Re: [Qemu-trivial] [PATCH 1/2] nbd: Don't export a block device with no medium.

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/05/2014 10:00, Michael Tokarev ha scritto:
> 18.05.2014 14:50, Hani Benhabiles wrote:
>> The device is exported with erroneous values and can't be read.
>>
>> Before the patch:
>> $ sudo nbd-client localhost -p 10809 /dev/nbd0 -name floppy0
>> Negotiation: ..size = 17592186044415MB
>> bs=1024, sz=18446744073709547520 bytes
>>
>> $ sudo mount /dev/nbd0 /mnt/tmp/
>> mount: block device /dev/nbd0 is write-protected, mounting read-only
>> mount: /dev/nbd0: can't read superblock
>>
>> After the patch:
>> (qemu) nbd_server_add ide0-hd0
>> (qemu) nbd_server_add floppy0
>> Device 'floppy0' has no medium
>
> I'm not sure this is the right direction.  Maybe we should actually
> allow exporting devices with no medium to be able to insert medium
> later without re-starting the server?  Especially with -t (persistent).
> I dunno.  At least this is the decision which should not be left to
> a -trivial tree.

The protocol has no support for marking a device as no-medium, or for 
reporting a medium change, so there's nothing sane that we can do. 
(Upon a medium change, for example, the embedded NBD server will shut 
down all connections).

In particular, for qemu-nbd there is no support for named exports, so 
even with "-t" it makes sense to just exit with an error condition and 
ask the user to reinvoke qemu-nbd later.

I'll revive the nbd-next branch for these two patches.  Thanks Michael 
for carrying the other two.

Paolo