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

On 03/05/2016 04:30 AM, Konrad Rzeszutek Wilk wrote:
> 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?

I don't know how to work with PV block backend. It is one reason that
why we only support pure HVM now.
For PV block backend, there is also other problem. For exampe resuming
it in the secondary side is very slow, because we need to disconnect and

Wen Congyang

>> 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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