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

[Avi Kivity <avi@redhat.com>]

On 08/10/2011 10:27 PM, Anthony Liguori wrote:
>> This may be acceptable, wait until the entire migration cluster is
>> xzbrle capable before enabling it. If not, add a monitor command.
>
>
> 1) xzbrle needs to be disabled by default.  That way management tools 
> don't unknowingly enable it by not passing -no-xzbrle.

We could hook it to -M, though it's a bit gross.  Otherwise we need to 
document this clearly in the management tool author's guide.

>
> 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.

>
> 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.

>
> 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.

Right.

>
> 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.

-- 
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