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@redhat.com> wrote:
Actually no, I had to revert this.
https://gitlab.com/nbdkit/libnbd/-/commit/fb42056929e9177e0de2603e2f430a8111...
I added a test:
https://gitlab.com/nbdkit/libnbd/-/commit/23b41ad0a2a6d6582a8e643e42960d2e3c...
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 Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org