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Date: Thu, 27 Jun 2013 11:01:39 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130627090139.GC14351@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [RFC] qemu-img: add option -d in convert
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To: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>, Dietmar Maurer <dietmar@proxmox.com>

On Tue, Jun 25, 2013 at 07:14:19PM +0800, Wenchao Xia wrote:
> 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
> >snapshots:
> >
> >>imageA(sn0)->imageB(sn0,sn1)->imageC(sn0)
> >
> >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.

Copying all the data out into external files in order to get
is_allocated information seems very inefficient.

We've discussed dirty block tracking APIs on the list several times.
This seems related.

Maybe it's time to tackle this properly for both external and internal
snapshots.

The reason why a dirty block tracking API is also helpful is that not
all tools want to learn how to open different image file formats.  (Even
with libqblock.)

The drity block tracking API gives them information which they can use
together with a single interface like NBD or a libvirt block pread-style
interface to read out dirty blocks.

BTW we already have qemu-io -c map which prints out allocation
information for an image file.  If that command is extended to support
-s then you can get your info easily.

Stefan