Re: [Qemu-devel] [Qemu-block] [PATCH v2 0/3] block: Warn about usage of growing formats over non-growable protocols

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/05/2015 18:37, Max Reitz wrote:
> Because qcow2 tries to write beyond the end of the file; the NBD client
> implementation passes that on to the server, and the server simply
> reports an error (which the NBD client turns into EIO).

Where?

    qemu_coroutine_yield();
    *reply = s->reply;
    if (reply->handle != request->handle) {
        reply->error = EIO;
    } else {
        if (qiov && reply->error == 0) {
            ret = qemu_co_recvv(s->sock, qiov->iov, qiov->niov,
                                offset, request->len);
            if (ret != request->len) {
                reply->error = EIO;
            }
        }

        /* Tell the read handler to read another header.  */
        s->reply.handle = 0;
    }

It should get into the "else" and then see reply->error != 0.  So the
guest should see ENOSPC.

>> Can you check if virtio-scsi
>> gives ENOSPC?
> 
> In which configuration? Using virtio-scsi on top of qcow2 on top of some
> SCSI passthrough block driver?  Sounds like we ought to make NBD return
> ENOSPC no matter the fate of this series.

virtio-scsi on top of qcow2 on top of NBD.  No passthrough: "-device
scsi-disk" should work.  It seems to me that NBD _should_ be getting
ENOSPC, unless it's nbd-server that throws away the error.

> The problem with only warning for a certain non-default configuration is
> that people who don't know what they are doing are more likely to use
> the default configuration, so I'd like the warning to appear then.

That's entirely true.  Perhaps the werror default should be changed from
enospc to report after all!

Paolo