Open Stack

IIRC, the meters discussed in the wiki [1] are supposed to show "delta"
values (usage since the last time an event was generated), although the
"Alternate Gauge Design" section discusses cumulative meters instead. The
libvirt pollsters we have now produce cumulative data, and it might be
complicated and error-prone to change them to compute the deltas internally
before generating events. On the other hand, some of the other counters may
be more complicated to generate as cumulative values.

Rather than taking an either/or approach, I propose we document for each
counter whether it uses the delta or cumulative approach. The API server
can ask the meter plugin for that piece of information when calculating the
aggregate value so that the caller does not have to keep track of the rules
for each meter.

Thoughts?

Doug

[1] http://wiki.openstack.org/EfficientMetering
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