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

[Kevin Wolf <kwolf@redhat.com>]

Am 23.04.2015 um 12:05 hat Paolo Bonzini geschrieben:
> 
> 
> On 23/04/2015 11:14, Wen Congyang wrote:
> > The bs->file->driver should support backing file, and use backing reference
> > already.
> > 
> > What about the primary side? We should control when to connect to NBD server,
> > not in nbd_open().

Why do you need to create the block device before the connection should
be made?

> My naive suggestion could be to add a BDRV_O_NO_CONNECT option to
> bdrv_open and a separate bdrv_connect callback.  Open would fail if
> BDRV_O_NO_CONNECT is specified and drv->bdrv_connect is NULL.
> 
> You would then need a way to have quorum pass BDRV_O_NO_CONNECT.

Please don't add new flags. If we have to, we can introduce a new option
(in the QDict), but first let's check if it's really necessary.

> 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.

What I think is really needed here is essentially an active mirror
filter.

Kevin