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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC] qemu-img: add option -d in convert
Date: Tue, 25 Jun 2013 11:13:00 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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

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

> 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

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

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 :).


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