[rrd-users] trying to understand the relationship between source data, what's in rrd and what gets plotted

Tobias Oetiker tobi at oetiker.ch
Fri Jul 20 21:20:57 CEST 2007

Hi Mark,

yes the 'lost' spike confuses people ... most, when they start
thinking about it, see that rrdtool does exactly the right thing,
it uses to consolidation method of the data being graphed to
further consolidate for the graph ...

so ifyou are using MAX as consolidation function for the RRA, the
grapher will use MAX too. If you are averaging the data, the
grapher will use the same function too ...

if you have textual suggestions for the grapher documentation I
will be glad to include tem

Today Mark Seger wrote:

> Alex van den Bogaerdt wrote:
> > On Fri, Jul 20, 2007 at 12:31:25PM -0400, Mark Seger wrote:
> >
> > > more experiments and I'm getting closer...  I think the problem is the
> > > AVERAGE in my DEF statements of the graphing command.  The only problem is
> > > I couldn't find any clear description or examples of how this works.  I
> > > did try using LAST (even though I have no idea what it does) and my plots
> > > got better, but I'm still missing data points and I want to see them all.
> > > Again, I have a step size of 1 second so I'd think everything should just
> > > be there...
> > >
> >
> > Last time I looked, which is several moons ago, the graphing part
> > would average different samples which needed to be "consolidated"
> > due to the fact that one was trying to display more rows than there
> > were pixel columns available.
> >
> Ahh yes, I think I see now.  However, and I simply point this out as an
> observation, it's never good to throw away or combine data points as you might
> lose something really important.  I don't know how gnuplot does it but I've
> never see it lose anything.  Perhaps when it sees multiple data points it just
> picks the maximum value.  hey - I just tried that and it worked!!!
> This may be obvious to everyone else but it sure wasn't to me.  I think the
> documentation could use some beefing up in this place as well as some
> examples.  At the very least I'd put in an example that shows a series that
> contains data with a lot of values <100 and a single point of 1000.  Then
> explain why you never see the spike! I'll bet a lot of people would be
> shocked.  I also wonder how many system managers are missing valuable data
> because it's simply getting dropped out off.
> -mark
> > (I wrote consolidated surrounded by quotation marks because it isn't
> > really consolidating what's happening)
> >
> > In other words: unless your graph is 50k pixels wide, you will have
> > to select which 400 out of 50k rates you would like to see, or you
> > will have to deal with the problem in a different way. For example:
> >
> > If you setup a MAX and MIN RRA, and you carefully craft their parameters,
> > you could do something like this:
> >
> > * Consolidate 60 rates (1 second each) into one (of 60 seconds).
> >   This means setting up an RRA with steps-per-row 60.
> > * Display 400 x 60 seconds on a graph (or adjust the graph width,
> >   together with the amount of CDPs to plot).
> > * Do this using (you fill in the blanks):
> >     DEF:MyValMin=my.rrd:minrra:...
> >     DEF:MyValMax=my.rrd:maxrra:...
> >     CDEF:delta=MyValMax,MyValMin,-
> >     AREA:MyValMin
> >     AREA:delta#FF0000:values:STACK
> >   (That first area does not plot anything, and it is not supposed to.
> >   The second area displays a line from min to max.)
> > * Do the same for 3600 steps per row, and 400x3600 seconds per graph
> >
> > and so on.  Of course you can adjust the numbers to your liking.
> >
> > HTH
> >

Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten
http://it.oetiker.ch tobi at oetiker.ch ++41 62 213 9902

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