


RRDTUTORIAL(1)               rrdtool               RRDTUTORIAL(1)


NAME
       rrdtutorial - Alex van den Bogaerdts RRDtool tutorial

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 and here we help you to
       understand the basics in order to prepare you to read the
       documentation yourself. It also explains the general
       things about statistics with a focus on networking.

TUTORIAL
       Important

       We first have to do some uninteresting reading folks,
       don't skip this part! Later on, in the examples, you need
       to know the basics.

       What is RRDtool ?

       RRDtool means 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.  RRDtool works with
       with Round Robin Databases (RRDs). It stores and retrieves
       data from them.

       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 to RRDtool. If you can do this, RRDtool will probably
       be able to store it.

       Many examples talk about SNMP which is an acronym for
       Simple Network Management Protocol. The "simple" is about
       the protocol, it does not mean it is simple to manage a
       network. After working your way through this document, you
       will know enough to be able to understand what people are
       talking about. For now, just assume SNMP is a way to talk
       to devices and ask those devices about counters they keep.
       It is the value from those counters that are kept in the



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       RRD.

       What can I do with this tool ?

       RRDtool originated from MRTG (multi router traffic
       grapher) which itself came from a tiny little script to
       monitor the performance of a connection to the Internet.
       Since then, MRTG has also been used for several other
       purposes including temperature, speed, voltage, number of
       printouts and many other things. 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 rather 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. Many devices have such a
       sensor.

       What if I still have problems after reading this document
       ?

       More exact, after re-reading this document :) It all
       depends on the kind of problems you have. 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 reading the
       list, your problem may already have been solved for
       somebody else and you will be helped without writing a new
       a message at all ... 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 <rrd-users-
       request@list.ee.ethz.ch> with a subject of "subscribe". If



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       you ever want to leave this list, you 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.  It is assumed that following the instructions
       in the order presented will build up enough knowledge of
       the program 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 "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. It is expected that you get the same results when
       you try these examples yourself so do that.  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 calculi:

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





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

       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 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: twelve 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:
       this is ninety MPH and therefore not recommended to try
       for yourself where I live :)

       Remember: these numbers are averages and there is no way
       to figure out from the numbers we got that you drove at a
       constant speed.

       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.




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       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 (so: first look if it's
       safe!) 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 (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 (so: 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 (so: 5 hours) The remaining
       options will be discussed later on.  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.

       We now have to fill our database with some numbers. We'll
       pretend to have read the following numbers:

















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        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 will be OS dependent.

       The time value may look strange to you, it is written in
       seconds since the first of January, 1970, midnight. Don't
       worry about this yet, just notice that there is a
       difference of 300 in between all values.

       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



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       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
       (Please, no discussions if black and white can be called
       colors)

          red    #FF0000
          green  #00FF00
          blue   #0000FF
          purple #FF00FF     (mixed red with blue)
          gray   #555555     (one third of all components)

       The GIF you just created can be displayed using a web
       browser or other software you like. I can not provide you
       with an example of that, because there are too many
       different setups, possibilities etc.

       Graphics with some math

       When looking at the image, you notice that the horizontal
       axis displays 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 RRDtool displays this as 40 m which
       means 40 milli (so: NOT 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 another 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



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       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 done with the CDEF part. What it says
       is: take the data source myspeed and the number 1000,
       multiply those. The calculations are done using Reverse
       Polish Notation. It is an easy way of performing
       calculations after you understand it. You will, eventually
       but for now assume it is correct what I write and just
       keep to the examples in this file. Read the documentation
       that came with RRDtool (look in rrdgraph.doc) when you're
       ready for it.

       Hang on! If we can multiply values with 1000, it should
       also be possible to display kilometers per hour from the
       same data !  What do we need to do ? If we have meters per
       second, we can make this meters per hour by multiplying
       the value with 3600 (there go 3600 seconds in one hour).
       To get kilometers per hour, we need to divide by 1000.  We
       end up with: value * 3600 / 1000 = value * 3.6 . Remember
       we also have to correct our mistake, so it is value * 3600
       for us.

       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:

       o Check if kmh is greater than 100    ( kmh,100 ) GT

       o If so, return 0, else kmh           ((( kmh,100 ) GT ),
         0, kmh) IF

       For the other speed:

       o Check if kmh is greater than 100    ( kmh,100 ) GT




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       o If so, return kmh, else return 0    ((( kmh,100) GT ),
         kmh, 0) IF

       Graphics Magic

       I like to believe there are virtually no limits about what
       RRDtool can do. 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. Every time you create these three
       GIFs and reload the page and the GIFs (better reread these
       last three words!) you know how fast you traveled.

       Updates in Reality

       You already looked at the "update" tool. 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, so I just give you an example script in a sort
       of pseudo code: (Do not try this with our test database,



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       it is used in further examples)

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

       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

       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 the laser-jet
       printer, the coffee made by the coffee machine, a device
       that counts the electricity used, whatever. Just as long
       as it is an incrementing counter that you can measure
       regularly, you can graph it against time as we did. Most
       people will use the counter that keeps track of octets
       (bytes) transfered by a routing 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. If you have to look why
       things went wrong, you need to know how the stuff works.

       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 of your device, for
       password you use the "community" as it is called. For most
       devices "public" will do but this can be disabled, altered
       or protected for privacy and security reasons. You will
       need to find this out as there is no way I can tell what
       you ( ...your device... ) are keeping a secret from me.

       Then there is this third parameter, called OID.  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 or in short: the MIB. It is an upside-
       down tree, 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.



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

       From there, we are especially interested in the branch
       "interfaces" which has number 2 (so: 1.3.6.1.2.1.2 or
       1.3.6.1.2.1.interfaces).

       The SNMP programs that I know can take above path as a
       default, so I could walk to interfaces without writing it
       in full. To do that, the path has to be written as
       interfaces, without a leading dot.  I could also use the
       long form, then I need to start with a dot.

       I can intermix numbers and names as I like.  1.3.6.1.2.1
       is the same as iso.org.dod.1.2.1 and
       1.3.6.internet.2.mib-2 For interfaces I could write
       "interfaces", 2, or a long version by prefixing it with
       ".1.3.6.1.2.1.". Just try what works for you and do read
       the manual for the version of snmpget you are using.

       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. You are on your own
       here, sorry.

       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:



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          [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, it is time to continue with the following. If not,
       then go back.

       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.






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        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

        There are 600 samples of five minutes, together just over two days
        There are 600 samples of 30 minutes, almost two weeks
        There are 600 samples of 2 hours, 50 hour or almost two months
        There are 732 samples of 1 day, just over two years

       For people used to MRTG that even have read the manuals,
       these numbers should look familiar...

          rrdtool create myrouter.rrd         \
                   DS:input:COUNTER:600:U:U   \
                   DS:output:COUNTER:600:U:U  \
                   RRA:AVERAGE:0.5:1:600      \
                   RRA:AVERAGE:0.5:6:600      \
                   RRA:AVERAGE:0.5:24:600     \
                   RRA:AVERAGE:0.5:288:732    \
                   RRA:MAX:0.5:1:600          \
                   RRA:MAX:0.5:6:600          \
                   RRA:MAX:0.5:24:600         \
                   RRA:MAX:0.5:288:732

       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



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       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 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 for a few more days.  If
       you like, you could try the examples from the test
       database and see if you can play around with the options
       and calculations.

       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+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



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       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 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 motor-ways 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.

       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.




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       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. If I take the difference of the
       two samples and divide that by 300 (seconds) there would
       not be very much to display ... If there would be anything
       to display it would be the fluctuations in the temperature
       and if they produce noticeable results when divided by
       300, I'd better look for a new computer room.

       So, what can we do ?  RRDtool can also store the values
       you measure directly as they are (this is not entirely
       true but close enough). The graphs we make 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:







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          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"


       RRDtool under the Microscope


       o 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).

       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 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




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       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:

       o 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 !

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

       o 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.

       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. 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.

       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



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       be:

        Delta = 7 - 999987 = -999980    (instead of 1000007-999987=20)

        Real delta = -999980 + 999999 + 1 = 20

       At the moment 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





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        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 fail and it is not uncommon to get wrong numbers
       occasionally. We can't prevent all errors but there are
       some things we could do. RRDtool create takes two special
       parameters for this. They define the minimum and maximum
       allowed value. Until now, we made them "U" for Unknown.
       You may (not: must) provide values for one or both of them
       and if RRDtool receives values that are outside these
       limits, it will ignore them. 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.

       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



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       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 real

        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















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          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.

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
       program. You could create easy graphics using just the
       examples provided and using only RRDtool. You could also
       use the front ends that are available.

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
       (so: read that too) 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 answer but at the same time learn a
       whole lot more.

       Alex van den Bogaerdt <alex@ergens.op.het.net>





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