[libvirt] [Block Replication] Question about supporting COLO in libvirt

[Hailiang Zhang]

On 2017/2/6 20:39, Daniel P. Berrange wrote:
> On Mon, Feb 06, 2017 at 08:34:28PM +0800, Hailiang Zhang wrote:
>> Hi,
>> I'm trying to implement supporting COLO in libvirt,
>> But i found an annoying problem that libvirt does not
>> support the command line option argument syntax we used
>> for block replication in QEMU.
>>
>> That is libvirt does not support the bellow syntax for block:
>> -drive driver=qcow2,file.filename=test:a.qcow2
>> -drive file=test.qcow2,\
>> backing.file.filename=/dev/fdset/1
>>
>> It seems to be a new syntax that libvirt does not support
>> thought it has been exist in QEMU for a time.
>> I found some introductions from
>> http://www.linux-kvm.org/images/3/34/Kvm-forum-2013-block-dev-configuration.pdf
>>
>> The command line we use for COLO is just like the above syntax,
>> For example, for the shared disk in COLO, it is:
>>   -drive if=none,driver=qcow2,file.filename=/mnt/ramfs/hidden_disk.img,id=hidden_disk0,\
>>          backing.driver=raw,backing.file.filename=1.raw \
>>   -drive if=virtio,id=active-disk0,driver=replication,mode=secondary,\
>>          file.driver=qcow2,top-id=active-disk0,\
>>          file.file.filename=/mnt/ramfs/active_disk.img,\
>>          file.backing=hidden_disk0,shared-disk=on
>>
>> For the none-shared disk in COLO, it is quite same with the shared-disk:
>>    -drive if=none,driver=raw,file.filename=1.raw,id=colo1 \
>>    -drive if=xxx,id=topxxx,driver=replication,mode=secondary,top-id=topxxx\
>>           file.file.filename=active_disk.qcow2,\
>>           file.driver=qcow2,\
>>           file.backing.file.filename=hidden_disk.qcow2,\
>>           file.backing.driver=qcow2,\
>>           file.backing.backing=colo1
>>
>> So there seems to be two ways to solve this problem.
>>
>> One is to support this new option argument syntax in libvirt,
>> but I'm not sure if it is difficult or not to implement it,
>> and i don't know where to start either.
>
> Libvirt has to start supporting this new syntax. It is required for
> many different use cases beyond just colo. For example, to be able
> to explicitly given qemu details about a backing file format to
> remove probing, or to be able to set LUKS passwords on backing files,
> and more beside
>

Thanks for your quick reply, do you or other developers in libvirt
have plan to implement it ?

Thanks,
Hailiang


> Regards,
> Daniel
>