[rrd-users] Re: about DS and RRA

Alex van den Bogaerdt alex at ergens.op.het.net
Fri Feb 6 10:13:41 MET 2004

On Fri, Feb 06, 2004 at 08:52:18AM +0100, Christophe Gauder wrote:

> Here is a newbie question : 
> I have a RRD structure which use the following :
> DS:a1:GAUGE:120:0:1500
> DS:a2:GAUGE:120:0:1500
> DS:a3:GAUGE:120:0:1500
> RRA:AVERAGE:0.1:3:90000
> RRA:AVERAGE:0.1:10:26000
> a1,a2 and a3 are read every 30s.

Can be, but what is your "step".  I'll assume it is set to 30 as well,
however you should explicitly say so.

> Suppose now it is no more possible to read them....
> After how many second will I have a white zone in the graphs ?

No sooner than that you write a grapher script that produces white
zones.  For instance, mine will produce bright red when unknown
data is encountered so I have _no_ white zones.

Anyway, you probably _are_ talking about unknown data so I'll answer
as if you had asked about that:

When an update is given to RRDtool and this update is more than your
heartbeat after the previous update, the calculated rate is discarded
and the interval will be filled with unknown.

In this example, this will happen after 120 seconds. This is four PDPs
in one update command.  The other possible reason for unknown data is
when the rate is too high or too low.

> Is this time different if the second RRA is used ?

The first RRA will allow ten percent of the PDPs to be unknown.  However,
you only use three at a time, so effectively you have no room for unknown
PDPs (you cannot have 0.3 PDP).

The second RRA will also allow ten percent of the PDPs to be unknown. Here
you use ten PDPs, so one of them may be unknown while still producing a
valid CDP.

When you get an unknown because of out-of-range data, it will be masked
out.  When you get an unknown because the heartbeat has expired, the
unknown will be accompanied by three other unknowns and the CDP will
also be unknown  UNLESS one of them is in one CDP and the other three
are in another CDP.

If you didn't understand what I wrote, I suggest you read some of the
docs I wrote and then, if still necessary, ask about the process again
(make sure to write which part you didn't understand). The key part
to understand is PDP vs. CDP.

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