[rrd-users] Absolute values..

[Alex van den Bogaerdt]

----- Original Message ----- 
From: "Alex van den Bogaerdt" <alex at vandenbogaerdt.nl>
To: <rrd-users at lists.oetiker.ch>
Sent: Tuesday, November 18, 2008 2:36 AM
Subject: Re: [rrd-users] Absolute values..


Sorry, part of it was confusing. Or rather I was confused.

I had "--step 1" and steps per RRA entry multiplied by 60 in my mind, but I 
did not write this down.

If you stay with "--step 60", the rate stays 0.5 even when using min or max.

When you change to step 1, and have 60 times more steps in each RRA entry, 
the same amount of time is kept per entry.  But when you change average to 
max (or min) the combined 60 entries will be the maximum (or minimum) rate 
seen.

But, depending on what you are actually monitoring, this may result in 
fooling yourself and/or others.  Why? Because you alter true data into 
something which is looking like, but not equal to, the truth. Data which 
should have been zero becomes one, or vice versa.

Perhaps you are monitoring an on-off switch or alike.  Or a door 
open/closed, true/false, and so on. It makes sense to have fractions in such 
a case.  The door was open during 10% of the day. The switch was on half of 
the time.

Take a step back and think about what it actually is you want to make 
visible.  Do so keeping in mind that RRDtool will consider numbers to be a 
rate, "whatever per second".  It doesn't know nor care what this "whatever" 
is, it could even be "degrees celsius times 1 second" which then results in 
"degrees celsius times 1 second per second -> degrees celsius".  It still is 
a rate.

Also think about consolidation. What will happen when you are looking at a 
graph representing a full year or more?  If you frequently see a 1 but also 
frequently a 0, MAX is going to make your data 1 throughout the year and MIN 
is going to make your data all zero.

If the amount of 1 is rather small, MAX will definately make it visible on 
the year graph but having one 1 during an hour on a day graph will show up 
as one 1 during a full day on a year graph.

If you do store rates like "the door was open 10% of the day", you (or 
rather rrdtool) can add such percentages and combine multiple rates into one 
rate valid during a larger time span. That's what consolidation is all 
about.  You usually don't want to know that the door was opened somewhere in 
the year. Usually you want to know it was open 10% of the year.


And what happens if the data can become more than 1?  Can it in your 
application, once you're out of the development phase?  If so, you better 
start using such rates right away IMHO.