RRDTUTORIAL(1) rrdtool RRDTUTORIAL(1)

NAME

rrdtutorial - Alex van den Bogaerdt's RRDtool tutorial

DESCRIPTION

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

or

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

+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
the lines.

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. 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.

+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.

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.

SEE ALSO

The RRDtool manpages

AUTHOR

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 <alex@vandenbogaerdt.nl>

1.8.0 2022-03-14 RRDTUTORIAL(1)