Re: [Qemu-devel] [PATCH][RFC] To mount qemu disk image on the host

[Anthony Liguori <anthony@codemonkey.ws>]

Laurent Vivier wrote:
> Hi,
>
> this patch allows to mount qemu disk images on the host.
>   

Note, the general problem with this approach is that mounting a NBD 
device locally with write access can lead to dead locks.  If you look 
through the mailing list archives, you'll find a number of conversations 
on the topic.

Regards,

Anthony Liguori

> It is based on the Network Block Device protocol and allows qemu-img to
> become an NBD server (Yes, Anthony, userspace block device is the right
> way to do that... :-P ).
>
> Once you've applied the attached patch to Qemu and build the binaries,
> you can use it like that:
>
> # ./qemu-img server -d 1234 etch.qcow2
>
> This starts an NBD server on port 1234. This server will expose
> the disk image etch.qcow2. "-d" means it will be daemonize and will run
> in background.
>
> Then you need to connect the block device to the server:
>
> # nbd-client localhost 1234 /dev/nbd0
> Negotiation: ..size = 4194304KB
> bs=1024, sz=4194304
>
> This will link etch.qcow2 to /dev/nbd0.
>
> Then to see partitions, you can use kpartx, as explained Daniel, or my
> patched loop modules (I can send an updated and bug free version).
> ...
> # kpartx -a /dev/nbd0
> ...
> or
> ...
> # rmmod loop
> # insmod drivers/block/loop.ko max_part=64
> # losetup -f /dev/nbd0
> ...
> # mount /dev/loop0p1 /mnt
> # ls /mnt
> bench  cdrom    etc     initrd.img  media  proc  selinux  tmp  vmlinuz
> bin    clients  home    lib         mnt    root  srv      usr
> boot   dev      initrd  lost+found  opt    sbin  sys      var
> # cd
> # umount /mnt
> # losetup -d  /dev/loop0
> # nbd-client -d /dev/nbd0
>
> TODO: security/host client checking, device lock...
>
> As usual all comments are welcome,
> have fun,
> Laurent
>