RRDTool Counter increment lower than time

Question

I create a standard RRDTool database with a default step of 5mn (300s). I have different types of values in it, some gauges which are easily processed, but I have other values I would have in COUNTER but here is my problem : I read the data in a program, and get the difference between values over two steps is good but the counter increment less than time (It can increment by less than 300 during a step), so my out value is wrong.

Is it possible to change the COUNTER for not be a number by second but by step or something like that, if it's not I suppose I have to calculate the difference in my program ?

Thank you for helping.

Answer 1

RRDTool is capable of handling fractional values, so there is no problem if the counter increments by less than the seconds interval since the last update.

RRDTool stores everything as a Rate. If your DS is of type GAUGE, then RRDTool assumes that the incoming value is alreayd a rate, and only applies Data Normalisation (more on this later). If the type is COUNTER or DERIVE, then the value/timepoint you are updating with is compared to the previous value/timepoint to obtain a rate thus: r=(x2 - x1)/(t2 - t1). The rate obtained is then Normalised. The other DS type is ABSOLUTE, which assumes the counter was reset on the last read, giving r=x2/(t2 - t1).

The Normalisation step adjusts the data point based on assuming a linear progression from the last data point so that it lies exactly on an interval boundary. For example, if your step is 5min, and you update at 12:06, the data point is adjusted back to what it would have been at 12:05, and stored against 12:05. However the last unadjusted DP is still preserved for use at the next update, so that overall rates are correct.

So, if you have a 300s (5min) interval, and the value increased by 150, the rate stored will be 0.5.

If the value you are graphing is something small, e.g. 'number of pages printed', this might seem counterintuitive, but it works well for large rates such as network traffic counters (which is what RRDTool was designed for).

If you really do not want to display fractional values in the generated graphs or output, then you can use a format string such as %.0f to enforce no decimal places and the displayed number will be rounded to the nearest integer.