Re: [Qemu-devel] [RFC] qemu-img: add option -d in convert

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Jun 20, 2013 at 04:59:17PM +0800, Wenchao Xia wrote:

If I understand correctly, you have a backing chain with internal
snapshots:

> imageA(sn0)->imageB(sn0,sn1)->imageC(sn0)

And you want to convert this to a chain of external snapshots:

> imageA_sn0.qcow2-->imageA.qcow2-->imageB_sn0.qcow2-->imageB_sn1.qcow2-
> ->imageB.qcow2-->imageC_sn0.qcow2-->imageC.qcow2

You also want each image file to contain only data that is not in the
parent.  So imageA.qcow2 would not contain identical clusters from
imageA_sn0.qcow2.

The most efficient way of doing this is by looking into the L1/L2 tables
of the imageA(sn0) and imageA so that you know which clusters are
different.

This is a weird operation.  It also has a dual, converting the external
snapshot chain into a chain of image files with internal snapshots -
again sharing clusters instead of duplicating them.

It's possible to implement this but I'm a little surprised that anyone
would want to do this.  And I wonder if management tools can cope with
this mix of internal and external snapshots :).

Stefan