Re: [Xen-devel] [PATCH v11 20/27] Support colo mode for qemu disk

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Mar 04, 2016 at 05:52:09PM +0000, Ian Jackson wrote:
> Changlong Xie writes ("[PATCH v11 20/27] Support colo mode for qemu disk"):
> > +Enable COLO HA for disk. For better understanding block replication on
> > +QEMU, please refer to:
> > +http://wiki.qemu.org/Features/BlockReplication
> 
> Sorry, I missed this link on my first pass.  I still think that at the
> very least this needs something more user-facing (ie, how should one
> set this up).
> 
> But, I'm kind of worried that qemu is the wrong place to be doing
> this.
> 
> How can this be made to work with PV guests ?

QEMU can also serve PV guests (qdisk).

I think your question is more of - what about making this work with
PV block backend?
> 
> What if an HVM guest has PV-on-HVM drivers ?  In this case there might
> be two relevant qemus, one for the qdisk Xen PV block backend, and one
> for the emulated IDE.

In both cases QEMU would use the same underlaying API to actually write/read
out the blocks. That API would then use NBD, etc to replicate writes.

Maybe a little ASCII art?

        qdisk  ide
          \    /
           \  /
           block API
            |
           QCOW2
            |
           NBD

Or such?

> 
> I don't understand how discrepant writes are detected.  Surely they
> might occur and should trigger a resynch ?
> 
> Ian.

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