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

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Apr 02, 2015 at 03:28:57PM +0200, Alberto Garcia wrote:
> Hi,
> 
> I'm interested in adding the possibility to mirror an intermediate
> node in a disk image chain, but I would like to have some feedback
> before sending any patches.
> 
> 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.

Note that the management tool needs to enforce the assumption since QEMU
cannot know whether other programs or QEMU instances are modifying one
of the backing files.

Stefan

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