[rrd-users] Re: I have been told the .RRD is like a circle.....

Alex van den Bogaerdt alex at ergens.op.het.net
Tue Feb 11 00:49:50 MET 2003

On Mon, Feb 10, 2003 at 12:39:51PM -0800, John Giordano wrote:
> no beginning and no end.  within is contained the knowledge we seek.

true, provided you also have a clue about the pointer to "now".

> what I am not sure on is what I have heard on how RRD "compresses" the data over time.  What does this mean?

It doesn't compress data.  It does however do the following:

inputs:  A, B, C, D, E, F, G, H, I, J, K, L ...
archives: p,q,r ...

in archive p, each individual input is stored
in archive q, the average of two inputs is stored
in archive r, the average of four inputs is stored

A --> p
B --> p  >and<  avg(A,B) --> q
C --> p
D --> p  >and<  avg(C,D) --> q  >and<  avg(A,B,C,D) --> r
E --> p
F --> p  >and<  avg(E,F) --> q
G --> p
H --> p  >and<  avg(G,H) --> q  >and<  avg(E,F,G,H) --> r
...and so on...

> I understand that the .RRD logs don't grow and this is one of the many things that are cool about RRD and the fact it only generates the graphs when need be.

If you need to graph one day per pixel (say for a yearly graph), it is
nice to have the data available >>>in that resolution<<<.  You don't
want to spend time on computing the average of (86400/300) samples
for each pixel at graph time.  RRDtool will do this averaging (when
instructed!) at storage.

Also, if you never need to graph the data of last year's January in
a high resolution image, why keep that data ?

What this means is that it usually is a good thing to have, for instance:

about 600 lines of 300 seconds per bucket
about 600 lines of 30 minutes per bucket
about 600 lines of 2 hours per bucket
about 600 lines of 1 day per bucket

This doesn't allow you to fetch data in a 300-second-per-bucket resolution
from more than (300*600) seconds ago.  It does allow you to get the
same average in a lower resolution.

If you want to know more, there's detailed documentation available.

Much of what looks like rudeness in hacker circles is not intended to give
offence. Rather, it's the product of the direct, cut-through-the-bullshit
communications style that is natural to people who are more concerned about
solving problems than making others feel warm and fuzzy.


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