[collectd] Plugin to calculate percentages?

[Russ Poyner]

The percentage checking code in thresholds seems to be at least 
partially broken.

Perhaps this is a good time to create a plugin to calculate percentages. 
The idea being that the percentages would be calculated and fed either 
to thresholds or to match_threshold as part of a filter chain. Doing the 
percentage conversion in a plugin would eliminate the need for multiple 
versions of the calculation in different plugins. (I see 2 in thresholds 
alone).

Percentages *might* be appropriate to add to the nascent aggregation 
plugin, or it could be part of a separate plugin. If the separate plugin 
seems correct I'd be willing to take a shot at it. One argument for a 
separate plugin would be the ability to feed percentages to aggregation 
itself.

In any event I think configurable handling of NaNs or missing values 
would be helpful.

Percentage is of couse 100*value/(sum(values))

Some times it's appropriate to skip or ignore missing values when 
calculating the sum, for example if you are looking for hot-spots in a 
set of load-balanced servers. 100*server_load/sum(server_loads....)) 
will be correct even as the number of server_loads in the sum changes. 
(yes I just made that example up)

In other cases, for example calculating used% from df values = 
100*used/sum(used+free) if free comes up NaN for some reason, dropping 
it from the sum will report 100% used which is wrong. In this case its 
better to fail with a loud bang, by reporting used% as NaN.

RP