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> If your pollster is not running to compute delta and you have
> no state stored, you'll miss a part of what has been used.

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

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.

Similarly, calculating the time delta as a gauge measure would
discount only the usage up to the first libvirt poll after each
ceilo agent restart.

As long as the ceilo compute agent was restarted only rarely, I'm
not sure the under-reporting would be a huge issue in either case.

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

Cheers,
Eoghan