First of all, thanks for your great post!, it helped me allot to configure my 1min interval polling.
I still have one question regarding Rows calculation.
I understand the math behind Rows calculation, but I did not undersand why you choose longer periods than the period represented by Timespan value.
For example: In your 1min poll / 24 hours view, why did you choose Rows value of 4320 which is equal to 3 days if due to the Timespan value only 24hours will be displayed in the graph? my common sense would be to choose Rows value of 1440 in this case. what am I missing?
Likewise, I believe I understand everything except this last bit... I'm not sure of the reasoning behind keeping extra rows beyond the timespan values Cacti will use when rendering graphs (in most cases double the number of rows needed to represent the specified timespan).
For example, with step adjusted for 1 minute polling, the default RRAs might look something like this:
Hourly (1 Minute Average) 1 500 14400
Daily (5 Minute Average) 6 600 86400
Weekly (30 Minute Average) 30 700 604800
Monthly (2 Hour Average) 120 775 2678400
Yearly (1 Day Average) 1440 797 33053184
Taking the Daily (5 Minute Average) as an example. The timespan specifies that when this graph is displayed, it will show 1 day's worth of data; so in theory, the number of rows needed to represent this would be 288 (the number of 5 minute intervals in one day) - but the RRA is storing 600 rows of data (giving it a history of over two days).
My question is, what is the reasoning behind Cacti storing twice the data needed (and tekbot 3 times or more) when the timespan setting means this superfluous data wont be used to render the standard graphs?
Could it be that this extra data is used when custom graph view settings are used. For example, if I ask Cacti to show me graphs spanning the past two days instead of one, will it use this extra data being stored when generating this custom graph for me?
If so, it got me thinking about this a bit further... Again using the RRAs above as an example; what if I asked Cacti to show me custom graphs spanning the past 4 days (exceeding the available information contained in the Hourly or Daily RRAs).
Does Cacti use the more accurate Hourly RRA data to render the first 8 hours or so, the Daily RRA data to render up until the past 2 days, and then fill in the rest using the less accurate Weekly (30 Minute Average) RRA? Or does it instead use the RRA data from the Weekly RRA because that's the only one that contains enough data spanning the requested timespan?
Thank you in advanced for your time and consideration.
Edit: I think this from the Cacti manual kind of answers my question: "The data retrieval function of RRDtool automatically selects the archive with the highest resolution that still covers the requested timespan. This mechanism is also used by RRDtool's graphing subsystem."