openstack team mailing list archive

[Julien Danjou <julien@xxxxxxxxxxx>]

On Wed, Oct 31 2012, Eoghan Glynn wrote:

> Would we have also have some 'misses' with the cumulative approach
> when the ceilometer agent was down?

No, unless the counter resets several times while your agent is down.
But delta has the same issue.

> If I understood the (\Sigma local maxima)-first idea correctly,
> the usage up to the first polling cycle would always be
> discounted from any duration.

No, because if you have:

Time | Value
0    | 10
1    | 30
2    | 50
3    | 80
4    | 100

If your delta-pollster is down at 1 and 2, you restart at 3, therefore
at 4 you'll send "20" as usage (100 minus 80). So you miss the delta
between 10 (time 0) and 80 (time 3) (therefore 70 for free!).
If you send right away 80 at time 3 when restarting, the API will be
able to guess that between 0 and 3 the value went from 10 to 80.
With delta approach, the API cannot guess that.

> A more pernicious problem would occur if the instance was being
> regularly restarted with a higher frequency than the polling period.

Yes, but in that case, whatever counting method you use, you're screwed.
:)

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

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