[collectd] Question about StoreRates option

[Jonathan Kinred]

On 9 May 2014 16:37, Jonathan Kinred <jonathan.kinred at gmail.com> wrote:

> 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
>

With the help of #collectd on FreeNode I think I've worked out the
confusion. It was based on my skimming over the DERIVE type (which are
derived into a per second value).

The other confusing part was the following section in the write_amqp plugin
documentation which made me think the DERIVE value was going through some
conversion to become a GAUGE:
"Determines whether or not COUNTER, DERIVE and ABSOLUTE data sources are
converted to a *rate* (i.e. a GAUGE value)."

Is this correct? I think that DERIVE and ABSOLUTE are already "rates".
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