RRD_PDPCALC(1) rrdtool RRD_PDPCALC(1)
NAME
PDP calculation explanation - PDP inner calculation logics with an
example by Tianpeng Xia
DESCRIPTION
This article explains how PDP are calculated in a detailed yet easy-to-
understand way, with an example.
Refreshing some basics about PDP
Fundamental knowledge
If you have not read the tutorials or man pages either on the official
site or those by others, then I strongly encourage you to do so. As
said in the description, this article will only explain how a PDP is
calculated, but not the definition of it. So please read the following
materials to get a basic understanding of PDP:
<http://rrdtool.vandenbogaerdt.nl/process.php> - By Alex van den
Bogaerdt. This article explained PDP in a very detailed and clear way,
however, it does not explain the "normalization process" in its
"Normalize interval" section in the right way( as opposed to the
official version I confirmed with @oetiker himself). The flaw can be
easily seen in the bar charts, discussed in the "Calculation logics"
section.
<https://oss.oetiker.ch/rrdtool/doc/rrdcreate.en.html> - This one is on
the official site. Actually it's the manual page for "rrdcreate", and
it reveals what's under the hood with regard to PDP calculation in its
"The HEARTBEAT and the STEP" section.
The text graph by Don Baarda provides a vivid explanation on how UNKOWN
data are produced and how heartbeat value can influence in the
sampling. Unfortunately, it fails to give a clear method by which PDPs
are calculated.
<https://oss.oetiker.ch/rrdtool/tut/rrdtutorial.en.html> - Another
detailed official tutorial by Alex van den Bogaerdt. Similarly, it only
provides examples with data evenly and exactly distributed according to
the step set.
If you don't like doing experiments or care about the inner mechanics
that much, you can just stop here and give more attention to more
practical topics like graph exports or command manual. But if you are
the sort of people like me who just care as much about the calculation
logics, please read on.
Calculation logics
Here begins the core part of this article. In the following content of
this section, I would like to give two versions of calculation methods,
one by Alex van den Bogaerdt and the other by @eotiker.
To provide an ASCII-friendly explanation, I will explain both versions
with the char below instead of a real image.
|
| (v1)
| _______ (v4) (v5)
| | | (v3) ____________
| | | ______________| || |
| | | | || || |
| | | | || || |
| | | (v2) | || || |
| | |________| || || |
--------------------------------------------->
0 1 3 7 17 20 21
The X axis means time slots( each second denotes one slot) and the Y
axis means the value.
Let's make everything a little clearer:
- The step is 5
- each PDP gets updated only if a value arrives at or after the last
slot of the PDP, for instance, the last slot of the PDP from 16 to 20
is 20
- The heartbeat is 20, so the samples during the entire 7-17 period is
not discarded
- At second 3, the first value comes in as v1, and so on
- Second 0 is the origin, and it does not count as a sample
Bogaerdt version
As can be seen on this page:
<http://rrdtool.vandenbogaerdt.nl/process.php>, after all the primary
data are transformed to rates( except for GAUGE, of course), they have
to go through a normalization process if they are not distributed
exactly according to the step or on well-defined boundaries in time, in
the words of the author.
What does that mean? Basically, if all the known (as opposed to an
unknown value) data make up at least 50% of all slots during a period,
then a PDP is calculated from them.
This version seems to go well until we reach the bar chart part.
According to the ASCII bar chart, we have the following results:
From second 1 on, the PDP of each period( 1-5,6-10, ...) is computed by
averaging all the values within it.
So: - the PDP from 1 to 5 is (v1*3+v2*2)/5
- the PDP from 6 to 10 is (v2*2+v3*3)/5
- the PDP from 11 to 15 is (v3*5)/5, since all the values in slots 11,
12, 13, 14 and 15 are the same, which is v3
- ...
The official version( also @oetiker version):
Using the same chart, this version suggests the following:
- the PDP from 1 to 5 is (v1*3+v2*2)/5
- the PDPs from 6 to 10 and 11 to 15 are the SAME, which is (v2*2+v3*8)
- ...
A Comparison and some explanation
So we have seen the above two versions and their PDPs from 6 to 10 and
11 to 15 do not comply with each other.
Why is that?
Because the difference between the official version and Bogaerdt
version stems from the way they do the calculation for PDP(6-10) and
PDP(11-15).
Let's discuss this in more detail using the above bar chart.
Bogaerdt's version,
PDPs are always computed individually no matter how values arrive.
For example, the value at slot 17 comes after the last slot of
PDP(11-15). Also, the immediate previous value before slot 17 is at 7.
All the slots from 7 to 17 are assigned v3. Since each PDP is computed
individually, PDP(6-10) is (v2*2+v3*3)/5 while the PDP(11-15) is
(v3*5)/5.
The official version
PDPs are always computed in terms of the steps which the next update
spans, be it 1 step, 2 steps or n steps; in other words, PDPs may be
computed together.
For example, the update at slot 17 spans PDP(6-10) and PDP(11-15)
because the immediate previous value is at 7 and 7 is within 6 and 10 ,
and 17 is after 15. PDP(1-5) and PDP(16-20) are not included since the
update at slot 7 has already triggered the calculation for PDP(1-5) and
the update at slot 17 comes before the last slot of PDP(16-20) which is
20.
That's the reason why PDP(6-10) and PDP(11-15) have the same value,
(v2*2+v3*8).
An example
If you are still confused, don't worry, an example is here to help you.
Let's get our hands dirty with some commands
rrdtool create target.rrd --start 1000000000 --step 5 DS:mem:GAUGE:20:0:100 RRA:AVERAGE:0.5:1:10
rrdtool update target.rrd 1000000003:8 1000000006:1 1000000017:6 \
1000000020:7 1000000021:7 1000000022:4 \
1000000023:3 1000000036:1 1000000037:2 \
1000000038:3 1000000039:3 1000000042:5
rrdtool fetch target.rrd AVERAGE --start 1000000000 --end 1000000045
Basically, the above codes contain 3 commands: create, update and
fetch. First create a new rrd file, and then we feed in some data and
last we fetch all the PDPs from the rrd.
Focus on single steps
In order to provide a detailed explanation, each the calculation
process of each PDP is provided.
Below is the output of the commands above:
1000000005: 5.2000000000e+00
1000000010: 5.5000000000e+00
1000000015: 5.5000000000e+00
1000000020: 6.6000000000e+00
1000000025: 1.7333333333e+00
1000000030: 1.7333333333e+00
1000000035: 1.7333333333e+00
1000000040: 2.8000000000e+00
1000000045: nan
1000000050: nan
NOTE: 1000000005 means the PDP from 1000000001 to 1000000005, and so
on. For concision and readability, we use only the last two digits, so
05 denotes 1000000005. We choose the type of the data source as gauge
because original values will be treated as rates, no additional
transformation is needed, see this article
<http://rrdtool.vandenbogaerdt.nl/process.php> for detail.
05: 5.2 = (8*3+1*2)/5
10: 5.5 = (1*1+6*9)/10
15: the same as the previous one
20: 6.6 = (6*2+7*3)/5
25: 1.73333 = (7+4+3+1*12)/15
...
45: nan, as the last value is at 42,which does not trigger the
calculation for PDP(41-45)
50: nan, why this unknown PDP is shown is explained in this article
<https://oss.oetiker.ch/rrdtool/tut/rrdtutorial.en.html>
SUMMARY
All that said, I hope you get a clear understanding of the inner
calculation "magic" for PDPs.
Other References
o A great PowerShell shell script for generating ASCII bar charts:
<https://gallery.technet.microsoft.com/scriptcenter/Sample-Script-to-Generate-59c80d4c>
o <https://stackoverflow.com/questions/18924450/rrd-wrong-values>
1.7.2 2020-04-11 RRD_PDPCALC(1)