[rrd-users] RRD shows unknown data in the graph

Alex van den Bogaerdt alex at ergens.op.het.net
Fri Jun 15 13:40:34 CEST 2007

On Fri, Jun 15, 2007 at 04:26:54AM -0700, VarmaKK wrote:

> I'm using RRD tool to monitor a group of hosts. We have scripts to fetch the
> time series data from all the hosts and that data is stored in an Oracle DB.
> And for every hour this time series data is transfered from Oracle DB to RRD
> database through a perl script. It's been working fine since long time.
> Now I have discovered a problem with values supposed to be unknown (
> Channels not being updated because the corresponding hosts are down for 3
> hours):
> But they don't show up as unknown/NaN in the database - instead it show a
> constant value let's say 150 in the graphs during that 3 hrs down time.

As with many other updates: the rate is computed delta(value)/delta(time).
If the timer increased by 1620000 during the last three hours, the rate
will be 150.

To solve your problem:

If you know a host is down, just feed "U" (unknown) to the database.
You will have to track up/down, store it in your database and use it
in your update script.

Another approach is to utilize RRDtool's heartbeat feature; if an
update is done too long after the previous one, RRDtool will set
the data to unknown.

In my opinion it is better to tell RRDtool than to rely on this
heartbeat feature.

Alex van den Bogaerdt

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