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

[Hailiang Zhang]

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.

Another way is to convert these command line options in QEMU totally,
I mean hidden the descriptions of 'active_disk' and 'hidden_disk' disks.
Create/add them dynamicly by qmp commands while users want to make VM goes
into COLO state. That's just like to support live image clone in QEMU.

Any ideas ?


Thanks,
Hailiang