Re: [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 09/07/2010 10:09 AM, Stefan Hajnoczi wrote:
>> Right, so that argues for an incremental interface like I started with :-)
>>
>> BTW, this whole discussion is also relevant for other background tasks like
>> online defragmentation so keep that use-case in mind too.
>>      
> Right, I'm a little hesitant to get too far into discussing the
> management interface because I remember long threads about polling and
> async.  I never fully read them but I bet some wisdom came out of them
> that applies here.
>
> There are two ways to do a long running (async?) task:
> 1. Multiple smaller pokes.  Perhaps completion of a single poke is
> async.  But the key is that the interface is incremental and driven by
> the management stack.
> 2. State.  Turn on streaming and watch it go.  You can find out its
> current state using another command which will tell you whether it is
> enabled/disabled and progress.  Use a command to disable it.
>    

If everyone is going to do (1) by just doing a tight loop or just using 
the same simple mechanism (a sleep(5)), then I agree, we should do (2).

I can envision people wanting to do very complex decisions about the 
right time to do the next poke though and I'm looking for feedback about 
what other people think.

I expected people to do complex heuristics with respect to migration 
convergence but in reality, I don't think anyone does today.  So while I 
generally like being flexible, I realize that too much flexibility isn't 
always a good thing :-)

Regards,

Anthony Liguori

> Stefan
>