RRDTUTORIAL(1)                       rrdtool                      RRDTUTORIAL(1)

NAME
     rrdtutorial - Alex van den Bogaerdt's RRDtool tutorial

DESCRIPTION
     RRDtool  is  written by Tobias Oetiker <tobi@oetiker.ch> with contributions
     from many people all around the world. This document is written by Alex van
     den Bogaerdt <alex@vandenbogaerdt.nl> to help you understand  what  RRDtool
     is and what it can do for you.

     The  documentation provided with RRDtool can be too technical for some peo‐
     ple. This tutorial is here to help you understand the basics of RRDtool. It
     should prepare you to read the documentation yourself.   It  also  explains
     the general things about statistics with a focus on networking.

TUTORIAL
   Important
     Please  don't skip ahead in this document!  The first part of this document
     explains the basics and may be boring.  But if you don't understand the ba‐
     sics, the examples will not be as meaningful to you.

     Sometimes things change.  This example used to provide numbers like  "0.04"
     instead  of "4.00000e-02".  Those are really the same numbers, just written
     down differently.  Don't be alarmed if a future version of rrdtool displays
     a slightly different form of output. The examples in this document are cor‐
     rect for version 1.2.0 of RRDtool.

     Also, sometimes bugs do occur. They may also influence the outcome  of  the
     examples.  Example  speed4.png was suffering from this (the handling of un‐
     known data in an if-statement was wrong). Normal data will be just fine  (a
     bug  in  rrdtool wouldn't last long) but special cases like NaN, INF and so
     on may last a bit longer.  Try another version if you  can,  or  just  live
     with it.

     I fixed the speed4.png example (and added a note). There may be other exam‐
     ples  which suffer from the same or a similar bug.  Try to fix it yourself,
     which is a great exercise. But please do not submit your result as a fix to
     the source of this document. Discuss it on the user's list, or write to me.

   What is RRDtool?
     RRDtool refers to Round Robin Database tool.  Round robin  is  a  technique
     that  works  with a fixed amount of data, and a pointer to the current ele‐
     ment. 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 cir‐
     cle  there  is neither a beginning nor an end, you can go on and on and on.
     After a while, all the available places will be used and the process  auto‐
     matically reuses old locations. This way, the dataset will not grow in size
     and  therefore  requires  no  maintenance.   RRDtool works with Round Robin
     Databases (RRDs). It stores and retrieves data from them.

   What data can be put into an RRD?
     You name it, it will probably fit as long as it is some sort of time-series
     data. This means you have to be able  to  measure  some  value  at  several
     points in time and provide this information to RRDtool. If you can do this,
     RRDtool  will  be  able to store it. The values must be numerical but don't
     have to be integers, as is the case with MRTG (the next section  will  give
     more details on this more specialized application).

     Many  examples below talk about SNMP which is an acronym for Simple Network
     Management Protocol. "Simple" refers to the protocol. It does not  mean  it
     is  simple  to  manage or monitor a network. After working your way through
     this document, you should know enough to be able to understand what  people
     are talking about. For now, just realize that SNMP can be used to query de‐
     vices  for  the  values  of  counters they keep. It is the value from those
     counters that we want to store in the RRD.

   What can I do with this tool?
     RRDtool originated from MRTG (Multi Router Traffic Grapher).  MRTG  started
     as  a  tiny little script for graphing the use of a university's connection
     to the Internet. MRTG was later (ab-)used as a tool for graphing other data
     sources including temperature, speed, voltage, number of printouts and  the
     like.

     Most  likely  you  will start to use RRDtool to store and process data col‐
     lected via SNMP. The data will most likely be bytes (or  bits)  transferred
     from  and  to  a network or a computer.  But it can also be used to display
     tidal waves, solar radiation, power consumption, number of visitors  at  an
     exhibition, noise levels near an airport, temperature on your favorite hol‐
     iday  location, temperature in the fridge and whatever your imagination can
     come up with.

     You only need a sensor to measure the data and be able to feed the  numbers
     into  RRDtool.  RRDtool  then lets you create a database, store data in it,
     retrieve that data and create graphs in PNG format for  display  on  a  web
     browser. Those PNG images are dependent on the data you collected and could
     be,  for  instance,  an overview of the average network usage, or the peaks
     that occurred.

   What if I still have problems after reading this document?
     First of all: read it again! You may have missed something.  If you are un‐
     able to compile the sources and you have a fairly common OS, it will proba‐
     bly not be the fault of RRDtool. There may be pre-compiled versions  around
     on the Internet. If they come from trusted sources, get one of those.

     If  on  the other hand the program works but does not give you the expected
     results, it will be a problem with configuring it. Review  your  configura‐
     tion 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 with‐
     out searching the archives: your problem may already have been  solved  for
     somebody  else!   This  is true for most, if not all, mailing lists and not
     only for this particular one. Look in the documentation that came with RRD‐
     tool for the location and usage of the list.

     I suggest you take a moment to subscribe to the mailing list right  now  by
     sending  an email to <rrd-users-request@lists.oetiker.ch> with a subject of
     "subscribe". If you ever want to leave this list, just write  an  email  to
     the same address but now with a subject of "unsubscribe".

   How will you help me?
     By  giving you some detailed descriptions with detailed examples.  I assume
     that following the instructions in the order presented will give you enough
     knowledge of RRDtool to experiment for yourself.  If it  doesn't  work  the
     first  time,  don't  give up. Reread the stuff that you did understand, you
     may have missed something.

     By following the examples you get some hands-on experience and,  even  more
     important, some background information of how it works.

     You  will  need  to know something about hexadecimal numbers. If you don't,
     start with reading bin_dec_hex before you continue here.

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

     We will start with some easy stuff and compare a car with a router, or com‐
     pare 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, in‐
     creasing with every byte that is transferred. This  counter  will  probably
     have  a  maximum  value.  If  this  value  is  reached and an extra byte is
     counted, the counter starts over at zero. This is the same as many counters
     in the world such as the mileage counter in a car.

     Most discussions about networking talk about bits per second so  let's  get
     used  to that right away. Assume a byte is eight bits and start to think in
     bits not bytes. The counter, however, still  counts  bytes!   In  the  SNMP
     world most of the counters are 32 bits. That means they are counting from 0
     to  4294967295. We will use these values in the examples.  The device, when
     asked, returns the current value of the counter. We know the time that  has
     passes  since  we last asked so we now know how many bytes have been trans‐
     ferred ***on average*** per second. This is not  very  hard  to  calculate.
     First in words, then in calculations:

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

     2. Do the same with the current time and the previous time (in seconds).

     3. Divide  the  outcome  of  (1)  by  the outcome of (2), the result is the
        amount of bytes per second. Multiply by eight to get the number of  bits
        per second (bps).

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

     For some people it may help to translate this to an automobile example.  Do
     not try this example, and if you do, don't blame me for the results!

     People  who are not used to think in kilometers per hour can translate most
     into miles per hour by dividing km by 1.6 (close enough).  I will  use  the
     following abbreviations:

      m:    meter
      km:   kilometer (= 1000 meters).
      h:    hour
      s:    second
      km/h: kilometers per hour
      m/s:  meters per second

     You  are  driving a car. At 12:05 you read the counter in the dashboard and
     it tells you that the car has moved 12345 km until that moment.   At  12:10
     you  look  again,  it reads 12357 km. This means you have traveled 12 km in
     five minutes. A scientist would translate that into meters per  second  and
     this makes a nice comparison toward the problem of (bytes per five minutes)
     versus (bits per second).

     We  traveled  12 kilometers which is 12000 meters. We did that in five min‐
     utes or 300 seconds. Our speed is 12000m / 300s or 40 m/s.

     We could also calculate the speed in km/h: 12 times 5 minutes is  an  hour,
     so we have to multiply 12 km by 12 to get 144 km/h.  For our native English
     speaking  friends: that's 90 mph so don't try this example at home or where
     I live :)

     Remember: these numbers are averages only.  There is no way to  figure  out
     from  the  numbers,  if you drove at a constant speed.  There is an example
     later on in this tutorial that explains this.

     I hope you understand that there is no difference  in  calculating  m/s  or
     bps; only the way we collect the data is different. Even the k from kilo is
     the same as in networking terms k also means 1000.

     We  will now create a database where we can keep all these interesting num‐
     bers. The method used to start the program may differ slightly from  OS  to
     OS, but I assume you can figure it out if it works different on yours. Make
     sure  you  do not overwrite any file on your system when executing the fol‐
     lowing command and type the whole line as one long line (I had to split  it
     for readability) and skip all of the '\' characters.

        rrdtool create test.rrd             \
                 --start 920804400          \
                 DS:speed:COUNTER:600:U:U   \
                 RRA:AVERAGE:0.5:1:24       \
                 RRA:AVERAGE:0.5:6:10

     (So enter: "rrdtool create test.rrd --start 920804400 DS ...")

   What has been created?
     We  created the round robin database called test (test.rrd) which starts at
     noon the day I started writing this document, 7th of March, 1999 (this date
     translates to 920804400 seconds as explained below). Our database holds one
     data source (DS) named "speed" that represents a counter. This  counter  is
     read  every  five  minutes (this is the default therefore you don't have to
     put "--step=300").  In the same database two round  robin  archives  (RRAs)
     are  kept, one averages the data every time it is read (i.e., there's noth‐
     ing to average) and keeps 24 samples (24 times 5 minutes is 2  hours).  The
     other  averages  6 values (half hour) and contains 10 such averages (e.g. 5
     hours).

     RRDtool works with special time stamps coming from the  UNIX  world.   This
     time stamp is the number of seconds that passed since January 1st 1970 UTC.
     The  time  stamp  value is translated into local time and it will therefore
     look different for different time zones.

     Chances are that you are not in the same part of the world as I  am.   This
     means your time zone is different. In all examples where I talk about time,
     the  hours  may  be wrong for you. This has little effect on the results of
     the examples, just correct the hours while reading.  As an example: where I
     will see "12:05" the UK folks will see "11:05".

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

      12:05  12345 km
      12:10  12357 km
      12:15  12363 km
      12:20  12363 km
      12:25  12363 km
      12:30  12373 km
      12:35  12383 km
      12:40  12393 km
      12:45  12399 km
      12:50  12405 km
      12:55  12411 km
      13:00  12415 km
      13:05  12420 km
      13:10  12422 km
      13:15  12423 km

     We fill the database as follows:

      rrdtool update test.rrd 920804700:12345 920805000:12357 920805300:12363
      rrdtool update test.rrd 920805600:12363 920805900:12363 920806200:12373
      rrdtool update test.rrd 920806500:12383 920806800:12393 920807100:12399
      rrdtool update test.rrd 920807400:12405 920807700:12411 920808000:12415
      rrdtool update test.rrd 920808300:12420 920808600:12422 920808900:12423

     This reads: update our test database with the following numbers

      time 920804700, value 12345
      time 920805000, value 12357

     etcetera.

     As  you  can see, it is possible to feed more than one value into the data‐
     base in one command. I had to stop at three for readability  but  the  real
     maximum per line is OS dependent.

     We can now retrieve the data from our database using "rrdtool fetch":

      rrdtool fetch test.rrd AVERAGE --start 920804400 --end 920809200

     It should return the following output:

                               speed

      920804700: nan
      920805000: 4.0000000000e-02
      920805300: 2.0000000000e-02
      920805600: 0.0000000000e+00
      920805900: 0.0000000000e+00
      920806200: 3.3333333333e-02
      920806500: 3.3333333333e-02
      920806800: 3.3333333333e-02
      920807100: 2.0000000000e-02
      920807400: 2.0000000000e-02
      920807700: 2.0000000000e-02
      920808000: 1.3333333333e-02
      920808300: 1.6666666667e-02
      920808600: 6.6666666667e-03
      920808900: 3.3333333333e-03
      920809200: nan
      920809500: nan

     Note  that  you might get more rows than you expect. The reason for this is
     that you ask for a time range that ends on 920809200. The  number  that  is
     written  behind  920809200:  in  the  list above covers the time range from
     920808900 to 920809200, EXCLUDING 920809200. Hence to be on the sure  side,
     you receive the entry from 920809200 to 920809500 as well since it INCLUDES
     920809200.  You  may  also see "NaN" instead of "nan" this is OS dependent.
     "NaN" stands for "Not A Number".  If your OS writes "U" or "UNKN" or  some‐
     thing similar that's okay.  If something else is wrong, it will probably be
     due  to  an  error you made (assuming that my tutorial is correct of course
     :-). In that case: delete the database and try again.

     The meaning of the above output will become clear below.

   Time to create some graphics
     Try the following command:

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

     This will create speed.png which starts at 12:00 and ends at 13:00.   There
     is  a  definition  of  a  variable  called myspeed, using the data from RRA
     "speed" out of database "test.rrd". The line drawn is  2  pixels  high  and
     represents  the  variable  myspeed.  The  color  is  red  (specified by its
     rgb-representation, see below).

     You'll notice that the start of the graph is not at  12:00  but  at  12:05.
     This  is  because we have insufficient data to tell the average before that
     time. This will only happen when you miss some samples, this will not  hap‐
     pen 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 compo‐
     nents, you specify how much to use in hexadecimal where 00  means  not  in‐
     cluded and FF means fully included.  The "color" white is a mixture of red,
     green and blue: FFFFFF The "color" black is all colors off: 000000

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

     Additionally  you can (with a recent RRDtool)  add an alpha channel (trans‐
     parency).  The default will be "FF" which means non-transparent.

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

   Graphics with some math
     When  looking  at the image, you notice that the horizontal axis is labeled
     12:10, 12:20, 12:30, 12:40 and 12:50. Sometimes a label doesn't fit  (12:00
     and 13:00 would be likely candidates) so they are skipped.

     The vertical axis displays the range we entered. We provided kilometers and
     when  divided  by  300 seconds, we get very small numbers. To be exact, the
     first value was 12 (12357-12345) and divided by 300 this makes 0.04,  which
     is  displayed  by  RRDtool as "40 m" meaning "40/1000". The "m" (milli) has
     nothing to do with meters (also  m),  kilometers  or  millimeters!  RRDtool
     doesn't  know  about the physical units of our data, it just works with di‐
     mensionless numbers.

     If  we  had  measured  our  distances  in  meters,  this  would  have  been
     (12357000-12345000)/300 = 12000/300 = 40.

     As  most people have a better feel for numbers in this range, we'll correct
     that. We could recreate our database and store the correct data, but  there
     is a better way: we do some calculations while creating the png file!

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

     Note:  I need to escape the multiplication operator * with a backslash.  If
     I don't, the operating system may interpret it and use it for file name ex‐
     pansion. You could also place the line within quotation marks like so:

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

     It boils down to: it is RRDtool which should see *, not your shell.  And it
     is your shell interpreting \, not RRDtool. You may need to adjust  examples
     accordingly if you happen to use an operating system or shell which behaves
     differently.

     After viewing this PNG, you notice the "m" (milli) has disappeared. This is
     what  the  correct result would be. Also, a label has been added to the im‐
     age.  Apart from the things mentioned above, the PNG should look the same.

     The calculations are specified in the CDEF part above and  are  in  Reverse
     Polish Notation ("RPN"). What we requested RRDtool to do is: "take the data
     source  myspeed and the number 1000; multiply those". Don't bother with RPN
     yet, it will be explained later on in more detail. Also, you  may  want  to
     read my tutorial on CDEFs and Steve Rader's tutorial on RPN. But first fin‐
     ish this tutorial.

     Hang on! If we can multiply values with 1000, it should also be possible to
     display kilometers per hour from the same data!

     To change a value that is measured in meters per second:

      Calculate meters per hour:     value * 3600
      Calculate kilometers per hour: value / 1000
      Together this makes:           value * (3600/1000) or value * 3.6

     In  our  example  database  we made a mistake and we need to compensate for
     this by multiplying with 1000. Applying that correction:

      value * 3.6  * 1000 == value * 3600

     Now let's create this PNG, and add some more magic ...

      rrdtool graph speed3.png                             \
           --start 920804400 --end 920808000               \
           --vertical-label km/h                           \
           DEF:myspeed=test.rrd:speed:AVERAGE              \
           "CDEF:kmh=myspeed,3600,*"                       \
           CDEF:fast=kmh,100,GT,kmh,0,IF                   \
           CDEF:good=kmh,100,GT,0,kmh,IF                   \
           HRULE:100#0000FF:"Maximum allowed"              \
           AREA:good#00FF00:"Good speed"                   \
           AREA:fast#FF0000:"Too fast"

     Note: here we use another means to escape the * operator by  enclosing  the
     whole string in double quotes.

     This  graph  looks much better. Speed is shown in km/h and there is even an
     extra line with the maximum allowed speed (on the road I travel on). I also
     changed the colors used to display speed and changed it from a line into an
     area.

     The calculations are more complex now. For speed  measurements  within  the
     speed limit they are:

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

     For values above the speed limit:

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

   Graphics Magic
     I  like  to  believe there are virtually no limits to how RRDtool graph can
     manipulate data. I will not explain how it works, but look at the following
     PNG:

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

     Remember the note in the beginning?  I had to remove unknown data from this
     example. The 'nonans' CDEF is new, and the 6th line (which used to  be  the
     5th line) used to read 'CDEF:kmh=myspeed,3600,*'

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

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

     Name the file "speed.html" or similar, and look at it in your web browser.

     Now,  all  you  have  to  do is measure the values regularly and update the
     database.  When you want to view the data, recreate the PNGs and make  sure
     to  refresh  them  in  your browser. (Note: just clicking reload may not be
     enough,  especially  when  proxies  are  involved.   Try  shift-reload   or
     ctrl-F5).

   Updates in Reality
     We've  already used the "update" command: it took one or more parameters in
     the form of "<time>:<value>". You'll be glad to know that you  can  specify
     the  current  time  by  filling in a "N" as the time.  Or you could use the
     "time" function in Perl (the shortest example in this tutorial):

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

     How to run a program on regular intervals is OS specific. But  here  is  an
     example in pseudo code:

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

     (do not try this with our test database, we'll use it in further examples)

     This is all. Run the above script every five minutes. When you need to know
     what  the graphs look like, run the examples above. You could put them in a
     script as well. After running that script, view the page speed.html we cre‐
     ated above.

   Some words on SNMP
     I can imagine very few people that will be able to get real data from their
     car every five minutes. All other people will have to settle for some other
     kind of counter. You could  measure  the  number  of  pages  printed  by  a
     printer,  for example, the cups of coffee made by the coffee machine, a de‐
     vice that counts the electricity used, whatever. Any  incrementing  counter
     can  be  monitored and graphed using the stuff you learned so far. Later on
     we will also be able to monitor other types of values like temperature.

     Many people interested in RRDtool will use the counter that keeps track  of
     octets (bytes) transferred by a network device. So let's do just that next.
     We will start with a description of how to collect data.

     Some  people will make a remark that there are tools which can do this data
     collection for you. They are right! However, I feel it  is  important  that
     you  understand  they  are  not  necessary.  When you have to determine why
     things went wrong you need to know how they work.

     One tool used in the example has been talked about very briefly in the  be‐
     ginning of this document, it is called SNMP. It is a way of talking to net‐
     worked  equipment. The tool I use below is called "snmpget" and this is how
     it works:

        snmpget device password OID

     or

        snmpget -v[version] -c[password] device OID

     For device you substitute the name, or the IP address, of your device.  For
     password you use the "community read string" as it is called  in  the  SNMP
     world.   For  some devices the default of "public" might work, however this
     can be disabled, altered or protected for  privacy  and  security  reasons.
     Read the documentation that comes with your device or program.

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

     When  you  start  to learn about SNMP it looks very confusing. It isn't all
     that difficult when you look at the Management  Information  Base  ("MIB").
     It  is  an  upside-down tree that describes data, with a single node as the
     root and from there a number of branches.  These branches end up in another
     node, they branch out, etc.  All the branches have a name and they form the
     path that we follow all the way down.  The  branches  that  we  follow  are
     named:  iso,  org,  dod, internet, mgmt and mib-2.  These names can also be
     written down as numbers and are 1 3 6 1 2 1.

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

     There is a lot of confusion about the leading dot that some  programs  use.
     There  is  *no*  leading dot in an OID.  However, some programs can use the
     above part of OIDs as a default.  To indicate the difference between abbre‐
     viated OIDs and full OIDs they need a leading dot when you specify the com‐
     plete OID.  Often those programs will leave out the  default  portion  when
     returning the data to you.  To make things worse, they have several default
     prefixes ...

     Ok,  lets  continue to the start of our OID: we had 1.3.6.1.2.1 From there,
     we are especially interested in the branch "interfaces" which has number  2
     (e.g., 1.3.6.1.2.1.2 or 1.3.6.1.2.1.interfaces).

     First,  we have to get some SNMP program. First look if there is a pre-com‐
     piled package available for your OS. This is the preferred  way.   If  not,
     you  will have to get the sources yourself and compile those.  The Internet
     is full of sources, programs etc. Find information using a search engine or
     whatever you prefer.

     Assume you got the program. First try to collect some data that  is  avail‐
     able  on  most systems. Remember: there is a short name for the part of the
     tree that interests us most in the world we live in!

     I will give an example which can be used on Fedora Core 3.  If  it  doesn't
     work  for you, work your way through the manual of snmp and adapt the exam‐
     ple to make it work.

        snmpget -v2c -c public myrouter system.sysDescr.0

     The device should answer with a description of  itself,  perhaps  an  empty
     one.  Until you got a valid answer from a device, perhaps using a different
     "password", or a different device, there is no point in continuing.

        snmpget -v2c -c public myrouter interfaces.ifNumber.0

     Hopefully you get a number as a result, the number of interfaces.   If  so,
     you can carry on and try a different program called "snmpwalk".

        snmpwalk -v2c -c public myrouter interfaces.ifTable.ifEntry.ifDescr

     If  it  returns  with a list of interfaces, you're almost there.  Here's an
     example:
        [user@host /home/alex]$ snmpwalk -v2c -c public cisco 2.2.1.2

        interfaces.ifTable.ifEntry.ifDescr.1 = "BRI0: B-Channel 1"
        interfaces.ifTable.ifEntry.ifDescr.2 = "BRI0: B-Channel 2"
        interfaces.ifTable.ifEntry.ifDescr.3 = "BRI0" Hex: 42 52 49 30
        interfaces.ifTable.ifEntry.ifDescr.4 = "Ethernet0"
        interfaces.ifTable.ifEntry.ifDescr.5 = "Loopback0"

     On this cisco equipment, I would like to monitor the "Ethernet0"  interface
     and from the above output I see that it is number four. I try:

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

        interfaces.ifTable.ifEntry.ifInOctets.4 = 2290729126
        interfaces.ifTable.ifEntry.ifOutOctets.4 = 1256486519

     So now I have two OIDs to monitor and they are (in full, this time):

        1.3.6.1.2.1.2.2.1.10

     and

        1.3.6.1.2.1.2.2.1.16

     both with an interface number of 4.

     Don't get fooled, this wasn't my first try. It took some time for me too to
     understand  what  all  these numbers mean. It does help a lot when they get
     translated into descriptive text... At least, when people are talking about
     MIBs and OIDs you know what it's all about.  Do not  forget  the  interface
     number  (0  if it is not interface dependent) and try snmpwalk if you don't
     get an answer from snmpget.

     If you understand the above section and get numbers from your device,  con‐
     tinue on with this tutorial. If not, then go back and re-read this part.

   A Real World Example
     Let  the fun begin. First, create a new database. It contains data from two
     counters, called input and output. The data is put into archives that aver‐
     age it. They take 1, 6, 24 or 288 samples at a time.   They  also  go  into
     archives  that  keep  the maximum numbers. This will be explained later on.
     The time in-between samples is 300 seconds, a good starting point, which is
     the same as five minutes.

      1 sample "averaged" stays 1 period of 5 minutes
      6 samples averaged become one average on 30 minutes
      24 samples averaged become one average on 2 hours
      288 samples averaged become one average on 1 day

     Lets try to be compatible with MRTG which stores about the following amount
     of data:

      600 5-minute samples:    2   days and 2 hours
      600 30-minute samples:  12.5 days
      600 2-hour samples:     50   days
      732 1-day samples:     732   days

     These ranges are appended, so the total amount of data stored in the  data‐
     base  is  approximately  797  days. RRDtool stores the data differently, it
     doesn't start the "weekly" archive where the "daily" archive  stopped.  For
     both archives the most recent data will be near "now" and therefore we will
     need to keep more data than MRTG does!

     We will need:

      600 samples of 5 minutes  (2 days and 2 hours)
      700 samples of 30 minutes (2 days and 2 hours, plus 12.5 days)
      775 samples of 2 hours    (above + 50 days)
      797 samples of 1 day      (above + 732 days, rounded up to 797)

        rrdtool create myrouter.rrd         \
                 DS:input:COUNTER:600:U:U   \
                 DS:output:COUNTER:600:U:U  \
                 RRA:AVERAGE:0.5:1:600      \
                 RRA:AVERAGE:0.5:6:700      \
                 RRA:AVERAGE:0.5:24:775     \
                 RRA:AVERAGE:0.5:288:797    \
                 RRA:MAX:0.5:1:600          \
                 RRA:MAX:0.5:6:700          \
                 RRA:MAX:0.5:24:775         \
                 RRA:MAX:0.5:288:797

     Next  thing  to do is to collect data and store it. Here is an example.  It
     is written partially in pseudo code,  you will have to find out what to  do
     exactly on your OS to make it work.

        while not the end of the universe
        do
           get result of
              snmpget router community 2.2.1.10.4
           into variable $in
           get result of
              snmpget router community 2.2.1.16.4
           into variable $out

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

           wait for 5 minutes
        done

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

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

     This should produce a picture with one day worth of traffic.  One day is 24
     hours  of  60  minutes of 60 seconds: 24*60*60=86400, we start at now minus
     86400 seconds. We define (with DEFs) inoctets and outoctets as the  average
     values from the database myrouter.rrd and draw an area for the "in" traffic
     and a line for the "out" traffic.

     View the image and keep logging data for a few more days.  If you like, you
     could  try the examples from the test database and see if you can get vari‐
     ous options and calculations to work.

     Suggestion: Display in bytes per second and in bits per  second.  Make  the
     Ethernet graphics go red if they are over four megabits per second.

   Consolidation Functions
     A  few  paragraphs  back I mentioned the possibility of keeping the maximum
     values instead of the average values. Let's go into this a bit more.

     Recall all the stuff about the speed of the car. Suppose we  drove  at  144
     km/h  during  5 minutes and then were stopped by the police for 25 minutes.
     At the end of the lecture we would take our laptop and create and view  the
     image  taken from the database. If we look at the second RRA we did create,
     we would have the average from 6 samples. The  samples  measured  would  be
     144+0+0+0+0+0=144,  divided by 30 minutes, corrected for the error by 1000,
     translated into km/h, with a result of 24 km/h.  I would still get a ticket
     but not for speeding anymore :)

     Obviously, in this case we shouldn't look at the averages.  In  some  cases
     they are handy. If you want to know how many km you had traveled, the aver‐
     aged  picture would be the right one to look at. On the other hand, for the
     speed that we traveled at, the maximum numbers seen is much more  interest‐
     ing. Later we will see more types.

     It  is the same for data. If you want to know the amount, look at the aver‐
     ages. 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 num‐
     bers.

     For my car this would translate in averages per day of 96/24=4 km/h  (as  I
     travel about 94 kilometers on a day) during working days, and maxima of 120
     km/h (my top speed that I reach every day).

     Big  difference.  Do not look at the second graph to estimate the distances
     that I travel and do not look at the first graph to estimate my speed. This
     will work if the samples are close together, as they are in  five  minutes,
     but not if you average.

     On some days, I go for a long ride. If I go across Europe and travel for 12
     hours, the first graph will rise to about 60 km/h. The second one will show
     180  km/h.  This  means  that I traveled a distance of 60 km/h times 24 h =
     1440 km. I did this with a higher speed and a maximum around 180 km/h. How‐
     ever, it probably doesn't mean that I traveled for 8 hours  at  a  constant
     speed of 180 km/h!

     This is a real example: go with the flow through Germany (fast!) and stop a
     few  times for gas and coffee. Drive slowly through Austria and the Nether‐
     lands. 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 dif‐
     ferent picture. You would see the same values on the  average  and  maximum
     graphs  (provided  I measured every 300 seconds).  You would be able to see
     when I stopped, when I was in top gear, when I  drove  over  fast  highways
     etc.  The granularity of the data is much higher, so you can see more. How‐
     ever, this takes 12 samples per hour, or 288 values per day, so it would be
     a lot of data over a longer period of time. Therefore we average it,  even‐
     tually  to  one  value per day. From this one value, we cannot see much de‐
     tail, of course.

     Make sure you understand the last few paragraphs. There is no value in only
     a line and a few axis, you need to know what they mean  and  interpret  the
     data in an appropriate way. This is true for all data.

     The biggest mistake you can make is to use the collected data for something
     that it is not suitable for. You would be better off if you didn't have the
     graph at all.

   Let's review what you now should know
     You  know  how to create a database and can put data in it. You can get the
     numbers out again by creating an image, do math on the data from the  data‐
     base  and view the result instead of the raw data.  You know about the dif‐
     ference between averages and maximum, and when to use which  (or  at  least
     you should have an idea).

     RRDtool  can  do  more than what we have learned up to now. Before you con‐
     tinue with the rest of this doc, I recommend that you reread from the start
     and try some modifications on the examples. Make sure you fully  understand
     everything.  It  will  be  worth the effort and helps you not only with the
     rest of this tutorial, but also in your day to day  monitoring  long  after
     you read this introduction.

   Data Source Types
     All  right, you feel like continuing. Welcome back and get ready for an in‐
     creased speed in the examples and explanations.

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

     So, what can we do?  We can tell RRDtool to store the values we measure di‐
     rectly as they are (this is not entirely true but close enough). The graphs
     we  make  will  look much better, they will show a rather constant value. I
     know when the router is busy (it works -> it uses more  electricity  ->  it
     generates  more  heat  -> the temperature rises). I know when the doors are
     left open (the room is air conditioned) -> the warm air from  the  rest  of
     the  building flows into the computer room -> the inlet temperature rises).
     Etc. The data type we use when creating the database before was counter, we
     now have a different data type and thus a different  name  for  it.  It  is
     called GAUGE. There are more such data types:

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

     The two additional types are DERIVE and ABSOLUTE. Absolute can be used like
     counter with one difference: RRDtool assumes the counter is reset when it's
     read.  That  is:  its delta is known without calculation by RRDtool whereas
     RRDtool needs to calculate it for the counter type.  Example: our first ex‐
     ample (12345, 12357, 12363, 12363) would read: unknown, 12, 6, 0. The  rest
     of the calculations stay the same.  The other one, derive, is like counter.
     Unlike  counter,  it  can  also  decrease  so it can have a negative delta.
     Again, the rest of the calculations stay the same.

     Let's try them all:

        rrdtool create all.rrd --start 978300900 \
                 DS:a:COUNTER:600:U:U \
                 DS:b:GAUGE:600:U:U \
                 DS:c:DERIVE:600:U:U \
                 DS:d:ABSOLUTE:600:U:U \
                 RRA:AVERAGE:0.5:1:10
        rrdtool update all.rrd \
                 978301200:300:1:600:300    \
                 978301500:600:3:1200:600   \
                 978301800:900:5:1800:900   \
                 978302100:1200:3:2400:1200 \
                 978302400:1500:1:2400:1500 \
                 978302700:1800:2:1800:1800 \
                 978303000:2100:4:0:2100    \
                 978303300:2400:6:600:2400  \
                 978303600:2700:4:600:2700  \
                 978303900:3000:2:1200:3000
        rrdtool graph all1.png -s 978300600 -e 978304200 -h 400 \
                 DEF:linea=all.rrd:a:AVERAGE LINE3:linea#FF0000:"Line A" \
                 DEF:lineb=all.rrd:b:AVERAGE LINE3:lineb#00FF00:"Line B" \
                 DEF:linec=all.rrd:c:AVERAGE LINE3:linec#0000FF:"Line C" \
                 DEF:lined=all.rrd:d:AVERAGE LINE3:lined#000000:"Line D"

   RRDtool under the Microscope
     • Line A is a COUNTER type, so it should continuously increment and RRDtool
       must calculate the differences. Also, RRDtool needs to divide the differ‐
       ence by the amount of time lapsed. This should end up as a straight  line
       at 1 (the deltas are 300, the time is 300).

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

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

     • Line  D  is of type ABSOLUTE. This is like counter but it works on values
       without calculating the difference. The numbers are the same and  as  you
       can see (hopefully) this has a different result.

     This  translates  in  the following values, starting at 23:10 and ending at
     00:10 the next day (where "u" means unknown/unplotted):

      - Line A:  u  u  1  1  1  1  1  1  1  1  1  u
      - Line B:  u  1  3  5  3  1  2  4  6  4  2  u
      - Line C:  u  u  2  2  2  0 -2 -6  2  0  2  u
      - Line D:  u  1  2  3  4  5  6  7  8  9 10  u

     If your PNG shows all this, you know you have entered the  data  correctly,
     the  RRDtool  executable is working properly, your viewer doesn't fool you,
     and you successfully entered the year 2000 :)

     You could try the same example four times, each time with only one  of  the
     lines.

     Let's go over the data again:

     • Line A: 300,600,900 and so on. The counter delta is a constant 300 and so
       is  the  time  delta. A number divided by itself is always 1 (except when
       dividing by zero which is undefined/illegal).

       Why is it that the first point is unknown? We do know what  we  put  into
       the  database,  right?  True, But we didn't have a value to calculate the
       delta from, so we don't know where we started. It would be wrong  to  as‐
       sume we started at zero so we don't!

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

     • Line  C: Again, the start-out value is unknown. This is the same story as
       for line A. In this case the deltas are not constant, therefore the  line
       is not either. If we would put the same numbers in the database as we did
       for line A, we would have gotten the same line. Unlike type counter, this
       type  can  decrease and I hope to show you later on why this makes a dif‐
       ference.

     • Line D: Here the device calculates the deltas. Therefore we DO  know  the
       first  delta and it is plotted. We had the same input as with line A, but
       the meaning of this input is different and thus the  line  is  different.
       In this case the deltas increase each time with 300. The time delta stays
       at  a constant 300 and therefore the division of the two gives increasing
       values.

   Counter Wraps
     There are a few more basics to show. Some important options are still to be
     covered and we haven't look at counter wraps yet. First the  counter  wrap:
     In our car we notice that the counter shows 999987. We travel 20 km and the
     counter  should  go to 1000007. Unfortunately, there are only six digits on
     our counter so it really shows 000007. If we would plot that on a type  DE‐
     RIVE, it would mean that the counter was set back 999980 km. It wasn't, and
     there has to be some protection for this. This protection is only available
     for type COUNTER which should be used for this kind of counter anyways. How
     does it work? Type counter should never decrease and therefore RRDtool must
     assume  it wrapped if it does decrease!  If the delta is negative, this can
     be compensated for by adding the maximum value of the counter + 1. For  our
     car this would be:

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

      Real delta = -999980 + 999999 + 1 = 20

     At  the  time  of writing this document, RRDtool knows of counters that are
     either 32 bits or 64 bits of size. These counters can handle the  following
     different values:

      - 32 bits: 0 ..           4294967295
      - 64 bits: 0 .. 18446744073709551615

     If  these numbers look strange to you, you can view them in their hexadeci‐
     mal form:

      - 32 bits: 0 ..         FFFFFFFF
      - 64 bits: 0 .. FFFFFFFFFFFFFFFF

     RRDtool handles both counters the same. If an overflow occurs and the delta
     would be negative, RRDtool first adds the maximum of a small counter + 1 to
     the delta. If the delta is still negative, it had to be the  large  counter
     that  wrapped.  Add the maximum possible value of the large counter + 1 and
     subtract the erroneously added small value.

     There is a risk in this: suppose the large counter wrapped while  adding  a
     huge  delta,  it could happen, theoretically, that adding the smaller value
     would make the delta positive. In this unlikely case the results would  not
     be  correct.  The  increase should be nearly as high as the maximum counter
     value for that to happen, so chances are you would have several other prob‐
     lems as well and this particular problem would not even be  worth  thinking
     about.  Even  though,  I did include an example, so you can judge for your‐
     self.

     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 hap‐
     pen. However, SNMP or whatever method you choose to collect the data, might
     also report wrong numbers occasionally.  We can't prevent all  errors,  but
     there  are  some  things  we can do. The RRDtool "create" command takes two
     special parameters for this. They define the minimum  and  maximum  allowed
     values.  Until  now,  we used "U", meaning "unknown". If you provide values
     for one or both of them and if RRDtool receives data points that  are  out‐
     side  these  limits,  it will ignore those values. For a thermometer in de‐
     grees Celsius, the absolute minimum is just under -273. For  my  router,  I
     can  assume  this  minimum is much higher so I would set it to 10, where as
     the maximum temperature I would set to 80. Any higher and the device  would
     be out of order.

     For  the  speed of my car, I would never expect negative numbers and also I
     would not expect a speed  higher than 230. Anything else,  and  there  must
     have been an error. Remember: the opposite is not true, if the numbers pass
     this  check,  it doesn't mean that they are correct. Always judge the graph
     with a healthy dose of suspicion if it seems weird to you.

   Data Resampling
     One important feature of RRDtool has not been explained yet: it  is  virtu‐
     ally  impossible  to  collect data and feed it into RRDtool on exact inter‐
     vals. RRDtool therefore interpolates the data, so they are stored on  exact
     intervals.  If you do not know what this means or how it works, then here's
     the help you seek:

     Suppose a counter increases by exactly one for every second.  You  want  to
     measure  it  in  300 seconds intervals. You should retrieve values that are
     exactly 300 apart. However, due to various circumstances you are a few sec‐
     onds late and the interval is 303. The delta will also be 303 in that case.
     Obviously, RRDtool should not put 303 in the database and make you  believe
     that  the  counter  increased by 303 in 300 seconds.  This is where RRDtool
     interpolates: it alters the 303 value as if it would have been stored  ear‐
     lier  and  it will be 300 in 300 seconds.  Next time you are at exactly the
     right time. This means that the current interval is 297  seconds  and  also
     the counter increased by 297. Again, RRDtool interpolates and stores 300 as
     it should be.

           in the RRD                 in reality

      time+000:   0 delta="U"   time+000:    0 delta="U"
      time+300: 300 delta=300   time+300:  300 delta=300
      time+600: 600 delta=300   time+603:  603 delta=303
      time+900: 900 delta=300   time+900:  900 delta=297

     Let's  create two identical databases. I've chosen the time range 920805000
     to 920805900 as this goes very well with the example numbers.

        rrdtool create seconds1.rrd   \
           --start 920804700          \
           DS:seconds:COUNTER:600:U:U \
           RRA:AVERAGE:0.5:1:24

     Make a copy

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

     Put in some data

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

     Create output

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

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

WRAPUP
     It's  time  now to wrap up this tutorial. We covered all the basics for you
     to be able to work with RRDtool and to read  the  additional  documentation
     available. There is plenty more to discover about RRDtool and you will find
     more  and  more  uses  for this package. You can easily create graphs using
     just the examples provided and using only RRDtool. You can also use one  of
     the front ends to RRDtool that are available.

MAILINGLIST
     Remember  to subscribe to the RRDtool mailing list. Even if you are not an‐
     swering to mails that come by, it helps both you and the rest of the users.
     A lot of the stuff that I know about MRTG  (and  therefore  about  RRDtool)
     I've  learned  while just reading the list without posting to it. I did not
     need to ask the basic questions as they are answered in the FAQ (read  it!)
     and  in  various mails by other users. With thousands of users all over the
     world, there will always be people who ask questions that  you  can  answer
     because you read this and other documentation and they didn't.

SEE ALSO
     The RRDtool manpages

AUTHOR
     I  hope  you  enjoyed  the examples and their descriptions. If you do, help
     other people by pointing them to this document when they are  asking  basic
     questions. They will not only get their answers, but at the same time learn
     a whole lot more.

     Alex van den Bogaerdt <alex@vandenbogaerdt.nl>

1.10.0                             2026-05-25                     RRDTUTORIAL(1)
