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

[Chun Yan Liu]

Thanks for your suggestions.

For the usage "qemu-nbd -f disk.img", adding some code could implement it. I think it could be like "losetup -f" usage.

#qemu-nbd -f

show the first free nbd device at this moment.

user can choose to issue "qemu-nbd -c THAT_DEVICE disk.img" or not.

#qemu-nbd -f disk.img

find a free nbd device and connect disk.img to that device.

How do you think?

For the race conditions caused by executing multiple qemu-nbd -f at the same time, I've tried both ways (1. lock; 2. if one device not work, trying other devices until one works).

In my testing, the 2nd way has problem. When issuing "qemu-nbd -c /dev/nbd0 disk.img -v" and "qemu-nbd -c /dev/nbd0 disk1.img -v" at the same time, the latter one will eventually exit with EXIT_FAILURE, but the first one cannot work normally as well, it cannot show disk partitions. Executing multiple "qemu-nbd -f" has same problem.

So, it seems using lock from a earlier time is more proper. In my testing, I'm using file lock (fcntl). For "qemu-nbd -c" case, if lock failed, qemu-nbd exits directly. For "qemu-nbd -f" case, if lock failed, redo find_free_nbd_device (there might be updates) and then try to connectdisk.img to the new free device.

Will post V2 soon.

>>> Ian Campbell <address@hidden> 11/17/2011 1:23 AM >>>
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.