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

[Paolo Bonzini]

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).

Paolo