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

From: Wenchao Xia
Subject: Re: [Qemu-devel] [RFC] qemu-img: add option -d in convert
Date: Tue, 25 Jun 2013 19:14:19 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130509 Thunderbird/17.0.6

于 2013-6-25 17:13, Stefan Hajnoczi 写道:
On Thu, Jun 20, 2013 at 04:59:17PM +0800, Wenchao Xia wrote:

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


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

  What it looks like is convert into external snapshots, actually
requirement is get delta data from internal snapshot, external snapshots
have delta data by nature so it is a way to orginize it. This patch
will help get that delta data.


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

  yes, it is the purpose, delta data.

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

  There would be two steps I think: 1st is simply compare the cluster
to get delta, 2nd is improve it: when 2 snapshots is in one qcow2,
introduce a function to looking into L1/L2.

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.

  Indeed, but converting external chain into internal snapshots, is not
that useful as its revert, since in most case qemu-img can directly
convert external chain back to restore a snapshot. It can be added if
some case appear.

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 :).

  I think it is useful: internal snapshot have advantage about
performance, but disadvantage for delta data retrieving. This patch
will help it. When it is online, the user, for example, libvirt,
can retrieve internal/external ones very similar, it simply calls
qemu-img to get the delta for both case.


Best Regards

Wenchao Xia

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