Re: [Libguestfs] [PATCH libnbd] copy: Always allow multiple connections
On Thu, May 27, 2021 at 02:13:24PM +0300, Nir Soffer wrote:
On Thu, May 27, 2021 at 1:49 PM Richard W.M. Jones
<rjones(a)redhat.com> wrote:
>
>
> Actually no, I had to revert this.
>
> https://gitlab.com/nbdkit/libnbd/-/commit/fb42056929e9177e0de2603e2f430a8...
>
> I added a test:
>
> https://gitlab.com/nbdkit/libnbd/-/commit/23b41ad0a2a6d6582a8e643e42960d2...
>
> However the test deadlocks when we try to open multiple connections to
> qemu-nbd (with your patch). As best I can tell this is because of
> qemu's file locking. qemu-nbd doesn't seem to support the -U option
> found in qemu-img, but even if it did I don't think it would be a good
> idea to tell users that they have to use it.
When using qemu-nbd you need to specify --shared=4 on both source and
destination.
Ah, I didn't realise that. So my explanation for why it failed was
wrong.
It has to be said qemu-nbd's defaults are a pain, especially having to
always specify the -t option.
Current nbdcopy depends on the server supporting multiple
connections, so we try to open the additional connections in the
main thread. If the server supports only one connection (--shared=4
not specified), this will deadlock.
One way to fix this is to open the extra connection in the worker
threads. If the server does not support more then one connection,
only one worker can make progress, and the other will just block on
nbd_connect(). When the first worker finishes all the work, the rest
of the workers will connect and find that they have no work to do
and exit.
Agreed.
> I think this needs to be fixed in qemu-nbd itself.
I don't think we can change qemu-nbd behavior at this point.
IIUC the reason why qemu-nbd doesn't support multi-conn is because of
the generality of the qemu block layer. If qemu-nbd is backed by --
for example -- a Ceph device then we'd need to check if Ceph was safe
for multi-conn.
However I wonder if it's possible to advertise multi-conn in some
limited scenarios. Could qemu-nbd check if the backing block device
is a plain raw file (or other limited cases) and offer multi-conn
then?
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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