[collectd] derivative - what EXACTLY does it measure?
smcracraft at me.com
Tue Oct 21 23:09:34 CEST 2014
The documentation is not specific.
What are the time sub new and time sub old in the formula below?
Are those the wakeup times of the current and immediate prior collectd data point?
If so, or if not, how do I find out what they are?
A GAUGE value is simply stored as-is. This is the right choice for values which may increase as well as decrease, such as temperatures or the amount of memory used.
These data sources assume that the change of the value is interesting, i.e. the derivative. Such data sources are very common with events that can be counted, for example the number of emails that have been received by an MTA since it was started. The total number of emails is not interesting, but the change since the value has been read the last time. The value is therefore converted to a rate using the following formula (see: Finite difference (Wikipedia)):
Please note that if valuenew < valueold, the resulting rate will be negative. If you set the minimum value to zero, such data points will be discarded. Using DERIVE data sources and a minimum value of zero is recommended for counters that rarely overflow, i.e. wrap-around after their maximum value has been reached. This data source type is available since version 4.8.
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