[rrd-users] Re: rrdcgi problem with picture.

[Olivier Nicole]

Hi,

> I started using rrdcgi today. I am planning to use rrdcgi to make "zooms" .
> The user will have a form with 2 fields, startdate and enddate, so he can
> create graphics from old values with very good precision.

It seems that you make a confusion and RRD is NOT  the tool you need.

You cannot make good precision zoom on old data.

Say you keep one day with no average, one week with average at 30
minutes, one month with average at 2 hours and one year with average
at one day. If you want to zoom in a day last week, you will only get
30 minutes precision, and if you zoom in november 1st, you get 2 hours
precision. If you zoom at january you get one day precision and if you
are interested inoctober 2000 you get nothing.

RRD keeps AVERAGE values, not the full precision values. And it keeps
just the exact amount of data that you asked him to keep: in my example
it will keep:

- 288 data full precision (one day, one sample every 5 minutes)
- 288 data at 30 minutes average (one week, one average every 30 minutes)
- 276 data at 2 hours average (one month, one average every 2 hours)
- 335 data at one day average (one year, one average per day)

(and yes, one year is 365 days, but it can use the month (30 days)
above to make it 335+30=365 :)

So RRD only keep one year worth of data, and only 1/4 of the data are
at full precision.

If youwant to use RRD to draw the graphics, you keep the full
precision data in a database, extract the data range you want, push
them in a RRD file and draw your graphic. But RRD cannot be kept for
full precision data conservation, it is the  contrary of its purpose.

Of course one dat,one week, one month or one year are there just for
explanation, you can define the time range you want.

Olivier

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