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.
>
> 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.
  Nice to have the dirty block tracking API. As the discuss before, it
may be implemented as best effort which kept in memory, and I think it
is right because dirty map already exist in some block format, do that
in block seems a duplicated implemention.
  If that API is online, the approach in this patch, would be the
backed way to get delta data, when qemu is not shutdown gracefully.
What I can think is later additional parameter: -dirty_out file,
which will rebuild the dirty info with -d file_compare.
  For internal snapshot, still I need an easy way to get delta data
of historic ones which may have no dirty block tracking info.


> 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.

> Stefan


--
Best Regards

Wenchao Xia