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

From: Paolo Bonzini
Subject: Re: [Qemu-trivial] [PATCH 1/2] nbd: Don't export a block device with no medium.
Date: Mon, 19 May 2014 13:03:39 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

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.


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