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Re: [Qemu-block] [RFC] Intermediate block mirroring
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-block] [RFC] Intermediate block mirroring |
Date: |
Fri, 10 Apr 2015 15:02:52 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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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