rrdtool RDTUTORIAL(1)
NAME
rrdtutorial - Alex van den Bogaerdt's RDtool tutorial
DESCRIPTION
RDtool is written by Tobias Oetiker with
contributions from many people all around the world. This
document is written by Alex van den Bogaerdt
to help you understand what RDtool
is and what it can do for you.
The documentation provided with RDtool can be too technical
for some people. This tutorial is here to help you
understand the basics of RDtool. It should prepare you to
read the documentation yourself. It also explains the
general things about statistics with a focus on networking.
TUTORIAL
Important
Please don't skip ahead in this document! The first part of
this document explains the basics and may be boring. But if
you don't understand the basics, the examples will not be as
meaningful to you.
What is RDtool?
RDtool refers to Round Robin Database tool. Round robin is
a technique that works with a fixed amount of data, and a
pointer to the current 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 maintenance.
RDtool works with with Round Robin Databases (RDs). It
stores and retrieves data from them.
What data can be put into an RD?
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 RDtool. If you can do this, RDtool
will be able to store it. The values must be numerical but
don't have to be integers, as is the case with MRTG (the
next section will give more details on this more specialized
application).
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Many examples below talk about SNMP which is an acronym for
Simple Network 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 RD.
What can I do with this tool?
RDtool originated from MRTG (Multi Router Traffic Grapher).
MRTG started as a tiny little script for graphing the use of
a university's connection to the Internet. MRTG was later
(ab-)used as a tool for graphing other data sources
including temperature, speed, voltage, number of printouts
and the like.
Most likely you will start to use RDtool to store and
process data collected via SNMP. The data will most likely
be bytes (or bits) transfered from and to a network or a
computer. But it can also be used to display tidal waves,
solar radiation, power consumption, number of visitors 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 numbers into RDtool. RDtool 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 collected 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
RDtool. There may be pre-compiled versions 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
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rude to just ask a question 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 RDtool 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 subject 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 RDtool to
experiment for yourself. If it doesn't work the first time,
don't 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 don't then start with reading bindechex 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. It's 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 probably 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
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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
4'294'967'295. We will use these values in the examples.
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 calculations:
1. Take the current counter, subtract the previous value
from it.
2. Do the same with the current time and the previous time
(in seconds).
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 = (counternow - counterbefore) / (timenow - timebefore) * 8
For some people it may help to translate this to an
automobile example. Do not try this example, and if you do,
don't 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 12'345 KM
until that moment. At 12:10 you look again, it reads 12'357
KM. This means you have traveled 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 12'000 meters. We did
that in five minutes or 300 seconds. Our speed is 12'000M /
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:
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that's 90 MPH so don't try this example 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 1'000.
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 your's. 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 '\'
characters.
rrdtool create test.rrd \
--start 920804400 \
DS:speed:COUNTER:600:U:U \
RA:AVERAGE:0.5:1:24 \
RA: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
920'804'400 seconds as explained below). Our database holds
one data source (DS) named "speed" that represents a
counter. This counter is read every five minutes (this is
the default therefore you don't have to put "--step=300").
In the same database two round robin archives (RAs) are
kept, one averages the data every time it is read (e.g.,
there's 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).
RDtool 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.
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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. We'll
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
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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 doesn't, 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 that's 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. Don't 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 RDtool.
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#F0000
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 RA "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).
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You'll 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 components, you specify how much to use in
hexadecimal where 00 means not included and F means fully
included. The "color" white is a mixture of red, green and
blue: F The "color" black is all colors off: 000000
red #F0000
green #00F00
blue #0000F
magenta #F00F (mixed red with blue)
gray #555555 (one third of all components)
Additionally you can add an alpha channel (transparency).
The default will be "F" 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 doesn't 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
(12'357-12'345) and divided by 300 this makes 0.04, which is
displayed by RDtool as "40 m" meaning "40/1'000". The "m"
(milli) has nothing to do with meters, kilometers or
millimeters! RDtool doesn't 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 (12'357'000-12'345'000)/300 = 12'000/300 = 40.
As most people have a better feel for numbers in this range,
we'll correct 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!
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rrdtool graph speed2.png \
--start 920804400 --end 920808000 \
--vertical-label m/s \
DEF:myspeed=test.rrd:speed:AVERAGE \
CDEF:realspeed=myspeed,1000,\* \
LINE2:realspeed#F0000
Note: Make sure not to forget the backslash \ in front of
the multiplication 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
RDtool to do is: "take the data source myspeed and the
number 1000; multiply those". Don't 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 Rader's tutorial
on RPN. But first finish this tutorial.
Hang on! If we can multiply values with 1'000, it should
also be possible 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 1'000. Applying that
correction:
value * 3.6 * 1000 == value * 3600
Now let's create this PNG, and add some more magic ...
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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#0000F:"Maximum allowed" \
AREA:good#00F00:"Good speed" \
AREA:fast#F0000:"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
RDtool 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#0000F:"Maximum allowed" \
AREA:good#00F00:"Good speed" \
AREA:fast#550000:"Too fast" \
STACK:over#F0000:"Over speed"
Let's create a quick and dirty HTML page to view the three
PNGs:
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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
We've 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
We've already used the "update" command: it took one or more
parameters in the form of "