Re: [Qemu-block] [PATCH COLO v3 01/14] docs: block replication's description

[Dr. David Alan Gilbert]

* Paolo Bonzini (address@hidden) wrote:
> 
> 
> On 23/04/2015 13:36, Kevin Wolf wrote:
> > Crap. Then we need to figure out dynamic reconfiguration for filters
> > (CCed Markus and Jeff).
> > 
> > And this is really part of the fundamental operation mode and not just a
> > way to give users a way to change their mind at runtime? Because if it
> > were, we could go forward without that for the start and add dynamic
> > reconfiguration in a second step.
> 
> I honestly don't know.  Wen, David?

As presented at the moment, I don't see there's any dynamic reconfiguration
on the primary side at the moment - it starts up in the configuration with
the quorum(disk, NBD), and that's the way it stays throughout the fault-tolerant
setup; the primary doesn't start running until the secondary is connected.

Similarly the secondary startups in the configuration and stays that way;
the interesting question to me is what happens after a failure.

If the secondary fails, then your primary is still quorum(disk, NBD) but
the NBD side is dead - so I don't think you need to do anything there
immediately.

If the primary fails, and the secondary takes over, then a lot of the
stuff on the secondary now becomes redundent; does that stay the same
and just operate in some form of passthrough - or does it need to
change configuration?

The hard part to me is how to bring it back into fault-tolerance now;
after a primary failure, the secondary now needs to morph into something
like a primary, and somehow you need to bring up a new secondary
and get that new secondary an image of the primaries current disk.

Dave

> Paolo
> 
> > Anyway, even if we move it to a second step, it looks like we need to
> > design something rather soon now.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK