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

[Anthony Liguori]

On 08/11/2011 03:03 AM, Shribman, Aidan wrote:
> > From: Anthony Liguori [mailto:address@hidden
> > Sent: Wednesday, August 10, 2011 10:28 PM
> > To: Avi Kivity
> > Cc: Blue Swirl; Stefan Hajnoczi; Shribman, Aidan; qemu-devel
> > Developers; address@hidden
> > Subject: Re: [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of
> > large memory apps
>
> > 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' : {} }
>
> We may want to further sub-divide capabilities into categories:
> { 'execute': 'query-migration-caps' }
> { 'return' :
>    { 'encoding' : { 'current', 'asn.1', 'proto2', 'thrift', etc. } },
>    { 'delta' : { 'xbzrle', "xdelta", ...} },
>    { 'compression' : { 'snappy', 'lzo' } } }
> This would help libvirt/management to select features automatically or manually 
> (via UI) without having to 'understand' the any given capability meaning.

I would prefer caps to be mostly transparent to libvirt.  In fact, I'd 
like to see exactly three caps: xbzrle, asn1, and autonegotiate.

I'd like to move the caps negotation into the protocol itself.

> > 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.
>
> Agreed. Therefore caps plus xbzrle could be added before ASN.1/v1.0 without 
> breaking anything as long as when 'set-migration-caps' is not issued Qemu uses 
> the current protocol.

Exactly.

Regards,

Anthony Liguori

> Aidan