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

[Wenchao Xia]

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:
> > >
> > > 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.
  Forget to say, my purpose is not get is_allocated info, but
get the different data between two snapshots or images. Now qemu-img
can do it in-directly as the example shows, the -d option will make it
simpler and faster.

for example imageA(sn0, sn1, sn2), it will help to get different data
between sn0 and sn2.

  An alternative way is let qemu-nbd export sn0 and sn2, then user
diff the data itself, but we lost the chance to use L1/L2 table
, allocated info, to accelerate it.

> 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


--
Best Regards

Wenchao Xia