On 12/05/2012 04:59 PM, Laszlo Hornyak wrote:
----- Original Message -----
From: "Dan Kenigsberg" <[email protected]>
To: "Yaniv Kaul" <[email protected]>
Cc: "Laszlo Hornyak" <[email protected]>, "engine-devel"
<[email protected]>
Sent: Wednesday, December 5, 2012 3:22:18 PM
Subject: Re: [Engine-devel] host cpu feature
On Wed, Dec 05, 2012 at 04:05:00PM +0200, Yaniv Kaul wrote:
On 12/05/2012 03:55 PM, Laszlo Hornyak wrote:
The nice thing about hostModel (unlike hostPassthrough) is that
once
we
created the VM we can migrate it to stronger hosts, and back to
the
original host. I suppose that it complicates the scheduler.
Yes with host-model you get the features that libvirt handles.
In
such cases the engine could decide, if you want this
functionality. Well the scheduler architecture is just being
reinvented.
For the host-passthrough, I think the AllowMigrateCPUHost
configuration option would be a simple decision for the
administrator: set it to true if all hosts are uniform.
If it is THAT simple, Engine could take this decision without human
intervension.
Is there a way engine can figure out if the cpu-models in all the hosts are the
same?
I mean even if some host flags are not handled by libvirt and therefore vdsm
and engine...
So I would really need that permission from the user.
If it is
not set to true, then we will not allow migration of such VMs.
That's not what I understood from libvirt's documentation. I
You may be right, could you send an URL to that point of the
documentation or copy-paste?
The link I followed from your feature page:
http://libvirt.org/formatdomain.html#elementsCPU :
host-model
The host-model mode is essentially a shortcut to copying host CPU
definition from capabilities XML into domain XML. Since the CPU
definition is copied just before starting a domain, exactly the
same
XML can be used on different hosts while still providing the best
guest CPU each host supports. Neither match attribute nor any
feature elements can be used in this mode. Specifying CPU model is
not supported either, but model's fallback attribute may still be
used. Libvirt does not model every aspect of each CPU so the guest
CPU will not match the host CPU exactly. On the other hand, the ABI
provided to the guest is reproducible. During migration, complete
CPU model definition is transferred to the destination host so the
migrated guest will see exactly the same CPU model even if the
destination host contains more capable CPUs for the running
instance
of the guest; but shutting down and restarting the guest may
present
different hardware to the guest according to the capabilities of
the
new host.
host-passthrough
With this mode, the CPU visible to the guest should be exactly the
same as the host CPU even in the aspects that libvirt does not
understand. Though the downside of this mode is that the guest
environment cannot be reproduced on different hardware. Thus, if
you
hit any bugs, you are on your own.
That's exactly where AllowMigrateCPUHost fits well: when a user ticks
this for a cluster he says "yeah, I like to be on my own."
cpu mode="host-passthrough" migration: I talked to the libvirt guys and they
said it is OK if the hardware and the software are the same, and it will work, but they
would not recommend.
So if they do not recommend it, I would drop this from the feature spec. Anyone
against it?
Laszlo
I'm a bit against it. I don't see why it's that complicated:
Allow migration -> use 'host-model'
Do not allow -> use 'host-passthrough'.
The reason of why we need host-passthrough is that otherwise I suspect
we depend on libvirt for newer features to be somehow exposed to the
guest (not sure about it).
Y.
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