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

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

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.

>> imageA_sn0.qcow2-->imageA.qcow2-->imageB_sn0.qcow2-->imageB_sn1.qcow2-
>> ->imageB.qcow2-->imageC_sn0.qcow2-->imageC.qcow2
>
> 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
> imageA_sn0.qcow2.
>
   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
> different.
>
   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.

> Stefan
>


-- 
Best Regards

Wenchao Xia