RRDTUTORIAL(1) rrdtool RRDTUTORIAL(1)
NAME
rrdtutorial - Alex van den Bogaerdts RRDtool tutorial
DESCRIPTION
RRDtool is written by Tobias Oetiker with contribu
tions from many people all around the world. This document is written
by Alex van den Bogaerdt to help you under
stand what RRDtool is and what it can do for you.
The documentation provided with RRDtool can be too technical for some
people. This tutorial is here to help you understand the basics of RRD
tool. It should prepare you to read the documentation yourself. It
also explains the general things about statistics with a focus on net
working.
TUTORIAL
Important
Please dont skip ahead in this document! The first part of this docu
ment explains the basics and may be boring. But if you dont under
stand the basics, the examples will not be as meaningful to you.
What is RRDtool?
RRDtool refers to Round Robin Database tool. Round robin is a tech
nique that works with a fixed amount of data, and a pointer to the cur
rent element. Think of a circle with some dots plotted on the edge --
these dots are the places where data can be stored. Draw an arrow from
the center of the circle to one of the dots -- this is the pointer.
When the current data is read or written, the pointer moves to the next
element. As we are on a circle there is neither a beginning nor an end,
you can go on and on and on. After a while, all the available places
will be used and the process automatically reuses old locations. This
way, the dataset will not grow in size and therefore requires no main
tenance. RRDtool works with with Round Robin Databases (RRDs). It
stores and retrieves data from them.
What data can be put into an RRD?
You name it, it will probably fit as long as it is some sort of time-
series data. This means you have to be able to measure some value at
several points in time and provide this information to RRDtool. If you
can do this, RRDtool will be able to store it. The values must be
numerical but dont have to be integers, as is the case with MRTG (the
next section will give more details on this more specialized applica
tion).
Many examples below talk about SNMP which is an acronym for Simple Net
work Management Protocol. "Simple" refers to the protocol -- it does
not mean it is simple to manage or monitor a network. After working
your way through this document, you should know enough to be able to
understand what people are talking about. For now, just realize that
SNMP can be used to query devices for the values of counters they keep.
It is the value from those counters that we want to store in the RRD.
What can I do with this tool?
RRDtool originated from MRTG (Multi Router Traffic Grapher). MRTG
started as a tiny little script for graphing the use of a universitys
connection to the Internet. MRTG was later (ab-)used as a tool for
graphing other data sources including temperature, speed, voltage, num
ber of printouts and the like.
Most likely you will start to use RRDtool to store and process data
collected via SNMP. The data will most likely be bytes (or bits) trans
fered from and to a network or a computer. But it can also be used to
display tidal waves, solar radiation, power consumption, number of vis
itors at an exhibition, noise levels near an airport, temperature on
your favorite holiday location, temperature in the fridge and whatever
you imagination can come up with.
You only need a sensor to measure the data and be able to feed the num
bers into RRDtool. RRDtool then lets you create a database, store data
in it, retrieve that data and create graphs in PNG format for display
on a web browser. Those PNG images are dependent on the data you col
lected and could be, for instance, an overview of the average network
usage, or the peaks that occurred.
What if I still have problems after reading this document?
First of all: read it again! You may have missed something. If you are
unable to compile the sources and you have a fairly common OS, it will
probably not be the fault of RRDtool. There may be pre-compiled ver
sions around on the Internet. If they come from trusted sources, get
one of those.
If on the other hand the program works but does not give you the
expected results, it will be a problem with configuring it. Review your
configuration and compare it with the examples that follow.
There is a mailing list and an archive of it. Read the list for a few
weeks and search the archive. It is considered rude to just ask a ques
tion without searching the archives: your problem may already have been
solved for somebody else! This is true for most, if not all, mailing
lists and not only for this particular one. Look in the documentation
that came with RRDtool for the location and usage of the list.
I suggest you take a moment to subscribe to the mailing list right now
by sending an email to with a sub
ject of "subscribe". If you ever want to leave this list, just write an
email to the same address but now with a subject of "unsubscribe".
How will you help me?
By giving you some detailed descriptions with detailed examples. I
assume that following the instructions in the order presented will give
you enough knowledge of RRDtool to experiment for yourself. If it
doesnt work the first time, dont give up. Reread the stuff that you
did understand, you may have missed something.
By following the examples you get some hands-on experience and, even
more important, some background information of how it works.
You will need to know something about hexadecimal numbers. If you dont
then start with reading bin_dec_hex before you continue here.
Your first Round Robin Database
In my opinion the best way to learn something is to actually do it.
Why not start right now? We will create a database, put some values in
it and extract this data again. Your output should be the same as the
output that is included in this document.
We will start with some easy stuff and compare a car with a router, or
compare kilometers (miles if you wish) with bits and bytes. Its all
the same: some number over some time.
Assume we have a device that transfers bytes to and from the Internet.
This device keeps a counter that starts at zero when it is turned on,
increasing with every byte that is transfered. This counter will proba
bly have a maximum value. If this value is reached and an extra byte is
counted, the counter starts over at zero. This is the same as many
counters in the world such as the mileage counter in a car.
Most discussions about networking talk about bits per second so lets
get used to that right away. Assume a byte is eight bits and start to
think in bits not bytes. The counter, however, still counts bytes! In
the SNMP world most of the counters are 32 bits. That means they are
counting from 0 to 4294967295. We will use these values in the exam
ples. The device, when asked, returns the current value of the
counter. We know the time that has passes since we last asked so we now
know how many bytes have been transfered ***on average*** per second.
This is not very hard to calculate. First in words, then in calcula
tions:
1. Take the current counter, subtract the previous value from it.
2. Do the same with the current time and the previous time (in sec
onds).
3. Divide the outcome of (1) by the outcome of (2), the result is the
amount of bytes per second. Multiply by eight to get the number of
bits per second (bps).
bps = (counter_now - counter_before) / (time_now - time_before) * 8
For some people it may help to translate this to an automobile example.
Do not try this example, and if you do, dont blame me for the results!
People who are not used to think in kilometers per hour can translate
most into miles per hour by dividing km by 1.6 (close enough). I will
use the following abbreviations:
M: meter
KM: kilometer (= 1000 meters).
H: hour
S: second
KM/H: kilometers per hour
M/S: meters per second
You are driving a car. At 12:05 you read the counter in the dashboard
and it tells you that the car has moved 12345 KM until that moment.
At 12:10 you look again, it reads 12357 KM. This means you have trav
eled 12 KM in five minutes. A scientist would translate that into
meters per second and this makes a nice comparison toward the problem
of (bytes per five minutes) versus (bits per second).
We traveled 12 kilometers which is 12000 meters. We did that in five
minutes or 300 seconds. Our speed is 12000M / 300S or 40 M/S.
We could also calculate the speed in KM/H: 12 times 5 minutes is an
hour, so we have to multiply 12 KM by 12 to get 144 KM/H. For our
native English speaking friends: thats 90 MPH so dont try this exam
ple at home or where I live :)
Remember: these numbers are averages only. There is no way to figure
out from the numbers, if you drove at a constant speed. There is an
example later on in this tutorial that explains this.
I hope you understand that there is no difference in calculating M/S or
bps; only the way we collect the data is different. Even the K from
kilo is the same as in networking terms k also means 1000.
We will now create a database where we can keep all these interesting
numbers. The method used to start the program may differ slightly from
OS to OS, but I assume you can figure it out if it works different on
yours. Make sure you do not overwrite any file on your system when
executing the following command and type the whole line as one long
line (I had to split it for readability) and skip all of the \ char
acters.
rrdtool create test.rrd \
--start 920804400 \
DS:speed:COUNTER:600:U:U \
RRA:AVERAGE:0.5:1:24 \
RRA:AVERAGE:0.5:6:10
(So enter: "rrdtool create test.rrd --start 920804400 DS ...")
What has been created?
We created the round robin database called test (test.rrd) which starts
at noon the day I started writing this document, 7th of March, 1999
(this date translates to 920804400 seconds as explained below). Our
database holds one data source (DS) named "speed" that represents a
counter. This counter is read every five minutes (default). In the
same database two round robin archives (RRAs) are kept, one averages
the data every time it is read (e.g., theres nothing to average) and
keeps 24 samples (24 times 5 minutes is 2 hours). The other averages 6
values (half hour) and contains 10 such averages (e.g., 5 hours).
RRDtool works with special time stamps coming from the UNIX world.
This time stamp is the number of seconds that passed since January 1st
1970 UTC. The time stamp value is translated into local time and it
will therefore look different for different time zones.
Chances are that you are not in the same part of the world as I am.
This means your time zone is different. In all examples where I talk
about time, the hours may be wrong for you. This has little effect on
the results of the examples, just correct the hours while reading. As
an example: where I will see "12:05" the UK folks will see "11:05".
We now have to fill our database with some numbers. Well pretend to
have read the following numbers:
12:05 12345 KM
12:10 12357 KM
12:15 12363 KM
12:20 12363 KM
12:25 12363 KM
12:30 12373 KM
12:35 12383 KM
12:40 12393 KM
12:45 12399 KM
12:50 12405 KM
12:55 12411 KM
13:00 12415 KM
13:05 12420 KM
13:10 12422 KM
13:15 12423 KM
We fill the database as follows:
rrdtool update test.rrd 920804700:12345 920805000:12357 920805300:12363
rrdtool update test.rrd 920805600:12363 920805900:12363 920806200:12373
rrdtool update test.rrd 920806500:12383 920806800:12393 920807100:12399
rrdtool update test.rrd 920807400:12405 920807700:12411 920808000:12415
rrdtool update test.rrd 920808300:12420 920808600:12422 920808900:12423
This reads: update our test database with the following numbers
time 920804700, value 12345
time 920805000, value 12357
etcetera.
As you can see, it is possible to feed more than one value into the
database in one command. I had to stop at three for readability but the
real maximum per line is OS dependent.
We can now retrieve the data from our database using "rrdtool fetch":
rrdtool fetch test.rrd AVERAGE --start 920804400 --end 920809200
It should return the following output:
speed
920804700: nan
920805000: 4.0000000000e-02
920805300: 2.0000000000e-02
920805600: 0.0000000000e+00
920805900: 0.0000000000e+00
920806200: 3.3333333333e-02
920806500: 3.3333333333e-02
920806800: 3.3333333333e-02
920807100: 2.0000000000e-02
920807400: 2.0000000000e-02
920807700: 2.0000000000e-02
920808000: 1.3333333333e-02
920808300: 1.6666666667e-02
920808600: 6.6666666667e-03
920808900: 3.3333333333e-03
920809200: nan
If it doesnt, something may be wrong. Perhaps your OS will print
"NaN" in a different form. "NaN" stands for "Not A Number". If your OS
writes "U" or "UNKN" or something similar thats okay. If something
else is wrong, it will probably be due to an error you made (assuming
that my tutorial is correct of course :-). In that case: delete the
database and try again. Sometimes things change. This example used to
provide numbers like "0.04" in stead of "4.00000e-02". Those are
really the same numbers, just written down differently. Dont be
alarmed if a future version of rrdtool displays a slightly different
form of output. The examples in this document are correct for version
1.2.0 of RRDtool.
The meaning of the above output will become clear below.
Time to create some graphics
Try the following command:
rrdtool graph speed.png \
--start 920804400 --end 920808000 \
DEF:myspeed=test.rrd:speed:AVERAGE \
LINE2:myspeed#FF0000
This will create speed.png which starts at 12:00 and ends at 13:00.
There is a definition of a variable called myspeed, using the data from
RRA "speed" out of database "test.rrd". The line drawn is 2 pixels high
and represents the variable myspeed. The color is red (specified by its
rgb-representation, see below).
Youll notice that the start of the graph is not at 12:00 but at 12:05.
This is because we have insufficient data to tell the average before
that time. This will only happen when you miss some samples, this will
not happen a lot, hopefully.
If this has worked: congratulations! If not, check what went wrong.
The colors are built up from red, green and blue. For each of the com
ponents, you specify how much to use in hexadecimal where 00 means not
included and FF means fully included. The "color" white is a mixture
of red, green and blue: FFFFFF The "color" black is all colors off:
000000
red #FF0000
green #00FF00
blue #0000FF
magenta #FF00FF (mixed red with blue)
gray #555555 (one third of all components)
Additionally you can add an alpha channel (transparency). The default
will be "FF" which means non-transparent.
The PNG you just created can be displayed using your favorite image
viewer. Web browsers will display the PNG via the URL
"file:///the/path/to/speed.png"
Graphics with some math
When looking at the image, you notice that the horizontal axis is
labeled 12:10, 12:20, 12:30, 12:40 and 12:50. Sometimes a label doesnt
fit (12:00 and 13:00 would be candidates) so they are skipped.
The vertical axis displays the range we entered. We provided kilometers
and when divided by 300 seconds, we get very small numbers. To be
exact, the first value was 12 (12357-12345) and divided by 300 this
makes 0.04, which is displayed by RRDtool as "40 m" meaning "40/1000".
The "m" (milli) has nothing to do with meters, kilometers or millime
ters! RRDtool doesnt know about the physical units of our data, it
just works with dimensionless numbers.
If we had measured our distances in meters, this would have been
(12357000-12345000)/300 = 12000/300 = 40.
As most people have a better feel for numbers in this range, well cor
rect that. We could recreate our database and store the correct data,
but there is a better way: we do some calculations while creating the
png file!
rrdtool graph speed2.png \
--start 920804400 --end 920808000 \
--vertical-label m/s \
DEF:myspeed=test.rrd:speed:AVERAGE \
CDEF:realspeed=myspeed,1000,\* \
LINE2:realspeed#FF0000
Note: Make sure not to forget the backslash \ in front of the multipli
cation operator * above. The backslash is needed to "escape" the * as
some operating systems might interpret and expand * instead of passing
it to the rrdtool command.
After viewing this PNG, you notice the "m" (milli) has disappeared.
This it what the correct result would be. Also, a label has been added
to the image. Apart from the things mentioned above, the PNG should
look the same.
The calculations are specified in the CDEF part above and are in
Reverse Polish Notation ("RPN"). What we requested RRDtool to do is:
"take the data source myspeed and the number 1000; multiply those".
Dont bother with RPN yet, it will be explained later on in more
detail. Also, you may want to read my tutorial on CDEFs and Steve
Raders tutorial on RPN. But first finish this tutorial.
Hang on! If we can multiply values with 1000, it should also be possi
ble to display kilometers per hour from the same data!
To change a value that is measured in meters per second:
Calculate meters per hour: value * 3600
Calculate kilometers per hour: value / 1000
Together this makes: value * (3600/1000) or value * 3.6
In our example database we made a mistake and we need to compensate for
this by multiplying with 1000. Applying that correction:
value * 3.6 * 1000 == value * 3600
Now lets create this PNG, and add some more magic ...
rrdtool graph speed3.png \
--start 920804400 --end 920808000 \
--vertical-label km/h \
DEF:myspeed=test.rrd:speed:AVERAGE \
"CDEF:kmh=myspeed,3600,*" \
CDEF:fast=kmh,100,GT,kmh,0,IF \
CDEF:good=kmh,100,GT,0,kmh,IF \
HRULE:100#0000FF:"Maximum allowed" \
AREA:good#00FF00:"Good speed" \
AREA:fast#FF0000:"Too fast"
Note: here we use another means to escape the * operator by enclosing
the whole string in double quotes.
This graph looks much better. Speed is shown in KM/H and there is even
an extra line with the maximum allowed speed (on the road I travel on).
I also changed the colors used to display speed and changed it from a
line into an area.
The calculations are more complex now. For speed measurements within
the speed limit they are:
Check if kmh is greater than 100 ( kmh,100 ) GT
If so, return 0, else kmh ((( kmh,100 ) GT ), 0, kmh) IF
For values above the speed limit:
Check if kmh is greater than 100 ( kmh,100 ) GT
If so, return kmh, else return 0 ((( kmh,100) GT ), kmh, 0) IF
Graphics Magic
I like to believe there are virtually no limits to how RRDtool graph
can manipulate data. I will not explain how it works, but look at the
following PNG:
rrdtool graph speed4.png \
--start 920804400 --end 920808000 \
--vertical-label km/h \
DEF:myspeed=test.rrd:speed:AVERAGE \
"CDEF:kmh=myspeed,3600,*" \
CDEF:fast=kmh,100,GT,100,0,IF \
CDEF:over=kmh,100,GT,kmh,100,-,0,IF \
CDEF:good=kmh,100,GT,0,kmh,IF \
HRULE:100#0000FF:"Maximum allowed" \
AREA:good#00FF00:"Good speed" \
AREA:fast#550000:"Too fast" \
STACK:over#FF0000:"Over speed"
Lets create a quick and dirty HTML page to view the three PNGs:
Speed
Name the file "speed.html" or similar, and look at it in your web
browser.
Now, all you have to do is measure the values regularly and update the
database. When you want to view the data, recreate the PNGs and make
sure to refresh them in your browser. (Note: just clicking reload may
not be enough, especially when proxies are involved. Try shift-reload
or ctrl-F5).
Updates in Reality
Weve already used the "update" command: it took one or more parameters
in the form of "
Name the file "speed.html" or similar, and look at it in your web
browser.
Now, all you have to do is measure the values regularly and update the
database. When you want to view the data, recreate the PNGs and make
sure to refresh them in your browser. (Note: just clicking reload may
not be enough, especially when proxies are involved. Try shift-reload
or ctrl-F5).
Updates in Reality
Weve already used the "update" command: it took one or more parameters
in the form of "