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Date: Tue, 2 Jul 2013 10:47:00 +0200
From: Stefan Hajnoczi <stefanha@gmail.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 Thu, Jun 27, 2013 at 08:30:04PM +0800, Wenchao Xia wrote:
> 于 2013-6-27 17:01, Stefan Hajnoczi 写道:
> >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:
> >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.
>   Do you mean use it like:
>   1 call qemu-io file -c map -s sn0
>   2 call qemu-io file -c map -s sn1
>   3 for (offset = 0; offset < len; offset +=512) {
>         if (allocated on sn0) {
>              call qemu-io file read -s sn0, into buf0
>         }
>         if (allocated on sn1) {
>              call qemu-io file read -s sn1, into buf1
>         }
>         if strcmp(buf0, buf1) {
>              write down the delta
>         }
>     }
> ?
>   I think it is workable and flex, the bottle neck is the string
> parsing of qemu-io. For delta data info retrieving purpose, qemu-img
> approach would faster.

It's also possible to put Step 3 into qemu-img so that this feature is
fast and easy to use with a single command.

Stefan