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RRDTUTORIAL(1) rrdtool RRDTUTORIAL(1)

NAME
rrdtutorial - Alex van den Bogaerdt's RRDtool tutorial

DESCRIPTION
RRDtool is written by Tobias Oetiker <tobi AT oetiker.ch> with contributions from many people
all around the world. This document is written by Alex van den Bogaerdt
<alex AT vandenbogaerdt.nl> 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
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.

Sometimes things change. This example used to provide numbers like "0.04" instead 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 application).

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 list.

I suggest you take a moment to subscribe to the mailing list right now by sending an email to
<rrd-users-request AT lists.ch> 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 let's 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 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 = (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:

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 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 '\' characters.

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 (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 (i.e., 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

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
920809500: nan

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 \
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).

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
off: 000000

red #FF0000
green #00FF00
blue #0000FF
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 "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 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 \
DEF:myspeed=test.rrd:speed:AVERAGE \
CDEF:realspeed=myspeed,1000,\* \
LINE2:realspeed#FF0000

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 like so:

"CDEF:realspeed=myspeed,1000,*" \

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 is 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 \
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:nonans=myspeed,UN,0,myspeed,IF \
CDEF:kmh=nonans,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"

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:

<HTML><HEAD><TITLE>Speed</TITLE></HEAD><BODY>
<IMG src="speed2.png" alt="Speed in meters per second">
<BR>
<IMG src="speed3.png" alt="Speed in kilometers per hour">
<BR>
<IMG src="speed4.png" alt="Traveled too fast?">
</BODY></HTML>

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
"<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 tutorial):

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 examples)

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 speed.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 or program.

Then there is this parameter, called OID, which means "object identifier".

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 2 1.

iso.org.dod.internet.mgmt.mib-2 (1.3.6.1.2.1)

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 1.3.6.1.2.1 From there, we are especially
interested in the branch "interfaces" which has number 2 (e.g., 1.3.6.1.2.1.2 or
1.3.6.1.2.1.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 continuing.

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 an example:
[user@host /home/alex]$ snmpwalk -v2c -c public cisco 2.2.1.2

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 try:

[user@host /home/alex]$ snmpget -v2c -c public cisco 2.2.1.10.4 2.2.1.16.4

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):

1.3.6.1.2.1.2.2.1.10

and

1.3.6.1.2.1.2.2.1.16

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 part.

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 \
DS:input:COUNTER:600:U:U \
DS:output:COUNTER:600:U:U \
RRA:AVERAGE:0.5:1:600 \
RRA:AVERAGE:0.5:6:700 \
RRA:AVERAGE:0.5:24:775 \
RRA:AVERAGE:0.5:288:797 \
RRA:MAX:0.5:1:600 \
RRA:MAX:0.5:6:700 \
RRA:MAX:0.5:24:775 \
RRA:MAX:0.5:288:797

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
do
get result of
snmpget router community 2.2.1.10.4
into variable $in
get result of
snmpget router community 2.2.1.16.4
into variable $out

rrdtool update myrouter.rrd N:$in:$out

wait for 5 minutes
done

Then, after collecting data for a day, try to create an image using:

rrdtool graph myrouter-day.png --start -86400 \
DEF:inoctets=myrouter.rrd:input:AVERAGE \
DEF:outoctets=myrouter.rrd:output:AVERAGE \
AREA:inoctets#00FF00:"In traffic" \
LINE1:outoctets#0000FF:"Out 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.

Consolidation Functions
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 more.

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 by 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 types:

- COUNTER we already know this one
- GAUGE we just learned this one
- DERIVE
- ABSOLUTE

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 \
DS:a:COUNTER:600:U:U \
DS:b:GAUGE:600:U:U \
DS:c:DERIVE:600:U:U \
DS:d:ABSOLUTE:600:U:U \
RRA:AVERAGE:0.5:1:10
rrdtool update all.rrd \
978301200:300:1:600:300 \
978301500:600:3:1200:600 \
978301800:900:5:1800:900 \
978302100:1200:3:2400:1200 \
978302400:1500:1:2400:1500 \
978302700:1800:2:1800:1800 \
978303000:2100:4:0:2100 \
978303300:2400:6:600:2400 \
978303600:2700:4:600:2700 \
978303900:3000:2:1200:3000
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
• 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).

• Line B is of type GAUGE. These are "real" values so they should match what we put in: a
sort of a wave.

• 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.

• 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 the lines.

Let's go over the data again:

• 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!

• Line B: There is nothing to calculate. The numbers are as they are.

• Line C: Again, the start-out value is unknown. This is the same story 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.

• 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.

Counter Wraps
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 hexadecimal form:

- 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 :)

Correction numbers:

- 32 bits: (4294967295 + 1) = 4294967296
- 64 bits: (18446744073709551615 + 1)
- correction1 = 18446744069414584320

Before: 4294967200
Increase: 100
Should become: 4294967300
But really is: 4
Delta: -4294967196
Correction1: -4294967196 + 4294967296 = 100

Before: 18446744073709551000
Increase: 800
Should become: 18446744073709551800
But really is: 184
Delta: -18446744073709550816
Correction1: -18446744073709550816
+ 4294967296 = -18446744069414583520
Correction2: -18446744069414583520
+ 18446744069414584320 = 800

Before: 18446744073709551615 ( maximum value )
Increase: 18446744069414584320 ( absurd increase, minimum for
Should become: 36893488143124135935 this example to work )
But really is: 18446744069414584319
Delta: -4294967296
Correction1: -4294967296 + 4294967296 = 0
(not negative -> no correction2)

Before: 18446744073709551615 ( maximum value )
Increase: 18446744069414584319 ( one less increase )
Should become: 36893488143124135934
But really is: 18446744069414584318
Delta: -4294967297
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 you.

Data Resampling
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 \
DS:seconds:COUNTER:600:U:U \
RRA:AVERAGE:0.5:1:24

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

Create output

rrdtool graph seconds1.png \
--start 920804700 --end 920806200 \
--height 200 \
--upper-limit 1.05 --lower-limit 0.95 --rigid \
DEF:seconds=seconds1.rrd:seconds:AVERAGE \
CDEF:unknown=seconds,UN \
LINE2:seconds#0000FF \
AREA:unknown#FF0000
rrdtool graph seconds2.png \
--start 920804700 --end 920806200 \
--height 200 \
--upper-limit 1.05 --lower-limit 0.95 --rigid \
DEF:seconds=seconds2.rrd:seconds:AVERAGE \
CDEF:unknown=seconds,UN \
LINE2:seconds#0000FF \
AREA:unknown#FF0000

View both images together (add them to your index.html file) and compare. Both graphs should
show the same, despite the input being different.

WRAPUP
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.

MAILINGLIST
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.