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

[Stefan Hajnoczi]

On Wed, Apr 22, 2015 at 05:28:01PM +0800, Wen Congyang wrote:
> On 04/22/2015 05:18 PM, Stefan Hajnoczi wrote:
> > 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.
> 
> Hmm, if you want to use iSCSI or other protocols, you should update the driver
> to implement block replication's control interface.
> 
> Currently, we only support nbd now.

I took a quick look at the NBD patches in this series, it looks like
they are a hacky way to make quorum dynamically reconfigurable.

In other words, what you really need is a way to enable/disable a quorum
child or even add/remove children at run-time.

NBD is not the right place to implement that.  Add APIs to quorum so
COLO code can use them.

Or maybe I'm misinterpreting the patches, I only took a quick look...

Stefan

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