RRDTUTORIAL(1) rrdtool RRDTUTORIAL(1)
rrdtutorial - Alex van den Bogaerdt's RRDtool tutorial
RRDtool is written by Tobias Oetiker <firstname.lastname@example.org> with
contributions from many people all around the world. This document is
written by Alex van den Bogaerdt <email@example.com> to help you
understand 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
RRDtool. It should prepare you to read the documentation yourself. It
also explains the general things about statistics with a focus on
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.
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 RRDtool.
Also, sometimes bugs do occur. They may also influence the outcome of
the examples. Example speed4.png was suffering from this (the handling
of unknown data in an if-statement was wrong). Normal data will be just
fine (a bug in rrdtool wouldn't last long) but special cases like NaN,
INF and so on may last a bit longer. Try another version if you can,
or just live with it.
I fixed the speed4.png example (and added a note). There may be other
examples which suffer from the same or a similar bug. Try to fix it
yourself, which is a great exercise. But please do not submit your
result as a fix to the source of this document. Discuss it on the
user's list, or write to me.
What is RRDtool?
RRDtool 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. RRDtool works 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 don't have to be integers, as is the case with MRTG (the
next section will give more details on this more specialized
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 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 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 RRDtool to store and process data
collected via SNMP. The data will most likely be bytes (or bits)
transferred 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 your imagination can come up with.
You only need a sensor to measure the data and be able to feed the
numbers 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 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 RRDtool. 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 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 RRDtool for the location and usage of the
I suggest you take a moment to subscribe to the mailing list right now
by sending an email to <firstname.lastname@example.org> 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 RRDtool 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, 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. 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 transferred. 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 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
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 transferred ***on average*** per second.
This is not very hard to calculate. First in words, then in
1. Take the current counter, subtract the previous value from it.
2. Do the same with the current time and the previous time (in
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, 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:
km: kilometer (= 1000 meters).
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 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 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: 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 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 '\'
rrdtool create test.rrd \
--start 920804400 \
(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 (this is the default
therefore you don't have to put "--step=300"). In the same database
two round robin archives (RRAs) 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).
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. 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
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:
Note that you might get more rows than you expect. The reason for this
is that you ask for a time range that ends on 920809200. The number
that is written behind 920809200: in the list above covers the time
range from 920808900 to 920809200, EXCLUDING 920809200. Hence to be on
the sure side, you receive the entry from 920809200 to 920809500 as
well since it INCLUDES 920809200. You may also see "NaN" instead of
"nan" this is OS dependent. "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.
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 \
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).
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 FF means fully included. The "color" white is a
mixture of red, green and blue: FFFFFF The "color" black is all colors
magenta #FF00FF (mixed red with blue)
gray #555555 (one third of all components)
Additionally you can (with a recent RRDtool) 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
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 likely 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 (also m), kilometers or
millimeters! RRDtool 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
(12357000-12345000)/300 = 12000/300 = 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!
rrdtool graph speed2.png \
--start 920804400 --end 920808000 \
--vertical-label m/s \
Note: I need to escape the multiplication operator * with a backslash.
If I don't, the operating system may interpret it and use it for file
name expansion. You could also place the line within quotation marks
It boils down to: it is RRDtool which should see *, not your shell.
And it is your shell interpreting \, not RRDtool. You may need to
adjust examples accordingly if you happen to use an operating system or
shell which behaves differently.
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".
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 1000, 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 1000. Applying that correction:
value * 3.6 * 1000 == value * 3600
Now let's create this PNG, and add some more magic ...
rrdtool graph speed3.png \
--start 920804400 --end 920808000 \
--vertical-label km/h \
HRULE:100#0000FF:"Maximum allowed" \
AREA:good#00FF00:"Good speed" \
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
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
rrdtool graph speed4.png \
--start 920804400 --end 920808000 \
--vertical-label km/h \
HRULE:100#0000FF:"Maximum allowed" \
AREA:good#00FF00:"Good speed" \
AREA:fast#550000:"Too fast" \
Remember the note in the beginning? I had to remove unknown data from
this example. The 'nonans' CDEF is new, and the 6th line (which used to
be the 5th line) used to read 'CDEF:kmh=myspeed,3600,*'
Let's create a quick and dirty HTML page to view the three PNGs:
<IMG src="speed2.png" alt="Speed in meters per second">
<IMG src="speed3.png" alt="Speed in kilometers per hour">
<IMG src="speed4.png" alt="Traveled too fast?">
Name the file "speed.html" or similar, and look at it in your web
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
Updates in Reality
We've already used the "update" command: it took one or more parameters
in the form of "<time>:<value>". You'll be glad to know that you can
specify the current time by filling in a "N" as the time. Or you could
use the "time" function in Perl (the shortest example in this
perl -e 'print time, "\n" '
How to run a program on regular intervals is OS specific. But here is
an example in pseudo code:
- Get the value and put it in variable "$speed"
- rrdtool update speed.rrd N:$speed
(do not try this with our test database, we'll use it in further
This is all. Run the above script every five minutes. When you need to
know what the graphs look like, run the examples above. You could put
them in a script as well. After running that script, view the page
index.html we created above.
Some words on SNMP
I can imagine very few people that will be able to get real data from
their car every five minutes. All other people will have to settle for
some other kind of counter. You could measure the number of pages
printed by a printer, for example, the cups of coffee made by the
coffee machine, a device that counts the electricity used, whatever.
Any incrementing counter can be monitored and graphed using the stuff
you learned so far. Later on we will also be able to monitor other
types of values like temperature.
Many people interested in RRDtool will use the counter that keeps track
of octets (bytes) transferred by a network device. So let's do just
that next. We will start with a description of how to collect data.
Some people will make a remark that there are tools which can do this
data collection for you. They are right! However, I feel it is
important that you understand they are not necessary. When you have to
determine why things went wrong you need to know how they work.
One tool used in the example has been talked about very briefly in the
beginning of this document, it is called SNMP. It is a way of talking
to networked equipment. The tool I use below is called "snmpget" and
this is how it works:
snmpget device password OID
snmpget -v[version] -c[password] device OID
For device you substitute the name, or the IP address, of your device.
For password you use the "community read string" as it is called in the
SNMP world. For some devices the default of "public" might work,
however this can be disabled, altered or protected for privacy and
security reasons. Read the documentation that comes with your device
Then there is this parameter, called OID, which means "object
When you start to learn about SNMP it looks very confusing. It isn't
all that difficult when you look at the Management Information Base
("MIB"). It is an upside-down tree that describes data, with a single
node as the root and from there a number of branches. These branches
end up in another node, they branch out, etc. All the branches have a
name and they form the path that we follow all the way down. The
branches that we follow are named: iso, org, dod, internet, mgmt and
mib-2. These names can also be written down as numbers and are 1 3 6 1
There is a lot of confusion about the leading dot that some programs
use. There is *no* leading dot in an OID. However, some programs can
use the above part of OIDs as a default. To indicate the difference
between abbreviated OIDs and full OIDs they need a leading dot when you
specify the complete OID. Often those programs will leave out the
default portion when returning the data to you. To make things worse,
they have several default prefixes ...
Ok, lets continue to the start of our OID: we had 22.214.171.124.2.1 From
there, we are especially interested in the branch "interfaces" which
has number 2 (e.g., 126.96.36.199.2.1.2 or 188.8.131.52.2.1.interfaces).
First, we have to get some SNMP program. First look if there is a pre-
compiled package available for your OS. This is the preferred way. If
not, you will have to get the sources yourself and compile those. The
Internet is full of sources, programs etc. Find information using a
search engine or whatever you prefer.
Assume you got the program. First try to collect some data that is
available on most systems. Remember: there is a short name for the part
of the tree that interests us most in the world we live in!
I will give an example which can be used on Fedora Core 3. If it
doesn't work for you, work your way through the manual of snmp and
adapt the example to make it work.
snmpget -v2c -c public myrouter system.sysDescr.0
The device should answer with a description of itself, perhaps an empty
one. Until you got a valid answer from a device, perhaps using a
different "password", or a different device, there is no point in
snmpget -v2c -c public myrouter interfaces.ifNumber.0
Hopefully you get a number as a result, the number of interfaces. If
so, you can carry on and try a different program called "snmpwalk".
snmpwalk -v2c -c public myrouter interfaces.ifTable.ifEntry.ifDescr
If it returns with a list of interfaces, you're almost there. Here's
[user@host /home/alex]$ snmpwalk -v2c -c public cisco 184.108.40.206
interfaces.ifTable.ifEntry.ifDescr.1 = "BRI0: B-Channel 1"
interfaces.ifTable.ifEntry.ifDescr.2 = "BRI0: B-Channel 2"
interfaces.ifTable.ifEntry.ifDescr.3 = "BRI0" Hex: 42 52 49 30
interfaces.ifTable.ifEntry.ifDescr.4 = "Ethernet0"
interfaces.ifTable.ifEntry.ifDescr.5 = "Loopback0"
On this cisco equipment, I would like to monitor the "Ethernet0"
interface and from the above output I see that it is number four. I
[user@host /home/alex]$ snmpget -v2c -c public cisco 220.127.116.11.4 18.104.22.168.4
interfaces.ifTable.ifEntry.ifInOctets.4 = 2290729126
interfaces.ifTable.ifEntry.ifOutOctets.4 = 1256486519
So now I have two OIDs to monitor and they are (in full, this time):
both with an interface number of 4.
Don't get fooled, this wasn't my first try. It took some time for me
too to understand what all these numbers mean. It does help a lot when
they get translated into descriptive text... At least, when people are
talking about MIBs and OIDs you know what it's all about. Do not
forget the interface number (0 if it is not interface dependent) and
try snmpwalk if you don't get an answer from snmpget.
If you understand the above section and get numbers from your device,
continue on with this tutorial. If not, then go back and re-read this
A Real World Example
Let the fun begin. First, create a new database. It contains data from
two counters, called input and output. The data is put into archives
that average it. They take 1, 6, 24 or 288 samples at a time. They
also go into archives that keep the maximum numbers. This will be
explained later on. The time in-between samples is 300 seconds, a good
starting point, which is the same as five minutes.
1 sample "averaged" stays 1 period of 5 minutes
6 samples averaged become one average on 30 minutes
24 samples averaged become one average on 2 hours
288 samples averaged become one average on 1 day
Lets try to be compatible with MRTG which stores about the following
amount of data:
600 5-minute samples: 2 days and 2 hours
600 30-minute samples: 12.5 days
600 2-hour samples: 50 days
732 1-day samples: 732 days
These ranges are appended, so the total amount of data stored in the
database is approximately 797 days. RRDtool stores the data
differently, it doesn't start the "weekly" archive where the "daily"
archive stopped. For both archives the most recent data will be near
"now" and therefore we will need to keep more data than MRTG does!
We will need:
600 samples of 5 minutes (2 days and 2 hours)
700 samples of 30 minutes (2 days and 2 hours, plus 12.5 days)
775 samples of 2 hours (above + 50 days)
797 samples of 1 day (above + 732 days, rounded up to 797)
rrdtool create myrouter.rrd \
Next thing to do is to collect data and store it. Here is an example.
It is written partially in pseudo code, you will have to find out what
to do exactly on your OS to make it work.
while not the end of the universe
get result of
snmpget router community 22.214.171.124.4
into variable $in
get result of
snmpget router community 126.96.36.199.4
into variable $out
rrdtool update myrouter.rrd N:$in:$out
wait for 5 minutes
Then, after collecting data for a day, try to create an image using:
rrdtool graph myrouter-day.png --start -86400 \
AREA:inoctets#00FF00:"In traffic" \
This should produce a picture with one day worth of traffic. One day
is 24 hours of 60 minutes of 60 seconds: 24*60*60=86400, we start at
now minus 86400 seconds. We define (with DEFs) inoctets and outoctets
as the average values from the database myrouter.rrd and draw an area
for the "in" traffic and a line for the "out" traffic.
View the image and keep logging data for a few more days. If you like,
you could try the examples from the test database and see if you can
get various options and calculations to work.
Suggestion: Display in bytes per second and in bits per second. Make
the Ethernet graphics go red if they are over four megabits per second.
A few paragraphs back I mentioned the possibility of keeping the
maximum values instead of the average values. Let's go into this a bit
Recall all the stuff about the speed of the car. Suppose we drove at
144 km/h during 5 minutes and then were stopped by the police for 25
minutes. At the end of the lecture we would take our laptop and create
and view the image taken from the database. If we look at the second
RRA we did create, we would have the average from 6 samples. The
samples measured would be 144+0+0+0+0+0=144, divided by 30 minutes,
corrected for the error by 1000, translated into km/h, with a result of
24 km/h. I would still get a ticket but not for speeding anymore :)
Obviously, in this case we shouldn't look at the averages. In some
cases they are handy. If you want to know how many km you had traveled,
the averaged picture would be the right one to look at. On the other
hand, for the speed that we traveled at, the maximum numbers seen is
much more interesting. Later we will see more types.
It is the same for data. If you want to know the amount, look at the
averages. If you want to know the rate, look at the maximum. Over
time, they will grow apart more and more. In the last database we have
created, there are two archives that keep data per day. The archive
that keeps averages will show low numbers, the archive that shows
maxima will have higher numbers.
For my car this would translate in averages per day of 96/24=4 km/h (as
I travel about 94 kilometers on a day) during working days, and maxima
of 120 km/h (my top speed that I reach every day).
Big difference. Do not look at the second graph to estimate the
distances that I travel and do not look at the first graph to estimate
my speed. This will work if the samples are close together, as they are
in five minutes, but not if you average.
On some days, I go for a long ride. If I go across Europe and travel
for 12 hours, the first graph will rise to about 60 km/h. The second
one will show 180 km/h. This means that I traveled a distance of 60
km/h times 24 h = 1440 km. I did this with a higher speed and a maximum
around 180 km/h. However, it probably doesn't mean that I traveled for
8 hours at a constant speed of 180 km/h!
This is a real example: go with the flow through Germany (fast!) and
stop a few times for gas and coffee. Drive slowly through Austria and
the Netherlands. Be careful in the mountains and villages. If you would
look at the graphs created from the five-minute averages you would get
a totally different picture. You would see the same values on the
average and maximum graphs (provided I measured every 300 seconds).
You would be able to see when I stopped, when I was in top gear, when I
drove over fast highways etc. The granularity of the data is much
higher, so you can see more. However, this takes 12 samples per hour,
or 288 values per day, so it would be a lot of data over a longer
period of time. Therefore we average it, eventually to one value per
day. From this one value, we cannot see much detail, of course.
Make sure you understand the last few paragraphs. There is no value in
only a line and a few axis, you need to know what they mean and
interpret the data in an appropriate way. This is true for all data.
The biggest mistake you can make is to use the collected data for
something that it is not suitable for. You would be better off if you
didn't have the graph at all.
Let's review what you now should know
You know how to create a database and can put data in it. You can get
the numbers out again by creating an image, do math on the data from
the database and view the result instead of the raw data. You know
about the difference between averages and maximum, and when to use
which (or at least you should have an idea).
RRDtool can do more than what we have learned up to now. Before you
continue with the rest of this doc, I recommend that you reread from
the start and try some modifications on the examples. Make sure you
fully understand everything. It will be worth the effort and helps you
not only with the rest of this tutorial, but also in your day to day
monitoring long after you read this introduction.
Data Source Types
All right, you feel like continuing. Welcome back and get ready for an
increased speed in the examples and explanations.
You know that in order to view a counter over time, you have to take
two numbers and divide the difference of them between the time lapsed.
This makes sense for the examples I gave you but there are other
possibilities. For instance, I'm able to retrieve the temperature from
my router in three places namely the inlet, the so called hot-spot and
the exhaust. These values are not counters. If I take the difference
of the two samples and divide that by 300 seconds I would be asking for
the temperature change per second. Hopefully this is zero! If not, the
computer room is probably on fire :)
So, what can we do? We can tell RRDtool to store the values we measure
directly as they are (this is not entirely true but close enough). The
graphs we make will look much better, they will show a rather constant
value. I know when the router is busy (it works -> it uses more
electricity -> it generates more heat -> the temperature rises). I know
when the doors are left open (the room is air conditioned) -> the warm
air from the rest of the building flows into the computer room -> the
inlet temperature rises). Etc. The data type we use when creating the
database before was counter, we now have a different data type and thus
a different name for it. It is called GAUGE. There are more such data
- COUNTER we already know this one
- GAUGE we just learned this one
The two additional types are DERIVE and ABSOLUTE. Absolute can be used
like counter with one difference: RRDtool assumes the counter is reset
when it's read. That is: its delta is known without calculation by
RRDtool whereas RRDtool needs to calculate it for the counter type.
Example: our first example (12345, 12357, 12363, 12363) would read:
unknown, 12, 6, 0. The rest of the calculations stay the same. The
other one, derive, is like counter. Unlike counter, it can also
decrease so it can have a negative delta. Again, the rest of the
calculations stay the same.
Let's try them all:
rrdtool create all.rrd --start 978300900 \
rrdtool update all.rrd \
rrdtool graph all1.png -s 978300600 -e 978304200 -h 400 \
DEF:linea=all.rrd:a:AVERAGE LINE3:linea#FF0000:"Line A" \
DEF:lineb=all.rrd:b:AVERAGE LINE3:lineb#00FF00:"Line B" \
DEF:linec=all.rrd:c:AVERAGE LINE3:linec#0000FF:"Line C" \
DEF:lined=all.rrd:d:AVERAGE LINE3:lined#000000:"Line D"
RRDtool under the Microscope
o Line A is a COUNTER type, so it should continuously increment and
RRDtool must calculate the differences. Also, RRDtool needs to divide
the difference by the amount of time lapsed. This should end up as a
straight line at 1 (the deltas are 300, the time is 300).
o Line B is of type GAUGE. These are "real" values so they should match
what we put in: a sort of a wave.
o Line C is of type DERIVE. It should be a counter that can decrease.
It does so between 2400 and 0, with 1800 in-between.
o Line D is of type ABSOLUTE. This is like counter but it works on
values without calculating the difference. The numbers are the same
and as you can see (hopefully) this has a different result.
This translates in the following values, starting at 23:10 and ending
at 00:10 the next day (where "u" means unknown/unplotted):
- Line A: u u 1 1 1 1 1 1 1 1 1 u
- Line B: u 1 3 5 3 1 2 4 6 4 2 u
- Line C: u u 2 2 2 0 -2 -6 2 0 2 u
- Line D: u 1 2 3 4 5 6 7 8 9 10 u
If your PNG shows all this, you know you have entered the data
correctly, the RRDtool executable is working properly, your viewer
doesn't fool you, and you successfully entered the year 2000 :)
You could try the same example four times, each time with only one of
Let's go over the data again:
o Line A: 300,600,900 and so on. The counter delta is a constant 300
and so is the time delta. A number divided by itself is always 1
(except when dividing by zero which is undefined/illegal).
Why is it that the first point is unknown? We do know what we put
into the database, right? True, But we didn't have a value to
calculate the delta from, so we don't know where we started. It would
be wrong to assume we started at zero so we don't!
o Line B: There is nothing to calculate. The numbers are as they are.
o Line C: Again, the start-out value is unknown. The same story is
holds as for line A. In this case the deltas are not constant,
therefore the line is not either. If we would put the same numbers in
the database as we did for line A, we would have gotten the same
line. Unlike type counter, this type can decrease and I hope to show
you later on why this makes a difference.
o Line D: Here the device calculates the deltas. Therefore we DO know
the first delta and it is plotted. We had the same input as with line
A, but the meaning of this input is different and thus the line is
different. In this case the deltas increase each time with 300. The
time delta stays at a constant 300 and therefore the division of the
two gives increasing values.
There are a few more basics to show. Some important options are still
to be covered and we haven't look at counter wraps yet. First the
counter wrap: In our car we notice that the counter shows 999987. We
travel 20 km and the counter should go to 1000007. Unfortunately, there
are only six digits on our counter so it really shows 000007. If we
would plot that on a type DERIVE, it would mean that the counter was
set back 999980 km. It wasn't, and there has to be some protection for
this. This protection is only available for type COUNTER which should
be used for this kind of counter anyways. How does it work? Type
counter should never decrease and therefore RRDtool must assume it
wrapped if it does decrease! If the delta is negative, this can be
compensated for by adding the maximum value of the counter + 1. For our
car this would be:
Delta = 7 - 999987 = -999980 (instead of 1000007-999987=20)
Real delta = -999980 + 999999 + 1 = 20
At the time of writing this document, RRDtool knows of counters that
are either 32 bits or 64 bits of size. These counters can handle the
following different values:
- 32 bits: 0 .. 4294967295
- 64 bits: 0 .. 18446744073709551615
If these numbers look strange to you, you can view them in their
- 32 bits: 0 .. FFFFFFFF
- 64 bits: 0 .. FFFFFFFFFFFFFFFF
RRDtool handles both counters the same. If an overflow occurs and the
delta would be negative, RRDtool first adds the maximum of a small
counter + 1 to the delta. If the delta is still negative, it had to be
the large counter that wrapped. Add the maximum possible value of the
large counter + 1 and subtract the erroneously added small value.
There is a risk in this: suppose the large counter wrapped while adding
a huge delta, it could happen, theoretically, that adding the smaller
value would make the delta positive. In this unlikely case the results
would not be correct. The increase should be nearly as high as the
maximum counter value for that to happen, so chances are you would have
several other problems as well and this particular problem would not
even be worth thinking about. Even though, I did include an example, so
you can judge for yourself.
The next section gives you some numerical examples for counter-wraps.
Try to do the calculations yourself or just believe me if your
calculator can't handle the numbers :)
- 32 bits: (4294967295 + 1) = 4294967296
- 64 bits: (18446744073709551615 + 1)
- correction1 = 18446744069414584320
Should become: 4294967300
But really is: 4
Correction1: -4294967196 + 4294967296 = 100
Should become: 18446744073709551800
But really is: 184
+ 4294967296 = -18446744069414583520
+ 18446744069414584320 = 800
Before: 18446744073709551615 ( maximum value )
Increase: 18446744069414584320 ( absurd increase, minimum for
Should become: 36893488143124135935 this example to work )
But really is: 18446744069414584319
Correction1: -4294967296 + 4294967296 = 0
(not negative -> no correction2)
Before: 18446744073709551615 ( maximum value )
Increase: 18446744069414584319 ( one less increase )
Should become: 36893488143124135934
But really is: 18446744069414584318
Correction1: -4294967297 + 4294967296 = -1
Correction2: -1 + 18446744069414584320 = 18446744069414584319
As you can see from the last two examples, you need strange numbers for
RRDtool to fail (provided it's bug free of course), so this should not
happen. However, SNMP or whatever method you choose to collect the
data, might also report wrong numbers occasionally. We can't prevent
all errors, but there are some things we can do. The RRDtool "create"
command takes two special parameters for this. They define the minimum
and maximum allowed values. Until now, we used "U", meaning "unknown".
If you provide values for one or both of them and if RRDtool receives
data points that are outside these limits, it will ignore those values.
For a thermometer in degrees Celsius, the absolute minimum is just
under -273. For my router, I can assume this minimum is much higher so
I would set it to 10, where as the maximum temperature I would set to
80. Any higher and the device would be out of order.
For the speed of my car, I would never expect negative numbers and also
I would not expect a speed higher than 230. Anything else, and there
must have been an error. Remember: the opposite is not true, if the
numbers pass this check, it doesn't mean that they are correct. Always
judge the graph with a healthy dose of suspicion if it seems weird to
One important feature of RRDtool has not been explained yet: it is
virtually impossible to collect data and feed it into RRDtool on exact
intervals. RRDtool therefore interpolates the data, so they are stored
on exact intervals. If you do not know what this means or how it works,
then here's the help you seek:
Suppose a counter increases by exactly one for every second. You want
to measure it in 300 seconds intervals. You should retrieve values that
are exactly 300 apart. However, due to various circumstances you are a
few seconds late and the interval is 303. The delta will also be 303 in
that case. Obviously, RRDtool should not put 303 in the database and
make you believe that the counter increased by 303 in 300 seconds.
This is where RRDtool interpolates: it alters the 303 value as if it
would have been stored earlier and it will be 300 in 300 seconds. Next
time you are at exactly the right time. This means that the current
interval is 297 seconds and also the counter increased by 297. Again,
RRDtool interpolates and stores 300 as it should be.
in the RRD in reality
time+000: 0 delta="U" time+000: 0 delta="U"
time+300: 300 delta=300 time+300: 300 delta=300
time+600: 600 delta=300 time+603: 603 delta=303
time+900: 900 delta=300 time+900: 900 delta=297
Let's create two identical databases. I've chosen the time range
920805000 to 920805900 as this goes very well with the example numbers.
rrdtool create seconds1.rrd \
--start 920804700 \
Make a copy
for Unix: cp seconds1.rrd seconds2.rrd
for Dos: copy seconds1.rrd seconds2.rrd
for vms: how would I know :)
Put in some data
rrdtool update seconds1.rrd \
920805000:000 920805300:300 920805600:600 920805900:900
rrdtool update seconds2.rrd \
920805000:000 920805300:300 920805603:603 920805900:900
rrdtool graph seconds1.png \
--start 920804700 --end 920806200 \
--height 200 \
--upper-limit 1.05 --lower-limit 0.95 --rigid \
rrdtool graph seconds2.png \
--start 920804700 --end 920806200 \
--height 200 \
--upper-limit 1.05 --lower-limit 0.95 --rigid \
View both images together (add them to your index.html file) and
compare. Both graphs should show the same, despite the input being
It's time now to wrap up this tutorial. We covered all the basics for
you to be able to work with RRDtool and to read the additional
documentation available. There is plenty more to discover about RRDtool
and you will find more and more uses for this package. You can easily
create graphs using just the examples provided and using only RRDtool.
You can also use one of the front ends to RRDtool that are available.
Remember to subscribe to the RRDtool mailing list. Even if you are not
answering to mails that come by, it helps both you and the rest of the
users. A lot of the stuff that I know about MRTG (and therefore about
RRDtool) I've learned while just reading the list without posting to
it. I did not need to ask the basic questions as they are answered in
the FAQ (read it!) and in various mails by other users. With thousands
of users all over the world, there will always be people who ask
questions that you can answer because you read this and other
documentation and they didn't.
The RRDtool manpages
I hope you enjoyed the examples and their descriptions. If you do, help
other people by pointing them to this document when they are asking
basic questions. They will not only get their answers, but at the same
time learn a whole lot more.
Alex van den Bogaerdt <email@example.com>
1.7.0 2018-06-30 RRDTUTORIAL(1)