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

[Mike Lovell]

On 07/24/2012 11:20 PM, Bing Bu Cao 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?

i'm guessing that you are mounting the 'shared disk' in both guests 
using a non-cluster aware filesystem. i.e. ext2/3/4, reiserfs, xfs, jfs, 
etc. these file systems are assumed that the system mounting the disk is 
the only system thats going to be writing to the disk. if two systems 
mounted the same disk as writable, each system could end up writing 
different data to the same blocks of storage or changing the metadata 
that is written and the other system is not expecting those changes to 
happen. this can very easily and quickly lead to data corruption in the 
filesystem. if you plan on mounting the same shared block device in 
multiple guests, you are going to need to use a cluster aware file 
system such as ocfs2 or gfs.

mike