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

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 :-)


Anthony Liguori


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