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Re: [Qemu-devel] export root node for write through NBD

From: Kevin Wolf
Subject: Re: [Qemu-devel] export root node for write through NBD
Date: Thu, 30 Nov 2017 15:33:06 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

Am 30.11.2017 um 15:21 hat Max Reitz geschrieben:
> On 2017-11-30 08:47, Vladimir Sementsov-Ogievskiy wrote:
> > Hi all.
> > 
> > We need the following option: start vm in stopped mode (-S) and write
> > it's disk before start through NBD.
> > It should be absolutely safe, but unfortunately it is disallowed by root
> > role of the disk.
> > Is there any workaround or if not, what is a true way to implement this
> > possibility?
> > 
> > ----
> > error message:
> >     unable to execute QEMU command 'nbd-server-add': Conflicts with use
> > by drive0 as 'root', which does not allow 'write' on #block100
> One thing that comes to mind is adding the guest device only after you
> are done with NBD.
> The other of course is to set share-rw=on for the device.

We already allow this with incoming migration, in that case the
BlockBackend for the guest device stays inactive and doesn't request
permissions yet.

I've been thinking for a while that it would be nice to unify the
startup of "normal" VMs and incoming migration, so that images are
always opened inactive first and then get activated.

The interesting point for you could be that opening the image happens
immediately when the process starts, but activation would only happen
when the VM actually starts running (i.e. on 'cont').


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