δΊ 2013-6-25 17:13, Stefan Hajnoczi ει:
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
