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

[Avi Kivity <avi@redhat.com>]

On 08/08/2011 06:10 PM, Anthony Liguori wrote:
> On 08/08/2011 09:47 AM, Avi Kivity wrote:
>> On 08/08/2011 05:46 PM, Avi Kivity wrote:
>>>
>>> Please provide documentation in docs/ of the compression format.
>>>
>>> IMO it should be disabled by default (with an option to disable it,
>>> via, sat, migrate-set-options, so we can migrate to older hosts).
>>>
>>> The protocol should allow XBZRLE to turn itself off if it detects that
>>> it isn't effective.
>>>
>>
>> IOW, this should be part of the standard migration protocol, not some
>> side option that is enabled if the user remembers. It should not be
>> mutually exclusive with future migration extensions, including 
>> compression.
>
> Are you thinking of a static decision or a dynamic decision?
>

Dynamic.  If the cache hit rate is too low, disable XBZRLE and eliminate 
the overhead of copying pages to the history buffer.

> I think feature negotiation would address static decision making.  For 
> dynamic decision making, you could look to something like the VNC 
> protocol and how it encodes pixel data.  The flow looks something like:
>
> 1) All clients/servers must support raw encoding
>
> 2) Client presents list of support encodings
>
> 3) Server takes intersection of client supported encodings and server 
> supported encodings.
>
> 4) Server can choose to encode updates using any encoding supported by 
> client and server.

Feature negotiation in the migration protocol itself would break exec: 
migration (and any existing single duplex proxies).

We can do a poor man's feature negotiation via capabilities, relying on 
management to disable features which don't exist on the other side.  It 
isn't pretty, but it's the best we can do at this point.

Real feature negotiation will likely have to wait until the next version 
of the migration protocol.

-- 
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