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Date: Thu, 9 Apr 2015 14:40:00 +0200
From: Alberto Garcia <berto@igalia.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [RFC] Intermediate block mirroring
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org

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().

Berto