Re: [Qemu-block] [RFC] Intermediate block mirroring

[Stefan Hajnoczi]

On Thu, Apr 09, 2015 at 02:40:00PM +0200, Alberto Garcia wrote:
> On Thu, Apr 09, 2015 at 11:39:28AM +0100, Stefan Hajnoczi wrote:
> 
> > > The goal would be to convert this:
> > > 
> > >    [A] -> [B] -> [C] -> [D]
> > > 
> > > into this:
> > > 
> > >    [A] -> [B] -> [X] -> [D]
> > > 
> > > where [D] is the active image and [X] would be a copy of [C]. The
> > > latter would be unlinked from the chain.
> > > 
> > > A use case would be to move disk images across different storage
> > > backends.
> > 
> > The simple solution to that problem is:
> > 
> > Assumption: backing files are read only.  (True in most cases.)
> > 
> > 1. Copy the backing files using cp(1) or another method.
> > 2. Issue QMP 'change-backing-file' command so that [D] uses [X] instead
> >    of [C].
> > 
> > So it can be done today already.
> 
> So do you think it would be better to implement this somewhere else?
> The code that I have for QEMU is quite simple, the actual algorithm
> doesn't change, it only needs to do the changing of backing files in
> mirror_exit().

I'll take a look at your patch series, maybe there are advantages to
doing it within QEMU.

Stefan

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