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Re: [libvirt] [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps

On 08/11/2011 03:17 AM, Avi Kivity wrote:
3) a management tool should be able to query the source and
destination, and then enable xzbrle if both sides support it.

You can argue that (3) could be static. A command could be added to
toggle it dynamically through the monitor.

But no matter what, someone has to touch libvirt and any other tool
that works with QEMU to make this thing work. But this is a general
problem. Any optional change to the migration protocol has exactly the
same characteristics whether it's XZBRLE, XZBRLE v2 (if there is a
v2), ASN.1, or any other form of compression that rolls around.

If we have two-way communication we can do this transparently in the
protocol itself.

Yes.  This should be one of the initial caps to introduce.

Instead of teaching management tools how to deal with all of these
things, let's just fix this problem once. It just takes:

a) A query-migration-caps command that returns a dict with two lists
of strings. Something like:

{ 'execute': 'query-migration-caps' }
{ 'return' : { 'capabilities': [ 'xbzrle' ], 'current': [] } }

b) A set-migration-caps command that takes a list of strings. It
simply takes the intersection of the capabilities set with the
argument and sets the current set to the result. Something like:

{ 'execute': 'set-migration-caps', 'arguments': { 'set': [ 'xbzrle' ] }}
{ 'return' : {} }

c) An internal interface to register a capability and an internal
interface to check if a capability is currently enabled. The xzbrle
code just needs to disable itself if the capability isn't set.

Then we teach libvirt (and other tools) to query the caps list on the
source, set the destination, query the current set on the destination,
and then set that set on the source.

This is only if the capability has no side effect.

Right, it can't change the output of any monitor commands or anything like that. It's strictly about the encoding of the wire protocol which ought to be transparent to libvirt.

As we introduce new things, like the next great compression protocol,
or ASN.1, we don't need to touch libvirt again. libvirt can still know
about the caps and selectively override QEMU if it's so inclined but
it prevents us from reinventing the same mechanisms over and over again.


Yes. But that negotiation needs to become part of the "protocol" for
migration. In the absence of that negotiation, we need to use the wire
protocol we use today. We cannot have ad-hoc feature negotiation for
every change we make to the wire protocol.

Okay, as long as we have someone willing to implement it.

Sounds like a good hackathon project :-)


Anthony Liguori

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