On Tue, Sep 20, 2016 at 15:21:16 +0100, Daniel Berrange wrote:
> On Tue, Sep 20, 2016 at 03:53:16PM +0200, Peter Krempa wrote:
> > On Tue, Sep 20, 2016 at 09:40:22 -0400, Corey S. McQuay wrote:
> > > Currently Libvirt allows attempts to migrate read only disks. Qemu cannot 
> > > handle this as read only
> > > disks cannot be written to on the destination system. The end result is a 
> > > cryptic error message
> > > and a failed migration.
> > 
> > This is not necessarily true. Read only disks can sometimes be in fact
> > backed by storage that is writable and it's desired to be migrated.
> 
> If 'def->readonly' is true, then the security drivers won't allow
> QEMU to write to the image, regardless of whether the underlying
> storage is writable.

That definitely would be just a bug in the implementation rather than a
reason to do this.

In fact the problem is that qemu itself forbids to write the data to a
readonly marked block backend, which indeed makes this impossible
although I'm pretty certain that it worked for me in some configuration.

At any rate, checking this on the source of migration is incorrect. The
destination should do such check if there's currently no way to persuade
qemu do do it. We still may later find a way and all hosts running older
versions would be hosed.

Peter

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