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On Mon, Nov 26 2012, Jiang, Yunhong wrote:

Hi,

[…]

> 	There are several method on this, like JD's MAX()- FIRST(), or
> asalkeld's idea of store the offset. Both try to detect if there is reset
> happen by checking if there is counter reverse. (again, Eglynn, please
> correct me if I'm wrong).

I've indeed already express my point of view of solving this issue, but
I can do it here again.:)

I'm against trying to do anything fancy on calculation in pollsters, and
want always see raw data sent to the collector. One promise of
Ceilometer has always been to do no aggregation of any kind.

The "final value" computing problem is a probably easily solved with the
good algorithm in the API, and more specifically via the storage backend
if it's capable of doing it (at least SQL is, and I'm pretty sure
MongoDB would be).

[…]

> 	My proposal is to fix this issue from nova side as it's nova bug.
> Per my understanding, the reason is different view of openstack/libvirt. For
> openstack, the instance is a logic object, thus still exist after
> suspend/resumt, while for libvirt, it's a real object, qemu process, which
> does not exist after suspend/resume. Nova should translate libvirt's view to
> openstack's view. And this issue happens to other functionality, like nova's
> get_diagnostic(). But I agree with Eglynn that there are several
> implementation difficulties to fix it from nova side.

Just to be clear, that's a totally different issue. The fact that you
want to fix this in Nova is understandable, but you're not unfortunately
not going to be able to fix it for every pollsters people will write in
the future.

So both problems have to be addressed:
1. how to deal with resetable counters
2. how to prevent counters to be reset

And I agree with you that 2. is more a Nova issue than a Ceilometer one,
since we want that value to be provided by Nova at some point and not by
libvirt directly IIUC, which is a good idea.

-- 
Julien Danjou
# Free Software hacker & freelance
# http://julien.danjou.info
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