Re: [Qemu-devel] [PATCH] Add -f option to qemu-nbd

[Stefan Hajnoczi]

On Wed, Nov 16, 2011 at 6:57 AM, Chunyan Liu <address@hidden> wrote:
> Currently qemu-nbd does not support finding free nbd device for users like
> "losetup -f" and issuing "qemu-nbd -c /dev/nbdX disk.img" won't report error
> message when /dev/nbd is already in use. It makes things a little confusing.
> This patch adds "-f" option to qemu-nbd to support finding a free nbd device
> for users. Please review and share your comments. Thanks.
>
> Signed-off-by: Chunyan Liu <address@hidden>
> ---
>  qemu-nbd.c |   65 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 64 insertions(+), 1 deletions(-)

This patch finds a free device but does not immediately attach to it
and use it.  Interfaces like this are prone to race conditions, I
think it would make more sense to combine the -f option with running
the actual NBD server.

I suggest:
qemu-nbd -f disk.img

That way it is safe to execute multiple qemu-nbd -f at the same time
without race conditions.  Plus it probably makes the user's life
easier than having to say qemu-nbd -c $(qemu-nbd -f) disk.img.

Stefan