Re: [Qemu-devel] How to share disk image between two kvm guests

[Stefan Hajnoczi]

On Thu, Jul 26, 2012 at 3:51 AM, Bing Bu Cao <address@hidden> wrote:
> Hi,all
>
> I want to use qemu-nbd to share disk between two kvm guests.
>
> I.
> # qemu-nbd -p 1024 -e 2 -n my-disk.qcow2
> # nbd-client localhost 1024 /dev/nbd0
> # nbd-client localhost 1024 /dev/nbd1
>
> II.
> guest1:
> # /usr/local/bin/qemu -enable-kvm -m 1024 -drive file=ubuntu.img -drive
> file=/dev/nbd0,cache=none -net user -net nic,model=virtio -chardev
> stdio,id=mon0 -mon chardev=mon0 -usb -device usb-tablet
>
> guest2:
> # /usr/local/bin/qemu -enable-kvm -m 1024 -drive file=ubuntu.img -drive
> file=/dev/nbd0,cache=none -net user -net nic,model=virtio -chardev
> stdio,id=mon0 -mon chardev=mon0 -usb -device usb-tablet
>
> Found one problem:
> No disk r/w sync between 2 virtual hosts.
> One write/change on the shared disk,
> another virtual host must remount the disk and can see the update from the
> other virtual host.
>
> For example,a simply test:
> In the guest1,mount the disk and create on text file add some line to it.
> In the guest2,mount the same disk and print the text file and found the
> content is not updated.
> But after after I remount the disk, the content will be updated.
>
>
>
> Is it a issue of guest OS or qemu-nbd? Can you help me?

It's a guest OS issue.  If you want to share the same disk between
multiple machines you need to use a cluster file system.  If you are
using something like ext4 which is not a cluster file system then
mounting it from two or more machines simultaneously may corrupt the
file system or at least give an inconsistent view.

Which file system are you using inside the guest?

Stefan