[rrd-users] Re: Finding time for latest primary data point

[Alex van den Bogaerdt]

On Thu, Jan 29, 2004 at 09:33:37AM -0600, Anders Nygren wrote:


> > > What I am planning to do is
> > > 1, "normal" data collection, and storing the data in rrdtool
> > > 2, a separate process that periodically, (after the update), gets the
> > > latest values from rrdtool and does threshold checks for generating
> > > alarms.
> >
> > Inbetween (1) and (2), time will move forward.
> >
> > If 't' is T modulo R, time _could_ go from (t+n) to (t) and you will
> > be looking at the wrong interval.  This won't happen often however it
> > is possible and it is something to take care of.
> 
> But in (2) I will know the time used in the update, so I can use the same
> time in the fetch. That would work, right?

_If_ you know, and remember, and use, the time of the last update then
yes, you will be taking care of the possible problem.

This means you cannot rely on "rrdtool update ....   N:$value".  You will
have to do either
a) time=....; rrdtool update ... $time:$value
or
b) rrdtool update ...; time=`rrdtool last|...`

In other words: using "N" (now) followed by "time=[somewaytogetcurrenttime]"
will eventually cause problems.

grtz
Alex
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