[rrd-users] Re: Question on rrdtool graph

Chang-Ping Hsiao Chang-PingH at corp.netzero.net
Mon Mar 19 23:03:16 MET 2001


> >     I am having a question on the rrdtool graph output for I don't know
> > to select an RRA for the output.  I hope I can get some help from here,
> > it is much appreciated.
> You don't select the correct RRA, let rrdtool do this for you.

	I guess so, according to the man page (HTML page).

> >     I am having different RRAs for an RRD.  All RRAs are using CF
> > you can see below.  In the document for rrdtool graph, in the DEF:
> > it says "... The DEF call automatically chooses an RRA which contains CF
> > consolidated data in a resolution appropriate for the size of the graph
> > be drawn."  Since I don't have enough data yet, I would have no way of
> > telling where or not I can retrieve data in the range at my will.
> 1st of all, an RRA must have the same CF properties.  In your case
> there's not much to choose from as you only have AVERAGE RRAs.
> Normally, you can fetch maximum values from the MAX RRA and average
> values from the AVERAGE RRA.  Which CF to use is selected by DEF.

	Well, :-), this "all-AVERAGE" design is to the request from above,
not up to me.  Since they don't need MAX or MIN, there are only AVERAGE's.

> Then, the closest match is found on the time frame you selected.
> > sub create_rrd {        # create an RRD for a production box
> >         my $start = "--start $time_s";
> >         my $step  = "--step 3600";
> Step time == 3600 --> one hour per PDP (primary data point)


> >         my $rra1  = "RRA:AVERAGE:0.5:1:8784";   # daily
> One PDP per CDP (consolidated data point, row) in this RRA
> and 8784 rows.  This makes a total of 366 days, or: a year.

	Exactly.  It's designed to be 366 days to cover leap years.

> >         my $rra2  = "RRA:AVERAGE:0.5:3:2928";   # weekly
> Three PDPs per CDP, totalling 2928 rows.  This is thus 2928
> rows of 3 hours per row = the same amount of time as the
> previous RRA. (I don't think this one gets used often...)

	Good point.  The request came with the ability to retrieve data for
graphing for daily, weekly, monthly, quarterly, and yearly (or annually)
pretty much like the graphs for stocks; without the data being saved in the
RRD and RRA, it would require the graphs to be generated predefined and
stored somewhere.  This doesn't fit the goal of saving disk space while
computation time is not a concern.

> >         my $rra3  = "RRA:AVERAGE:0.5:6:1464";   # monthly
> Ditto
> >         my $rra4  = "RRA:AVERAGE:0.5:12:732";   # quarterly
> Ditto
> >         my $rra5  = "RRA:AVERAGE:0.5:24:366";   # yearly
> Ditto
> The purpose of having multiple RRAs is that you can store
> data for a longer time, with a lower resolution.  There's
> nothing agains having such a long "daily" RRA but it makes
> no sense to have the "yearly" RRA with the same amount of
> data.  It may speed up graphing a bit but I don't think
> this is noticeable.

	For this point of view, I need to ask more if I may.  So if I want
to have lower resolution for a year's worth of data, without saving them
into RRA, how may I proceed and process?  I would love to learn how to save
space and time for having the same result.  :-)

> However, you can run rrdtool fetch with different resolution
> settings and see what consolidation does with your data.
> Do, for instance:
> rrdtool fetch --resolution 3600 --start '00:00' --end start+6h
> and 
> rrdtool fetch --resolution 10800 --start '00:00' --end start+6h
> on the same RRD so see the differences.

	Thank you for this idea.  I never tried.  For some reason, the RRDs
I have don't pop up information on these queries.  Maybe something is not
right on them.  I'll check.

Chang-Ping Hsiao

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