=head1 NAME

rrdtutorial - Alex van den Bogaerdt's RRDtool tutorial

=for html <div align="right">Go <a href="rrdtutorial.es.html">Spanish</a></div> 

=for html <div align="right"><a href="rrdtutorial.pdf">PDF</a> version.</div> 

=head1 DESCRIPTION

RRDtool is written by Tobias Oetiker <oetiker@ee.ethz.ch> with
contributions from many people all around the world. This document is
written by Alex van den Bogaerdt <alex@ergens.op.het.net> 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.

=head1 TUTORIAL

=head2 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.

=head2 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 no beginning nor an end, you can
go on and on. After a while, all the available places will be used and
the process automatically reuses old locations. This way, the database
will not grow in size and therefore requires no mainenance.
RRDtool works with with Round Robin Databases (RRDs). It stores and retrieves
data from them.

=head2 What data can be put into an RDD ?

You name it, it will probably fit. You should 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 need to be
numerical but don't have to be, as opposed to MRTG, integers.

Many examples 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 is a way to
ask devices for the values of counters they keep.
It is the value from those counters that are kept in the RRD.

=head2 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 connection
to the Internet. MRTG evolved into a tool for graphing other data sources
including temperature, speed, voltage, number of printouts and
the like. Most likely you will start to use the RRDtool to store
and process data collected via SNMP. The data will most likely be bytes
(or bits) transfered from and to a network or a computer.
RRDtool lets you create a database, store data in it, retrieve that data
and create graphs in GIF format for display on a web browser. Those GIF
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.
It can also be used to display tidal waves, solar radiation, power
consumption, number of visitors at an exhibition, noise levels near an
airport, temperature on your favorite holiday location, temperature in the
fridge and whatever you imagination can come up with. You need a sensor to
measure the data and be able to feed the numbers to RRDtool.

=head2 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 precompiled
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 list! 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 E<lt>rrd-users-request@list.ee.ethz.chE<gt> with a
subject of "subscribe". If you ever want to leave this list, you write
an email to the same address but now with a subject of "unsubscribe".

=head2 How will you help me ?

By giving you some detailed descriptions with detailed examples.
It is assumed 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
then start with reading L<bin_dec_hex> before you continue here.

=head2 Your first Round Robin Database

In my opinion the best way to learn something is to actually do it.
Why not start right now?  We will create a database, put some values
in it and extract this data again.  Your output should be the same
as the output that is included in this document.

We will start with some easy stuff and compare a car with a router,
or compare kilometers (miles if you wish) with bits and bytes. It's
all the same: some number over some time.

Assume we have a device that transfers bytes to and from the Internet.
This device keeps a counter that starts at zero when it is turned on,
increasing with every byte that is transfered. This counter will have
a maximum value, if that value is reached and an extra byte is counted,
the counter starts all over at zero. This is the same as many counters
in the world such as the mileage counter in a car.
Most discussions about networking talk about bits per second so lets
get used to that right away. Assume a byte is eight bits and start to
think in bits not bytes. The counter, however, still counts bytes !
In the SNMP world most of the counters are 32 bits. That means they are
counting from 0 to 4294967295. We will use these values in the examples.
The device, when asked, returns the current value of the counter. We
know the time that has passes since we last asked so we now know how
many bytes have been transfered ***on average*** per second. This is
not very hard to calculate. First in words, then in calculations:

=over 3

=item 1.

Take the current counter, subtract the previous value from it.

=item 2.

Do the same with the current time and the previous time.

=item 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).

=back

  bps = (counter_now - counter_before) / (time_now - time_before) * 8

For some people it may help to translate this to a 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're 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 towards 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 which translates into 300 seconds. Our speed is 12000M / 300S
equals 40 M/S.

We could also calculate the speed in KM/H: 12 times five 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
your OS. 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: C<rrdtool create test.rrd --start 920804400 DS ...>)

=head2 What has been created ?

We created the round robin database called test (test.rrd)
which starts at noon the day I started (7th of march, 1999) writing
this document. It holds one data source (DS) named "speed" that gets
built from a counter. This counter is read every five minutes (default)
In the same database two round robin archives (RRAs) are kept, one
averages the data every time it is read (eg 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 of such averages (eg 5 hours)
The remaining options will be discussed later on.

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.  This time stamp is translated into local time and
it will therefore look different for the 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 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:      0.04
 920805300:      0.02
 920805600:      0.00
 920805900:      0.00
 920806200:      0.03
 920806500:      0.03
 920806800:      0.03
 920807100:      0.02
 920807400:      0.02
 920807700:      0.02
 920808000:      0.01
 920808300:      0.02
 920808600:      0.01
 920808900:      0.00
 920809200:       NaN

If it doesn't, something may be wrong.  Perhaps your OS will print
"NaN" in a different form.  It represents "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.

What this output represents will become clear in the rest of the tutorial.

=head2 It is time to create some graphics

Try the following command:

 rrdtool graph speed.gif                                 \
         --start 920804400 --end 920808000               \
         DEF:myspeed=test.rrd:speed:AVERAGE              \
         LINE2:myspeed#FF0000

This will create speed.gif which starts at 12:00 and ends at 13:00.
There is a definition of variable myspeed, it is the data from RRA
"speed" out of database "test.rrd". The line drawn is 2 pixels high,
and comes from variable myspeed. The color is red.
You'll notice that the start of the graph is not at 12:00 but at 12:05
and 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)

The GIF you just created can be displayed using your favorite image
viewer.  Web browsers will display the GIF via the URL
"file://the/path/to/speed.gif"

=head2 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. The two remaining times (12:00 and
13:00) would not be displayed nicely 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" has
nothing to do with meters, kilometers or millimeters! RRDtool doesn't
know about all this, it just works with numbers and not with meters...

What we did wrong was that we should have measured in meters, this would
have been (12357000-12345000)/300 = 12000/300 = 40.

Let's correct that. We could recreate our database and store the correct
data but there is a better way: do some calculations while creating the
gif file !

   rrdtool graph speed2.gif                           \
      --start 920804400 --end 920808000               \
      --vertical-label m/s                            \
      DEF:myspeed=test.rrd:speed:AVERAGE              \
      CDEF:realspeed=myspeed,1000,*                   \
      LINE2:realspeed#FF0000

After viewing this GIF, you notice the "m" has disappeared. This it what
the correct result would be. Also, a label has been added to the image.
Apart from the things mentioned above, the GIF should be the same.

The calculations are in the CDEF part and are in Reverse Polish Notation
("RPN"). What it says 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) == 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 GIF, and add some more magic ...

   rrdtool graph speed3.gif                           \
      --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"

This looks much better. Speed in KM/H and even an extra line with the
maximum allowed speed (on the road I travel at). 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 the "good" speed 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 the other speed:

   Check if kmh is greater than 100    ( kmh,100 ) GT
   If so, return kmh, else return 0    ((( kmh,100) GT ), kmh, 0) IF

=head2 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 GIF:

   rrdtool graph speed4.gif                           \
      --start 920804400 --end 920808000               \
      --vertical-label km/h                           \
      DEF:myspeed=test.rrd:speed:AVERAGE              \
      "CDEF:kmh=myspeed,3600,*"                       \
      CDEF:fast=kmh,100,GT,100,0,IF                   \
      CDEF:over=kmh,100,GT,kmh,100,-,0,IF             \
      CDEF:good=kmh,100,GT,0,kmh,IF                   \
      HRULE:100#0000FF:"Maximum allowed"              \
      AREA:good#00FF00:"Good speed"                   \
      AREA:fast#550000:"Too fast"                     \
      STACK:over#FF0000:"Over speed"

Let's create a quick and dirty HTML page to view three GIFs:

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

Name the file "speed.html" or similar, and view it.

Now, all you have to do is measure the values regularly and update the
database.  When you want to view the data, recreate the GIFs and make
sure to refresh them in your browser. (Note: just clicking reload may
not be enough; Netscape in particular has a problem doing so and you'll
need to click reload while pressing the shift key).

=head2 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
get the current time by filling in a "N" as the time.
If you wish, you can also use the "time" function in perl.
The shortest example in this doc :)

   perl -e 'print time, "\n" '

How you can run a program on regular intervals is OS specific. But here's
an example in pseudo code:

   Get the value, put it in variable "$speed"
   rrdtool update speed.rrd N:$speed

(Do not try this with our test database, it is used in further examples)

This is all. Run this script every five minutes. When you need to know
what the graphics look like, run the examples above. You could put them
in a script. After running that script, view index.html

=head2 Some words on SNMP

I can imagine very few people 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, the 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 until now. Later on we will also
be able to monitor other types of values like temperature.
Most people will use the counter that keeps track
of octets (bytes) transfered by a network device so we have to do just
that. We will start with a description of how to collect data.
Some people will make a remark that there are tools who 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
equipment. The tool I use below is called "snmpget" and this is how it
works:

   snmpget device password 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 third 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 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 ...

Right, 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 (eg 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 yourself the sources and compile those.
The Internet is full of sources, programs etc. Find information using
a search engine or whatever you prefer. As a suggestion: look for
CMU-SNMP.  It is commonly used.

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 use the short version as I think this document is large enough
as it is. If that doesn't work for you, prefix with .1.3.6.1.2.1 and
try again.  Also, Read The Fine Manual.  Skip the parts you cannot
understand yet, you should be able to find out how to start the
program and use it.

   snmpget myrouter public system.sysDescr.0

The device should answer with a description of itself, perhaps empty.
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 myrouter public 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 myrouter public 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 cisco public 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 see that it is number four. I try:

   [user@host /home/alex]$ snmpget cisco public 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 above part, and get numbers from your device, continue
on with this tutorial. If not, then go back and re-read this part.

=head2 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:
MRTG 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 kept 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 collect data and store it. Here is an example.
It is written partially in pseudo code so 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.gif --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 working.

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.

=head2 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+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 much KM you had traveled,
the picture would be the right one to look at. On the other hand, for
the speed that we traveled at, the maximum number seen is much more
valuable. (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 week days, and
maximum of 120 KM/H on weekdays (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 over 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. This 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 hiways 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 too much to keep for a long
period of time. Therefore we average it, eventually to one value per
day. From this one value, we cannot see much detail.

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 a good 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 would not have the graphics at all in that case.


=head2 Let's review what you now should know.

You now know how to create a database. You can put the numbers in it,
get them out again by creating an image, do math on the data from the
database and view the outcome instead of the raw data.
You know about the difference between averages and maxima, and when
to use which (or at least you 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 doc but also in your day to day
monitoring long after you read this introduction.

=head2 Data Source Types

All right, you feel like continuing. Welcome back and get ready
for an increased speed in the examples and explanation.

You know that in order to view a counter over time, you have to
take two numbers and divide the difference of them between the
time lapsed.  This makes sense for the examples I gave you but there
are other possibilities.  For instance, I'm able to retrieve the
temperature from my router in three places namely the inlet, the
so called hot-spot and the exhaust.  These values are not counters.
If I take the difference of the two samples and divide that by
300 seconds I would be asking for the temperature change per second.
Hopefully this is zero! If not, the computerroom is 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
cooled -> 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 new 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.gif -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"

=head2 RRDtool under the Microscope

=over 2

=item *

Line A is a counter so it should continuously increment and RRDtool
should 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).

=item *

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

=item *

Line C is derive. It should be a counter that can decrease. It does
so between 2400 and 0, with 1800 in-between.

=item *

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.

=back

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 GIF shows all this, you know you have typed the data correct,
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:

=over 2

=item *

Line A: 300,600,900 and so on. The counter delta is a constant 300 and
so it 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 ? 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 !

=item *

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

=item *

Line C: Again, the start-out value is unknown. The same story is valid
like for line A. In this case the deltas are not constant so the line
is not. 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
there is a difference.

=item *

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. Therefore 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 results.

=back

=head2 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 our 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 would like to 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 falsely added small value.
There is a risk in this: suppose the large counter wrapped while adding
a huge delta, it could happen in theory 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 of it so you
can judge that 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 value. Until now, we used "U", meaning
"unknown". If you provide values for one or both of them and if RRDtool
receives values 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 say it is 10. The maximum temperature for my router I would
state as 80. Any higher and the device would be out of order.
For my car, I would never expect negative numbers and also I would not
expect numbers to be 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 paranoia if it looks weird.

=head2 Data Resampling

One important feature of RRDtool has not been explained yet:
It is virtually impossible to collect the data and feed it into RRDtool
on exact intervals. RRDtool therefore interpolates the data so it is 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 with 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 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 with 297. Again
RRDtool alters the value and stores 300 as it should be.

      in the RDD                 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

   for Unix: cp seconds1.rrd seconds2.rrd
   for Dos:  copy seconds1.rrd seconds2.rrd
   for vms:  how would I know :)

   rrdtool update seconds1.rrd \
      920805000:000 920805300:300 920805600:600 920805900:900
   rrdtool update seconds2.rrd \
      920805000:000 920805300:300 920805603:603 920805900:900

   rrdtool graph seconds1.gif                       \
      --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.gif                       \
      --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

Both graphs should show the same.

=head1 WRAPUP

It's time to wrap up this document. You now know all the basics to be
able to work with RRDtool and to read the documentation available.
There is plenty more to discover about RRDtool and you will find more and
more uses for the package. You could create easy graphics using just the
examples provided and using only RRDtool. You could also use the front
ends that are available.

=head1 MAILINGLIST

Remember to subscribe to the mailing-list. Even if you are not answering
the mails that come by, it helps both you and the rest. 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.

=head1 SEE ALSO

The RRDtool manpages

=head1 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 answer but at the same
time learn a whole lot more.

Alex van den Bogaerdt
E<lt>alex@ergens.op.het.netE<gt>

