Xen users - Aug 2010 - Can Dom0 be replaced after installation?

Hi,

Agonizing about a strategy for installing, when I know nothing about it.
I suspect that in spite of the flexibility in choices for a dom0
operating system, I think it''s probably really important to get it all
right.

I guess there are some burning questions here that might clear
everything up if I could answer:

1. Assuming I install Xen, a handful of VMs and have it all set up, is
there a way to replace dom0 with some other dom0?  If I knew I could
change my mind later it would be much easier to set this up.

2. There seems to be a way to have a kernel which works either on the
bare metal or as a guest, but it doesn''t seem to have been tested much.
Do I ignore this for now, and if I do can I start using it later?  If I
change it on Dom0 do I need to recompile kernels on the other guests?

3. I keep expecting to need to install the hypervisor somewhere.  I
don''t see the hypervisor anywhere in Gentoo, but there''s
xen-sources
which I suspect contains it.  Or do I need the tarball from
www.xen.org/products/xen_source.html?

4. In my understanding the dom0 needs to be extremely stable and have
good driver support.  What distro works best for that?  I''m currently
using Gentoo, but I would think Debian would be most stable.  If so, is
the current difficulty with Debian an unhappy coincidence or is it
consistent?  How about Ubuntu Server, or is that still the same problem
since it''s also a Debian?  What do people use if they just want a
small,
stable dom0 with good driver support?

5. Is there a way to "loan" processors to a guest if nobody else is
using them?

Thanks.



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On Thu, Aug 12, 2010 at 12:30 PM, Ken <kr.xen@hoverclub.net>
wrote:
> Hi,
>
> Agonizing about a strategy for installing, when I know nothing about it.
> I suspect that in spite of the flexibility in choices for a dom0
> operating system, I think it''s probably really important to get it
all
> right.
At this point I usually say "it depends".
xenserver/XCP is designed for those who wants a system that "just
works" without having to worry about what the best dom0 setup is.
> 1. Assuming I install Xen, a handful of VMs and have it all set up, is
> there a way to replace dom0 with some other dom0?  If I knew I could
> change my mind later it would be much easier to set this up.
In principal, yes. A domU is basically defined by its configuration
file and backing storage. If you have both, then you should be able to
(re)create an identical domU using whatever dom0 hardware/OS.

So assuming that the old and new OS both have access to the same
storage (possibly include support for the same partition/LVM and
filesystem type) and domU configuration, changing dom0 OS is possible.
Moving from (for example) Centos to Opensuse is easy enough, but
moving from Centos to opensolaris is somewhat hard.
>
> 2. There seems to be a way to have a kernel which works either on the
> bare metal or as a guest, but it doesn''t seem to have been tested
much.
> Do I ignore this for now, and if I do can I start using it later?
Since you''re new to Xen, I suggest using dom0 with bundled Xen. My
favorite is RHEL5. Although its version of Xen is slightly out of date
in terms of features, it''s enough for most common use, and kept
up-to-date in terms of bugfix/security update.
>  If I
> change it on Dom0 do I need to recompile kernels on the other guests?
No. domU can be independent of dom0 kernel.
>
> 3. I keep expecting to need to install the hypervisor somewhere.  I
> don''t see the hypervisor anywhere in Gentoo, but there''s
xen-sources
> which I suspect contains it.
Try asking on Gentoo list
on RHEL/Centos it''s simply "yum groupinstall Virtualization"
>  Or do I need the tarball from
> www.xen.org/products/xen_source.html?
Possible, but it might not be so straightforward. See
http://wiki.xensource.com/xenwiki/Xen4.0
>
> 4. In my understanding the dom0 needs to be extremely stable and have
> good driver support.  What distro works best for that?  I''m
currently
> using Gentoo, but I would think Debian would be most stable.  If so, is
> the current difficulty with Debian an unhappy coincidence or is it
> consistent?  How about Ubuntu Server, or is that still the same problem
> since it''s also a Debian?  What do people use if they just want a
small,
> stable dom0 with good driver support?
I like RHEL.
>
> 5. Is there a way to "loan" processors to a guest if nobody else
is
> using them?
By default all domUs have the same priority. See
http://wiki.xensource.com/xenwiki/CreditScheduler

-- 
Fajar

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Am 12.08.2010 09:00, schrieb Fajar A. Nugraha:
> On Thu, Aug 12, 2010 at 12:30 PM, Ken<kr.xen@hoverclub.net>  wrote:
>> Hi,
>>
>> Agonizing about a strategy for installing, when I know nothing about
it.
>> I suspect that in spite of the flexibility in choices for a dom0
>> operating system, I think it''s probably really important to
get it all
>> right.
>
> At this point I usually say "it depends".
> xenserver/XCP is designed for those who wants a system that "just
> works" without having to worry about what the best dom0 setup is.
>
>> 1. Assuming I install Xen, a handful of VMs and have it all set up, is
>> there a way to replace dom0 with some other dom0?  If I knew I could
>> change my mind later it would be much easier to set this up.
>
> In principal, yes. A domU is basically defined by its configuration
> file and backing storage. If you have both, then you should be able to
> (re)create an identical domU using whatever dom0 hardware/OS.
>
> So assuming that the old and new OS both have access to the same
> storage (possibly include support for the same partition/LVM and
> filesystem type) and domU configuration, changing dom0 OS is possible.
> Moving from (for example) Centos to Opensuse is easy enough, but
> moving from Centos to opensolaris is somewhat hard.
>
>>
>> 2. There seems to be a way to have a kernel which works either on the
>> bare metal or as a guest, but it doesn''t seem to have been
tested much.
>> Do I ignore this for now, and if I do can I start using it later?
>
> Since you''re new to Xen, I suggest using dom0 with bundled Xen. My
> favorite is RHEL5. Although its version of Xen is slightly out of date
> in terms of features, it''s enough for most common use, and kept
> up-to-date in terms of bugfix/security update.
>
>>   If I
>> change it on Dom0 do I need to recompile kernels on the other guests?
>
> No. domU can be independent of dom0 kernel.
>
>>
>> 3. I keep expecting to need to install the hypervisor somewhere.  I
>> don''t see the hypervisor anywhere in Gentoo, but
there''s xen-sources
>> which I suspect contains it.
Try: http://en.gentoo-wiki.com/wiki/Xen
>
> Try asking on Gentoo list
> on RHEL/Centos it''s simply "yum groupinstall
Virtualization"
>
>>   Or do I need the tarball from
>> www.xen.org/products/xen_source.html?
>
> Possible, but it might not be so straightforward. See
> http://wiki.xensource.com/xenwiki/Xen4.0
>
>>
>> 4. In my understanding the dom0 needs to be extremely stable and have
>> good driver support.  What distro works best for that?  I''m
currently
>> using Gentoo, but I would think Debian would be most stable.  If so, is
>> the current difficulty with Debian an unhappy coincidence or is it
>> consistent?  How about Ubuntu Server, or is that still the same problem
>> since it''s also a Debian?  What do people use if they just
want a small,
>> stable dom0 with good driver support?
>
> I like RHEL.
>
>>
>> 5. Is there a way to "loan" processors to a guest if nobody
else is
>> using them?
>
> By default all domUs have the same priority. See
> http://wiki.xensource.com/xenwiki/CreditScheduler
>


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1.  Yes, what you would be essentially doing is migrating the VMs to a new
dom0, and as long as the xen version is the same; otherwise, you may have to
do some tweaking.

2. I think you''re referring to pvops kernel, which is the default in
many
distributions for kernels after 2.6.31.  To be honest, I would recommend
against pvops due to some instability in networking, but I have an edge case
for my xen networking (if you are considering using a DHCP/DNS/firewall in a
domU, do not use pvops as networking attempts will fail).

3.  See 4

4.  After attempting Xen on debian, I moved to OpenSUSE and had a much
easier time.  Debian currently is in a weird state where stable has very
outdated versions of xen.  I went to OpenSUSE as they are the only distro
that uses non-pvops kernels after 2.6.31, which I needed, plus they have
good support for xen4.0.  I was a fan of debian because of the minimal
install, and although OpenSUSE''s "minimal install" is much
more bloated than
debian, it doesn''t add too much.

5.  I was under the impression that the hypervisor automatically did this
unless specified otherwise.

On Thu, Aug 12, 2010 at 12:30 AM, Ken <kr.xen@hoverclub.net> wrote:
> Hi,
>
> Agonizing about a strategy for installing, when I know nothing about it.
> I suspect that in spite of the flexibility in choices for a dom0
> operating system, I think it''s probably really important to get it
all
> right.
>
> I guess there are some burning questions here that might clear
> everything up if I could answer:
>
> 1. Assuming I install Xen, a handful of VMs and have it all set up, is
> there a way to replace dom0 with some other dom0?  If I knew I could
> change my mind later it would be much easier to set this up.
>
> 2. There seems to be a way to have a kernel which works either on the
> bare metal or as a guest, but it doesn''t seem to have been tested
much.
> Do I ignore this for now, and if I do can I start using it later?  If I
> change it on Dom0 do I need to recompile kernels on the other guests?
>
> 3. I keep expecting to need to install the hypervisor somewhere.  I
> don''t see the hypervisor anywhere in Gentoo, but there''s
xen-sources
> which I suspect contains it.  Or do I need the tarball from
> www.xen.org/products/xen_source.html?
>
> 4. In my understanding the dom0 needs to be extremely stable and have
> good driver support.  What distro works best for that?  I''m
currently
> using Gentoo, but I would think Debian would be most stable.  If so, is
> the current difficulty with Debian an unhappy coincidence or is it
> consistent?  How about Ubuntu Server, or is that still the same problem
> since it''s also a Debian?  What do people use if they just want a
small,
> stable dom0 with good driver support?
>
> 5. Is there a way to "loan" processors to a guest if nobody else
is
> using them?
>
> Thanks.
>
>
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
>

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