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

[Bing Bu Cao <mars@linux.vnet.ibm.com>]

On 07/26/2012 05:30 PM, Stefan Hajnoczi wrote:
> On Thu, Jul 26, 2012 at 3:51 AM, Bing Bu Cao<mars@linux.vnet.ibm.com>  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?
Now using ext4 as file system inside the both guests.
Got it.
Thank you,Stefan.
>
> Stefan
>


-- 
Best Regards,
Bing Bu Cao