Re: Sharing disks between two kvm guests

[Laurent Vivier <Laurent.Vivier@bull.net>]

Le vendredi 20 juin 2008 à 09:07 -0500, Javier Guerra a écrit :
> On Fri, Jun 20, 2008 at 7:23 AM, carlopmart <carlopmart@gmail.com> wrote:
> > Felix Leimbach wrote:
> >>
> >>>  This is my first post to this list. I have already installed kvm-70
> >>> under rhel5.2. My intention is to share on disk image betwwen two rhel5.2
> >>> kvm guests. Is it possible to accomplish this in kvm like xen or vmware
> >>> does?? How can I do?? I didn't find any reference abou this on kvm
> >>> documentation ...
> 
> i tried this looong ago and didn't really work because there was some
> userspace cache on each QEMU instance.  but the -drive option has a
> 'cache=off' setting that should be enough.
> 
> in theory (i haven't tested, but Avi 'blessed' it):
> - create a new image with qemu-img
> - add it to the command line using -drive file=xxx,cache=off on both
> KVM instances
> - use a cluster filesystem!

RFC:

Well, well, perhaps it is delusions of a sick mind but since the
introduction of qemu-nbd I think we can develop easily something to
share a disk between several virtual hosts:

I- in a first step, we can modify qemu-nbd to accept several connections
for one disk image, for instance:

# qemu-nbd my-disk.qcow2
# nbd-client localhost 1024 /dev/nbd0
# nbd-client localhost 1024 /dev/nbd1

and start two virtual hosts:

"qemu -hda v1.img -hdb /dev/nbd0" and "qemu -hda v2.img -hdb /dev/nbd1"

Of course the filesystem must know how to share the access to the disk
with others (-> "cluster filesystem")

II- in a second step, we can include directly the nbd protocol in qemu
(block-nbd.c, "-drive file=nbd:localhost:1024") to connect to the
server. We can also add some commands to the protocol to manage lock,
HA, "what else ?" (Hi George).

Any comments ?

Cheers,
Laurent
-- 
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