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

[Kevin Wolf <kwolf@redhat.com>]

Am 23.04.2015 um 12:44 hat Paolo Bonzini geschrieben:
> On 23/04/2015 12:40, Kevin Wolf wrote:
> > The question that is still open for me is whether it would be a colo.c
> > or an active-mirror.c, i.e. if this would be tied specifically to COLO
> > or if it could be kept generic enough that it could be used for other
> > use cases as well.
> 
> Understood (now).
> 
> >>> What I think is really needed here is essentially an active mirror
> >>> filter.
> >>
> >> Yes, an active synchronous mirror.  It can be either a filter or a
> >> device.  Has anyone ever come up with a design for filters?  Colo
> >> doesn't need much more complexity than a "toy" blkverify filter.
> > 
> > I think what we're doing now for quorum/blkverify/blkdebug is okay.
> > 
> > The tricky and yet unsolved part is how to add/remove filter BDSes at
> > runtime (dynamic reconfiguration), but IIUC that isn't needed here.
> 
> Yes, it is.  The "defer connection to NBD when replication is started"
> is effectively "add the COLO filter" (with the NBD connection as a
> children) when replication is started.
> 
> Similarly "close the NBD device when replication is stopped" is
> effectively "remove the COLO filter" (which brings the NBD connection
> down with it).

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.

Anyway, even if we move it to a second step, it looks like we need to
design something rather soon now.

Kevin