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

[Kevin Wolf]

Am 10.04.2015 um 11:56 hat Stefan Hajnoczi geschrieben:
> 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.

If the patches are simple, I think consistency and avoiding arbitrary
restrictions are good enough reasons for having the support in qemu.
Eventually we want all nodes to be equal anyway.

Kevin

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