Chris, 'Nodepools' is not a breakout but a filter, so it takes multiple queries in sequence.
In the WebGUI the "Apply" button seems to be broken, I see filters applied only
when I change selections further down the page. And the pie chart and per directory stats right to it
seem never to get filtered.
Have a look at the usage histograms for logical or physical size to see the
effect of multiple filters applied (directory, nodepools).
(I have no nodepools on my virtual Isilon at the moment.)
Calculate the approximate usage by multiplying file count by avg 'bucket' value,
and accumulate over all buckets;
probably not exact enough for chargebacks, though.
As for really exact 'isi filepool apply':
Each file pool policy has either a nodepool, a tier, or an implicit default target.
Extracting these from the SmartPools config programatically would require some effort for sure.
For a quick start, I'd inspect the SmartPools config in the WebGUI, and extract by hand
the rule->pool mappings that can apply to the directories in question.
This little table can get hardcoded into a small proof-of-concept script
that parses the 'isi filepool apply' statistics. Makes sense?