# [rrd-users] Re: Data granularity

Serge Maandag serge.maandag at staff.zeelandnet.nl
Thu Jan 20 12:35:15 MET 2005

```> I have a monitor scripts that gets invoked every time a user
> interacts with a Web service.  There are several Web services
> that we are collecting data from.  Each one goes to a
> different RRD.  Some services are very busy (several hits per
> second) and some are not (a few hits per
> week).   There is no way for me to identify which is which...

I cannot see how this works. Rrdtool collects rates, while you use
It as a counting mechanism. But as long as it works for you..

> I would like to use 5sec STEP so that there is enough detail
> for busy services.  A few questions to RRD gurus:
>
> (1) What is best value for HEARTBEAT?  Is 10 seconds too low?

That depends on what you want to see in your database.
If you set it to 10:
- updates that are longer apart than 10 seconds, will cause NaNs
- updates that are between 5 and 10 seconds apart, will cause
2 consecutive rows to be influenced, it will be averaged out.

If you set heartnbeat to N x step, N rows will be affected by
An update, provided your high res RRA has a row time that is equal
To the step size.

> (2) What is the best value for XFF?

That also totally depends.

Say you have these RRA's:

RRA:AVERAGE:0.5:1:1000
RRA:AVERAGE:0.5:10:300

Every step tim, one value will be put in the first RRA.
Every 10 step times, 10 values from the first RRA will be averaged
And the result will be put in the second RRA. Unless more than 50%
Of the 10 values are NaNs. If that is the case NaN will be put in the
Second RRA.

If the second RRA would read:

RRA:AVERAGE:0.9:10:300

Then the average of the 10 values would still be put in the second
RRA if 9 out of 10 values would be NaN.

So it all depends on what you want to see.

In your case I'd probably just dump the hits with a timestamp in a
log file and setup a cron job that would do every 5 minutes:
- read back all lines and count the hits that were within the last
5 minutes
- update the RRD database with the sum of the counted hits as a gauge
value.

Serge.

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