in means less resources for
KVM, so the KVM users' opinions matter quite a bit.

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Old 04-04-2011, 10:31 AM
Soren Hansen
 
Default Revisit Xen support

2011/4/2 Serge van Ginderachter <serge@vanginderachter.be>:
> On 2 April 2011 16:58, Clint Byrum <clint@ubuntu.com> wrote:
>> Serge, would you mind elaborating on that? I'm looking for facts.
>
> I tested several virtualisation technologies last year.

I'm terribly sorry, but this information is practically useless. There
are no version numbers, no information about configuration, about
backing stores, disk image formats, cache settings, and very little
about hardware, etc. I can't e.g. tell if your factor 8 drop i
performance on Ubuntu for small writes is due to the virtual disk
being backed by a qcow2 on ext4, for instance, and as such, I can't
use the data (and much less the conclusions) for anything.

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Old 04-04-2011, 11:08 AM
"Michael Zoet"
 
Default Revisit Xen support

Am Mo, 4.04.2011, 11:40 schrieb Clint Byrum:
> Excerpts from Soren Hansen's message of Mon Apr 04 01:40:39 -0700 2011:
>> 2011/4/3 Clint Byrum <clint@ubuntu.com>:
>> > Excerpts from Clint Byrum's message of Fri Apr 01 16:51:04 -0700 2011:
>> >> Other than people already having familiarity with Xen, what is a
>> >> compelling reason to support it in favor of, or in addition to, KVM?
>> > Not one person has stood up and said that KVM blows Xen away, or is
>> even
>> > "better".
>>
>> Um, no... because you didn't ask.
>>
>
> Fair enough.
>
> Maybe we should ask though. Adding Xen back in means less resources for
> KVM, so the KVM users' opinions matter quite a bit.
>

In my opinion asking "is some software better than another software" is
the wrong approach. KVM has advantages over Xen and Xen has advantages
over KVM. It depends on a lot of factors which is an appropriate solution
for a given task. Sometimes KVM wins and sometimes Xen and sometimes
VMware and so on.

KVM should not degraded in favor of Xen. Never!

What SysAdmins need are options to choose from to fit the best in their
networks. If Xen is available in the vanilla kernel a Xen kernel should be
available. But never in favor of a good KVM support.

It is the same for MTAs: we have among postfix exim, sendmail, qmail and a
lot of other MTAs in the package repository. One MTA might work better for
a given situation than the others.


Michael


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Old 04-04-2011, 11:16 AM
Serge van Ginderachter
 
Default Revisit Xen support

On 4 April 2011 12:31, Soren Hansen <soren@linux2go.dk> wrote:


2011/4/2 Serge van Ginderachter <serge@vanginderachter.be>:



> On 2 April 2011 16:58, Clint Byrum <clint@ubuntu.com> wrote:

>> Serge, would you mind elaborating on that? I'm looking for facts.

>

> I tested several virtualisation technologies last year.



I'm terribly sorry, but this information is practically useless. There

Yes, I very well realise that.*I only gave some general concusions to give an idea of what we were looking at.

I was only trying to slightly elaborate on the matter as an answer to an earlier question on the list.
Unfortunately, the reports of those tests are private and not published, and I'm not allowed to do that (don't ask), so I can't fully disclose them.

*
are no version numbers, no information about configuration, about
backing stores, disk image formats, cache settings, and very little

about hardware, etc. I can't e.g. tell if your factor 8 drop i

performance on Ubuntu for small writes is due to the virtual disk

being backed by a qcow2 on ext4, for instance, and as such, I can't

use the data (and much less the conclusions) for anything.
Yes, lots of things could be optimised, that's for sure. But the main aim*of the tests were primarily about comparing Xen and KVM, and as such, similar setups (LVM backed disks, Virtio/HVM hardware, using the same startup scripts on all platforms, ... ) and pretty much most default settings were used. The VM images were all identical, Debian Lenny with ext3.


So, while different settings might not be fully optimized in those tests - at the time we were pretty new with this stuff - we did made several tests which could compare different platforms. And our conclusion to that was that KVM in general was less performant than Xen.


That is the only point I wanted to make. Obviously, YMMV.*
Also note that these conclusions don't stop me from still using Ubuntu+KVM for lots of setups, but more because of ease of use than performance.


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Old 04-04-2011, 11:20 AM
Chuck Short
 
Default Revisit Xen support

On Mon, 04 Apr 2011 02:40:25 -0700
Clint Byrum <clint@ubuntu.com> wrote:

> Excerpts from Soren Hansen's message of Mon Apr 04 01:40:39 -0700
> 2011:
> > 2011/4/3 Clint Byrum <clint@ubuntu.com>:
> > > Excerpts from Clint Byrum's message of Fri Apr 01 16:51:04 -0700
> > > 2011:
> > >> Other than people already having familiarity with Xen, what is a
> > >> compelling reason to support it in favor of, or in addition to,
> > >> KVM?
> > > Not one person has stood up and said that KVM blows Xen away, or
> > > is even "better".
> >
> > Um, no... because you didn't ask.
> >
>
> Fair enough.
>
> Maybe we should ask though. Adding Xen back in means less resources
> for KVM, so the KVM users' opinions matter quite a bit.
>

I totally disagree with this. Adding Xen back would take little effort
to do so.

Regards
chuck

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Old 04-04-2011, 11:20 AM
Serge van Ginderachter
 
Default Revisit Xen support

On 4 April 2011 13:08, Michael Zoet <Michael.Zoet@zoet.de> wrote:


> Maybe we should ask though. Adding Xen back in means less resources for

> KVM, so the KVM users' opinions matter quite a bit.



In my opinion asking "is some software better than another software" is

the wrong approach. KVM has advantages over Xen and Xen has advantages

over KVM. It depends on a lot of factors which is an appropriate solution

for a given task. Sometimes KVM wins and sometimes Xen and sometimes

VMware and so on.

Ack. +1*

KVM should not degraded in favor of Xen. Never!

At least not in the current state of things. One of the major advatages of KVM is its stimplicity, and the fact it's in streamline Linux.

As I understood, as well in Debian as In ubuntu, the problem with Xen was keeping it supported in more recent kernels, and managing the whole thing.


What SysAdmins need are options to choose from to fit the best in their

networks. If Xen is available in the vanilla kernel a Xen kernel should be

available. But never in favor of a good KVM support.

When Xen gets vanilla support for Dom0, it definitely could get some renewed attention, and things need to be evaluated again.*



It is the same for MTAs: we have among postfix exim, sendmail, qmail and a

lot of other MTAs in the package repository. One MTA might work better for

a given situation than the others.

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Serge van Ginderachter




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Old 04-04-2011, 11:34 AM
Soren Hansen
 
Default Revisit Xen support

2011/4/4 Clint Byrum <clint@ubuntu.com>:
> Maybe we should ask though. Adding Xen back in means less resources
> for KVM, so the KVM users' opinions matter quite a bit.

The very short version: I'm fine with Ubuntu getting Xen support again,
but I don't think it needs to be in main.

We chose KVM as our preferred, supported hypervisor a long time ago.
We've been telling people for years that it's what they should be using,
and lots of great effort has been put into the integration work. The
arguments against KVM were mostly about the hardware requirements, but
if we could live with that in 2007, I'd be surprised if we couldn't
today, since the percentage of server hardware that doesn't work with
KVM has severely declined.

Any decision will have supporters and opponents, and I firmly believe
that making a decision is the right thing to do. I believe there's a lot
of value (for everyone involved) in having firm answers to even tough
questions. Ubuntu, for instance, is a free operating system. No-one has
to ask over and over whether it's still free, because we've been very
clear from the beginning that that's how we roll. Similarly, you won't
find any closed-source applications on an Ubuntu CD. If you're wanting
to distribute closed source applications, don't bother asking if you can
put it on one of the Ubuntu CD's. No matter how popular your software
is, or how many people vote for it on a mailing list or on Ubuntu
Brainstorm, it's not going to happen. We're also not going to switch to
the FreeBSD kernel on a whim. Every decision we make defines us, whether
it's an additive or subtractive one. Every decision we fail to make,
weakens us.

When we chose KVM as our preferred hypervisor, it wasn't a decision to
use it in Hardy and revisit that decision every release following it
(that would have made it almost a non-decision). It wasn't a decision to
run this or that benchmark every 6 months, and whichever was in the lead
would be the preferred, supported hypervisor that we'd go out and
praise, and the rest would be deprecated until 6 months later when the
numbers would be slightly different. We made the decision even though
KVM was still quite young, and none of the other major distros were
shipping it. We made the decision to ship it, support it, stand behind
it, and help it grow. Ubuntu's hypervisor was KVM.

I happily stand by that decision.

I believe KVM's design is superior. KVM immediately benefits from
improvements made to the Linux kernel. If power management improves in
the Linux kernel, your KVM host's power management improves. If the
scheduler improves, KVM benefits. If memory management improves, KVM
benefits.

KVM is part of the Linux kernel, while Xen has its own kernel. I'm not
talking about the dom0, I'm talking about the Xen hypervisor on top of
which the dom0 and domU's run. This difference means that many
improvements in Linux need to be accommodated for or mimicked in Xen
before you get the benefits there[1]. To use KVM, you load a module that
turns your regular Linux kernel into a hypervisor. To run Xen, you boot
a completely different kernel on top of which you run a dom0. For the
most part, you don't see the difference, because distributors have put a
lot of work into making this change seamless, but effectively, you're
not running Linux anymore as your kernel. Anthony Liguori (one of the
KVM and QEmu developers) said it quite well[2]: "The whole situation is
somewhat absurd though. It's like if the distributions shipped a NetBSD
kernel automatically and switched to using it when you wanted to run a
LAMP stack." Linux is a fine hypervisor on its own. It may not be
perfect, but I'd prefer we focus on identifying and fixing those issues

If someone thinks Xen is sufficiently cool, I'd encourage them to put
some effort getting it into shape in Ubuntu. I don't think we should
divert any of the existing attention on kvm/libvirt/friends to Xen. I
don't think we can afford it.


[1]: This page on power management with Xen is a good example:
http://wiki.xensource.com/xenwiki/xenpm

[2]: http://blog.codemonkey.ws/2008/05/truth-about-kvm-and-xen.html

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Old 04-04-2011, 03:28 PM
"Michael Zoet"
 
Default Revisit Xen support

Am Mo, 4.04.2011, 13:34 schrieb Soren Hansen:
> 2011/4/4 Clint Byrum <clint@ubuntu.com>:
>> Maybe we should ask though. Adding Xen back in means less resources
>> for KVM, so the KVM users' opinions matter quite a bit.
>
> The very short version: I'm fine with Ubuntu getting Xen support again,
> but I don't think it needs to be in main.
>

I think most people here think the same. Xen do not need to be in main.
Xen is only needed if there is a demand for a feature Xen has and KVM not.

And most people will agree in everything else you wrote in your mail. I
have several KVM and Xen server up and running and I think KVM will be (or
is, if you prefer ;-) ) the virtualisation technology for the future. But
sometimes an admin needs Xen for various reasons and then they get driven
away from Ubuntu for now. Something that is not so good for the wide
deployment of Ubuntu server.


Michael


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Old 04-05-2011, 05:24 PM
Jorge Armando Medina
 
Default Revisit Xen support

On 04/01/2011 06:05 PM, Raphaël Pinson wrote:
> On Sat, Apr 2, 2011 at 1:51 AM, Clint Byrum <clint@ubuntu.com> wrote:
>> Excerpts from Chuck Short's message of Wed Mar 30 07:27:50 -0700 2011:
>>> Hi,
>>>
>>> In the past Xen support in Ubuntu as a host has been difficult for a
>>> variety of reasons most notably no upstream kernel support. Now that
>>> dom0 should be coming into the vanilla kernel soon. I think its time to
>>> revisit supporting Xen as a hypervisor as well.
>> Just playing devil's advocate here.
>>
>> Other than people already having familiarity with Xen, what is a
>> compelling reason to support it in favor of, or in addition to, KVM?
>
> Familiarity is a good reason I think, but also industry standards, and
> hardware considerations. I think a lot of big companies expect major
> distributions such as Ubuntu to provide a proper support for such a
> standard as Xen. I know it came as a disappointment for us that using
> Lucid as a (production) Xen dom0 was nearly impossible. Also, afair,
> KVM requires hardware support. Most recent machines provide it, but
> it's not rare to find servers that are too old to use it, and then
> you'd rather use Xen for servers than VMWare...

That is right, We have customers with big servers with lots of ram using
xen for paravirtulized machines, xen is a good product and we have
really good performance.

By the way, Xen is supported with the OpenStack.

Best regards.
>
> Raphaël
>


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