[rrd-users] Re: Is graphing the exact number possible?

[Alex van den Bogaerdt]

BAARDA, Don wrote:
> 	[...]
> > If you're (also) interested in the maximum number of lines in
> > use, a good way of doing so is to define:
> > -  an RRD with step time of one second, and a heartbeat of > 300.
> > -  an RRA with averages for 300 PDPs per CDP.
> > -  an RRA with maxima for 300 PDPs per CDP.
> > 
> 	This is the solution I would prefer for accurate graphing of
> "events", rather than "intervals". If you extend the heartbeat out to a day
> or two, the only time you need to feed data to rrd is when the status
> changes (provided the status changes at least every day or two). However,
> it's probably a good idea to use a low-ish heartbeat and include periodic
> updates as well as "status change events" incase you miss an event. The best
> way to do this is use snmp for the periodic samples, and snmp-trap for the
> "events".

So, you agree with me.

I have one addition to this: If possible, use the time on the device
and not the time on the monitoring box.  This prevents delay (which
is not nice but also not a big problem) and jitter (which can be a big
problem).

> > Everytime something changes in the line status, remember the number
> > of lines.  The *next* time something changes, or the next 5-minute
> > boundary, you enter the remembered value (which will be valid from
> > the previous update to the current one).
> 	[...]
> 
> 	This is the tricky bit. RRD with assumes a sample refers to the
> previous interval between samples, not the next interval. This means at each
> change in status, you need to tell rrd what it _was_, not what it now _is_.

Don't forget the periodic update. First of all you need this when there
is no change during a long time.  You can *guess* that your heartbeat is
long enough, or you can *make sure* it is by forcing an update every 5
minutes or so.  This is also good for a second purpose: you don't want
a long series of unknown at the end of your database while in fact you
know that nothing changed and thus you *do* know the number of lines
in use.

cheers
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