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

[Ian Campbell]

On Wed, 2011-11-16 at 10:34 +0000, Stefan Hajnoczi wrote:
> 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.

I agree, but you'd also need some locking inside qemu-nbd wouldn't you?
Or have it just keep trying devices until one works perhaps.

>   Plus it probably makes the user's life
> easier than having to say qemu-nbd -c $(qemu-nbd -f) disk.img.

Absolutely.

Ian.