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

[Stefan Hajnoczi]

On Tue, Apr 21, 2015 at 05:28:01PM +0200, Paolo Bonzini wrote:
> On 21/04/2015 03:25, Wen Congyang wrote:
> >> > Please do not introduce "<name>+colo" block drivers.  This approach is
> >> > invasive and makes block replication specific to only a few block
> >> > drivers, e.g. NBD or qcow2.
> > NBD is used to connect to secondary qemu, so it must be used. But the 
> > primary
> > qemu uses quorum, so the primary disk can be any format.
> > The secondary disk is nbd target, and it can also be any format. The cache
> > disk(active disk/hidden disk) is an empty disk, and it is created before run
> > COLO. The cache disk format is qcow2 now. In theory, it can be ant format 
> > which
> > supports backing file. But the driver should be updated to support colo 
> > mode.
> > 
> > > A cleaner approach is a QMP command or -drive options that work for any
> > > BlockDriverState.
> > 
> > OK, I will add a new drive option to avoid use "<name>+colo".
> 
> Actually I liked the "foo+colo" names.
> 
> These are just internal details of the implementations and the
> primary/secondary disks actually can be any format.
> 
> Stefan, what was your worry with the +colo block drivers?

Why does NBD need to know about COLO?  It should be possible to use
iSCSI or other protocols too.

Stefan

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