[rrd-users] Re: Getting unknown value :-?

Alex van den Bogaerdt alex at ergens.op.HET.NET
Fri Jul 19 13:29:09 MEST 2002

Carlos Costa Portela wrote:

> I'm trying to play with RRDs, trying to do a script that saves the
> load average of my machine. I create the file with:
>      RRDs::create ("$workdir/$rrd", "--step","$update",
> 	"DS:load:GAUGE:864:U:U",

Time between two updates may not be more than 864 seconds.
If you update at, for instance, 13:00 and 13:15, the update
is ignored.

> 	"RRA:AVERAGE:0.5:1:864",
> 	"RRA:MIN:0.5:1:864",
> 	"RRA:MAX:0.5:1:864",
> 	"RRA:LAST:0.5:1:864");

I think you think 864 is the xff here.  It isn't.  0.5 is.
864 in these four lines are the amounts of rows per CDP.

> 	So, as far I understand, with one only value, I must see some
> information when I request it:
> 	RRDs::update ("$workdir/$rrd","N:$load");
> 	But, unfortunately:
> ds[load].min = NaN
> ds[load].max = NaN
> ds[load].last_ds = "UNKN"

Indeed, no information.  Either you updated with a NaN value (this
can be done), or the update is ignored.  Make sure you log $load
to a separate log/debug file for a while.

> 	I am missing something. I don't understand what xff (xfiles
> factor) means (I've tried with several values).

xff comes into play when the data is moved to the RRA.  At this
stage, it plays no role so your problem must be elsewhere.

xff determines the amount of allowable unknown PDPs per CDP.

Example:  6 PDPs are needed for 1 CDP.  3 of them are NaN.
xff needs to be 0.5 or higher (upto but not including 1.0).

If xff is less than 0.5, the CDP will become unknown in this

You probably need to find the proper values for "--step",
"heartbeat" (currently 864 in "DS:load:GAUGE:864:U:U") and
your update schedular.

To get on track again:

set update to something sane (such as 300, or 900)
set heartbeat to $update * 1.1
set rows to a number of your liking

RRDs::create ("$workdir/$rrd", "--step","$update",

Then, every $update seconds, run the update command.

This database will store $update * $rows seconds of data.

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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