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

[Kevin Wolf]

Am 23.04.2015 um 12:33 hat Paolo Bonzini geschrieben:
> On 23/04/2015 12:17, Kevin Wolf wrote:
> > > Perhaps quorum is not a great match after all, and it's better to add a
> > > new "colo" driver similar to quorum but simpler and only using the read
> > > policy that you need for colo.  The new driver would also know how to
> > > use BDRV_O_NO_CONNECT.  In any case the amount of work needed would not
> > > be too big.
> >
> > I thought the same, but haven't looked at the details yet. But if I
> > understand correctly, the plan is to take quorum and add options to turn
> > off the functionality of using a quorum - that's a bit odd.
> 
> Yes, indeed.  Quorum was okay for experimenting, now it's better to "cp
> quorum.c colo.c" and clean up the code instead of adding options to
> quorum.  There's not going to be more duplication between quorum.c and
> colo.c than, say, between colo.c and blkverify.c.

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.

> > 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.

Kevin