[collectd] Question about StoreRates option

[Jonathan Kinred]

I've done a fairly lengthy search and have had a glance over the code but I
still don't fully understand how the StoreRates works. I understand that it
takes COUNTER, DERIVE and ABSOLUTE types and converts then to GAUGE.

To make it more specific, can someone help me understand the disk plugin
disk_ops/read metric, passed through the write_graphite plugin with
"StoreRates true".

Looking at the disk plugin source code, it reads /proc/diskstats and
produces a disk_ops/read DERIVE value (e.g. 34500).

The write_graphite plugin then calls uc_get_rate when StoreRates is true
and a GAUGE value is returned.

My specific questions are:
* How does the conversion work?
* Does it know about the previous value (e.g. 340000)
* Does the Interval parameter play a role?

Jonathan
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