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       You provide a question and I will try to provide an answer
       in the next release. No feedback equals no changes!

       Additions to this document are also welcome.

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

       Why this tutorial ?

       One of the powerful parts of RRDtool is its ability to do
       all sorts of calculations on the data retrieved from it's
       databases. However RRDtool's many options and syntax make
       it difficult for the average user to understand. The manu­
       als are good at explaining what these options do; however
       they do not (and should not) explain in detail why they
       are useful. As with my RRDtool tutorial: if you want a
       simple document in simple language you should read this
       tutorial.  If you are happy with the official documenta­
       tion, you may find this document too simple or even bor­
       ing. If you do choose to read this tutorial, I also expect
       you to have read and fully understand my other tutorial.

       More reading

       If you have difficulties with the way I try to explain it
       please read Steve Rader's rpntutorial. It may help you
       understand how this all works.

What are CDEFs ?

       When retrieving data from an RRD, you are using a "DEF" to
       work with that data. Think of it as a variable that
       changes over time (where time is the x-axis). The value of
       this variable is what is found in the database at that
       particular time and you can't do any modifications on it.
       This is what CDEFs are for: they takes values from DEFs
       and perform calculations on them.



       You first define "var_name_1" to be data collected from
       data source "ds_name" found in RRD "some.rrd" with consol­
       idation function "CF".

       Assume the ifInOctets SNMP counter is saved in mrtg.rrd as
       the DS "in".  Then the following DEF defines a variable
       for the average of that data source:



       RPN is short-hand for Reverse Polish Notation. It works as
       follows.  You put the variables or numbers on a stack. You
       also put operations (things-to-do) on the stack and this
       stack is then processed. The result will be placed on the
       stack. At the end, there should be exactly one number
       left: the outcome of the series of operations. If there is
       not exactly one number left, rrdtool will complain loudly.

       Above multiplication by eight will look like:

       1.  Start with an empty stack

       2.  Put the content of variable inbytes on the stack

       3.  Put the number eight on the stack

       4.  Put the operation multiply on the stack

       5.  Process the stack

       6.  Retrieve the value from the stack and put it in vari­
           able inbits

       We will now do an example with real numbers. Suppose the
       variable inbytes would have value 10, the stack would be:

       1.  ||

       2.  |10|

       3.  |10|8|

       4.  |10|8|*|

       5.  |80|

       6.  ||

       Processing the stack (step 5) will retrieve one value from
       the stack (from the right at step 4). This is the opera­
       tion multiply and this takes two values off the stack as
       input. The result is put back on the stack (the value 80
       in this case). For multiplication the order doesn't matter
       but for other operations like subtraction and division it
       does.  Generally speaking you have the following order:

          y = A - B  -->  y=minus(A,B)  -->  CDEF:y=A,B,-

       This is not very intuitive (at least most people don't
       think so). For the function f(A,B) you reverse the posi­

          router1.rrd with link1in link2in
          router2.rrd with link1in link2in
          router3.rrd with link1in link2in

       Suppose you would like to add up all these counters,
       except for link2in inside router2.rrd. You need to do:

       (in this example, "router1.rrd:link1in" means the DS
       link1in inside the RRD router1.rrd)

          --------------------   +
          (outcome of the sum)

       As a mathmatical function, this could be written:

       "add(router1.rrd:link1in , router1.rrd:link2in ,
       router2.rrd:link1in , router3.rrd:link1in ,

       With RRDtool and RPN, first, define the inputs:


       Now, the mathematical function becomes: "add(a,b,c,d,e)"

       In RPN, there's no operator that sums more than two values
       so you need to do several additions. You add a and b, add
       c to the result, add d to the result and add e to the

          push a:         a     stack contains the value of a
          push b and add: b,+   stack contains the result of a+b
          push c and add: c,+   stack contains the result of a+b+c
          push d and add: d,+   stack contains the result of a+b+c+d
          push e and add: e,+   stack contains the result of a+b+c+d+e

       What was calculated here would be written down as:

          ( ( ( (a+b) + c) + d) + e) >

       This is in RPN:  "CDEF:result=a,b,+,c,+,d,+,e,+"

          push operator + on the stack:          a b c P +
          and process it:                        a b Q     (where Q == c+P)
          push operator + on the stack:          a b Q +
          and process it:                        a R       (where R == b+Q)
          push operator + on the stack:          a R +
          and process it:                        S         (where S == a+R)

       As you can see the RPN expression "a,b,c,d,e,+,+,+,+,+"
       will evaluate in "((((d+e)+c)+b)+a)" and it has the same
       outcome as "a,b,+,c,+,d,+,e,+" According to Steve Rader
       this is called the commutative law of addition but you may
       forget this right away, as long as you remember what it

       Now look at an expression that contains a multiplication:

       First in normal math: "let result = a+b*c". In this case
       you can't choose the order yourself, you have to start
       with the multiplication and then add a to it. You may
       alter the position of b and c, you may not alter the posi­
       tion of a and b.

       You have to take this in consideration when converting
       this expression into RPN. Read it as: "Add the outcome of
       b*c to a" and then it is easy to write the RPN expression:
       "result=a,b,c,*,+" Another expression that would return
       the same: "result=b,c,*,a,+"

       In normal math, you may encounter something like "a*(b+c)"
       and this can also be converted into RPN. The parenthesis
       just tell you to first add b and c, and then multiply a
       with the result. Again, now it is easy to write it in RPN:
       "result=a,b,c,+,*". Note that this is very similar to one
       of the expressions in the previous paragraph, only the
       multiplication and the addition changed places.

       When you have problems with RPN or when rrdtool is com­
       plaining, it's usually a Good Thing to write down the
       stack on a piece of paper and see what happens. Have the
       manual ready and pretend to be rrdtool.  Just do all the
       math by hand to see what happens, I'm sure this will solve
       most, if not all, problems you encounter.

Some special numbers

       The unknown value

       Sometimes collecting your data will fail. This can be very
       common, especially when querying over busy links. RRDtool
       can be configured to allow for one (or even more) unknown
       value and calculate the missing update. You can, for
       instance, query your device every minute. This is creating
       one so called PDP or primary data point per minute. If you
       unknown, the CDP is unknown as well.  This is determined
       by the xff factor. Please note that one unknown counter
       update can result in two unknown PDPs! If you only allow
       for one unknown PDP per CDP, this makes the CDP go

       Suppose the counter increments with one per second and you
       retrieve it every minute:

          counter value    resulting rate
          10060            1; (10060-10000)/60 == 1
          10120            1; (10120-10060)/60 == 1
          unknown          unknown; you don't know the last value
          10240            unknown; you don't know the previous value
          10300            1; (10300-10240)/60 == 1

       If the CDP was to be calculated from the last five
       updates, it would get two unknown PDPs and three known
       PDPs. If xff would have been set to 0.5 which by the way
       is a commonly used factor, the CDP would have a known
       value of 1. If xff would have been set to 0.2 then the
       resulting CDP would be unknown.

       You have to decide the proper values for heartbeat, number
       of PDPs per CDP and the xff factor. As you can see from
       the previous text they define the behavior of your RRA.

       Working with unknown data in your database

       As you have read in the previous chapter, entries in an
       RRA can be set to the unknown value. If you do calcula­
       tions with this type of value, the result has to be
       unknown too. This means that an expression such as
       "result=a,b,+" will be unknown if either a or b is
       unknown.  It would be wrong to just ignore the unknown
       value and return the value of the other parameter. By
       doing so, you would assume "unknown" means "zero" and this
       is not true.

       There has been a case where somebody was collecting data
       for over a year.  A new piece of equipment was installed,
       a new RRD was created and the scripts were changed to add
       a counter from the old database and a counter from the new
       database. The result was disappointing, a large part of
       the statistics seemed to have vanished mysteriously ...
       They of course didn't, values from the old database (known
       values) were added to values from the new database
       (unknown values) and the result was unknown.

       In this case, it is fairly reasonable to use a CDEF that
       alters unknown data into zero. The counters of the device
       depending on the position relative to zero.

       RRDtool is capable of representing (-not- graphing!)
       infinity by stopping at its current maximum (for positive
       infinity) or minimum (for negative infinity) without know­
       ing this maximum (minimum).

       Infinity in rrdtool is mostly used to draw an AREA without
       knowing its vertical dimensions. You can think of it as
       drawing an AREA with an infinite height and displaying
       only the part that is visible in the current graph. This
       is probably a good way to approximate infinity and it sure
       allows for some neat tricks. See below for examples.

       Working with unknown data and infinity

       Sometimes you would like to discard unknown data and pre­
       tend it is zero (or any other value for that matter) and
       sometimes you would like to pretend that known data is
       unknown (to discard known-to-be-wrong data).  This is why
       CDEFs have support for unknown data. There are also exam­
       ples available that show unknown data by using infinity.

Some examples

       Example: using a recently created RRD

       You are keeping statistics on your router for over a year
       now. Recently you installed an extra router and you would
       like to show the combined throughput for these two

       If you just add up the counters from router.rrd and
       router2.rrd, you will add known data (from router.rrd) to
       unknown data (from router2.rrd) for the bigger part of
       your stats. You could solve this in a few ways:

       ·   While creating the new database, fill it with zeros
           from the start to now.  You have to make the database
           start at or before the least recent time in the other

       ·   Alternately you could use CDEF and alter unknown data
           to zero.

       Both methods have their pros and cons. The first method is
       troublesome and if you want to do that you have to figure
       it out yourself. It is not possible to create a database
       filled with zeros, you have to put them in on purpose.
       Implementing the second method is described next:

       What we want is: "if the value is unknown, replace it with
       zero". This could be writte in pseudo-code as:  if (value

          if true  return a
          if false return b

       In RPN:  "result=x,a,b,IF" where "x" is either true or

       Now we have to fill in "x", this should be the "(value is
       unknown)" part and this is in RPN:  "result=value,UN"

       We now combine them: "result=value,UN,a,b,IF" and when we
       fill in the appropriate things for "a" and "b" we're fin­


       You may want to read Steve Raders RPN guide if you have
       difficulties with the way I explained this last example.

       If you want to check this RPN expression, just mimic rrd­
       tools behavior:

          For any known value, the expression evaluates as follows:
          CDEF:result=value,UN,0,value,IF  (value,UN) is not true so it becomes 0
          CDEF:result=0,0,value,IF         "IF" will return the 3rd value
          CDEF:result=value                The known value is returned

          For the unknown value, this happens:
          CDEF:result=value,UN,0,value,IF  (value,UN) is true so it becomes 1
          CDEF:result=1,0,value,IF         "IF" sees 1 and returns the 2nd value
          CDEF:result=0                    Zero is returned

       Of course, if you would like to see another value instead
       of zero, you can use that other value.

       Eventually, when all unknown data is removed from the RRD,
       you may want to remove this rule so that unknown data is
       properly displayed.

       Example: better handling of unknown data, by using time

       Above example has one drawback. If you do log unknown data
       in your database after installing your new equipment, it
       will also be translated into zero and therefore you won't
       see that there was a problem. This is not good and what
       you really want to do is:

       ·   If there is unknown data, look at the time that this
           sample was taken

       ·   If the unknown value is before time xxx, make it zero

          date -d "19990917 00:35:57" +%s

       You could also dump the database and see where the data
       starts to be known. There are several other ways of doing
       this, just pick one.

       Now we have to create the magic that allows us to process
       unknown values different depending on the time that the
       sample was taken.  This is a three step process:

       1.  If the timestamp of the value is after 937521357,
           leave it as is

       2.  If the value is a known value, leave it as is

       3.  Change the unknown value into zero.

       Lets look at part one:

           if (true) return the original value

       We rewrite this:

           if (true) return "a"
           if (false) return "b"

       We need to calculate true or false from step 1. There is a
       function available that returns the timestamp for the cur­
       rent sample. It is called, how surprisingly, "TIME". This
       time has to be compared to a constant number, we need
       "GT". The output of "GT" is true or false and this is good
       input to "IF". We want "if (time > 937521357) then (return
       a) else (return b)".

       This process was already described toroughly in the previ­
       ous chapter so lets do it quick:

          if (x) then a else b
             where x represents "time>937521357"
             where a represents the original value
             where b represents the outcome of the previous example

          time>937521357       --> TIME,937521357,GT

          if (x) then a else b --> x,a,b,IF
          substitute x         --> TIME,937521357,GT,a,b,IF
          substitute a         --> TIME,937521357,GT,value,b,IF
          substitute b         --> TIME,937521357,GT,value,value,UN,0,value,IF,IF

       We end up with:
       almost 10mb/s while the rest of your network activity does
       not produce numbers higher than 100kb/s.

       There are two options:

       1.  If the number exceeds 100kb/s it is wrong and you want
           it masked out by changing it into unknown

       2.  You don't want the graph to show more than 100kb/s

       Pseudo code: if (number > 100) then unknown else number or
       Pseudo code: if (number > 100) then 100 else number.

       The second "problem" may also be solved by using the rigid
       option of rrdtool graph, however this has not the same
       result. In this example you can end up with a graph that
       does autoscaling. Also, if you use the numbers to display
       maxima they will be set to 100kb/s.

       We use "IF" and "GT" again. "if (x) then (y) else (z)" is
       written down as "CDEF:result=x,y,z,IF"; now fill in x, y
       and z.  For x you fill in "number greater than 100kb/s"
       becoming "number,100000,GT" (kilo is 1000 and b/s is what
       we measure!).  The "z" part is "number" in both cases and
       the "y" part is either "UNKN" for unknown or "100000" for

       The two CDEF expressions would be:


       Example: working on a certain time span

       If you want a graph that spans a few weeks, but would only
       want to see some routers data for one week, you need to
       "hide" the rest of the time frame. Don't ask me when this
       would be useful, it's just here for the example :)

       We need to compare the time stamp to a begin date and an
       end date.  Comparing isn't difficult:


       These two parts of the CDEF produce either 0 for false or
       1 for true.  We can now check if they are both 0 (or 1)
       using a few IF statements but, as Wataru Satoh pointed
       out, we can use the "*" or "+" functions as locical AND
       and locical OR.

       For "*", the result will be zero (false) if either one of
       return true. You could also do it the other way around:


       This will return an UNKNOWN if either comparison returns

       Example: You suspect to have problems and want to see
       unknown data.

       Suppose you add up the number of active users on several
       terminal servers.  If one of them doesn't give an answer
       (or an incorrect one) you get "NaN" in the database ("Not
       a Number") and NaN is evaluated as Unknown.

       In this case, you would like to be alerted to it and the
       sum of the remaining values is of no value to you.

       It would be something like:


       If you now plot allusers, unknown data in one of
       users1..users4 will show up as a gap in your graph. You
       want to modify this to show a bright red line, not a gap.

       Define an extra CDEF that is unknown if all is okay and is
       infinite if there is an unknown value:


       "allusers,UN" will evaluate to either true or false, it is
       the (x) part of the "IF" function and it checks if
       allusers is unknown.  The (y) part of the "IF" function is
       set to "INF" (which means infinity) and the (z) part of
       the function returns "UNKN".

       The logic is: if (allusers == unknown) then return INF
       else return UNKN.

       You can now use AREA to display this "wrongdata" in bright
       red. If it is unknown (because allusers is known) then the
       red AREA won't show up.  If the value is INF (because
       allusers is unknown) then the red AREA will be filled in
       on the graph at that particular time.

          AREA:allusers#0000FF:combined user count
          AREA:users1#0000FF:users at ts1
          STACK:users2#00FF00:users at ts2
          STACK:users3#00FFFF:users at ts3
          STACK:users4#FFFF00:users at ts4
          AREA:wrongdata#FF0000:unknown data

       If there is unknown data in one of users1..users4, the
       "wrongdata" AREA will be drawn and because it starts at
       the X-axis and has infinite height it will effectively
       overwrite the STACKed parts.

       You could combine the two CDEF lines into one (we don't
       use "allusers") if you like.  But there are good reasons
       for writting two CDEFS:

       ·   It improves the readability of the script

       ·   It can be used inside GPRINT to display the total num­
           ber of users

       If you choose to combine them, you can substitute the
       "allusers" in the second CDEF with the part after the
       equal sign from the first line:


       If you do so, you won't be able to use these next GPRINTs:

          COMMENT:"Total number of users seen"
          GPRINT:allusers:MAX:"Maximum: %6.0lf"
          GPRINT:allusers:MIN:"Minimum: %6.0lf"
          GPRINT:allusers:AVERAGE:"Average: %6.0lf"
          GPRINT:allusers:LAST:"Current: %6.0lf\n"

The examples from the rrd graph manual page

       Degrees Celcius vs. Degrees Fahrenheit

          rrdtool graph demo.gif --title="Demo Graph" \
             DEF:cel=demo.rrd:exhaust:AVERAGE \
             CDEF:far=cel,32,-,0.55555,* \
             LINE2:cel#00a000:"D. Celsius" \
             LINE2:far#ff0000:"D. Fahrenheit\c"

       This example gets the DS called "exhaust" from database
       "demo.rrd" and puts the values in variable "cel". The CDEF
       used is evaluated as follows:
          6. the resulting value is now "(cel-32)*0.55555"

       Note that if you take the celcius to fahrenheit function
       you should be doing "5/9*(cel-32)" so 0.55555 is not
       exactly correct. It is close enough for this purpose and
       it saves a calculation.

       Changing unknown into zero

          rrdtool graph demo.gif --title="Demo Graph" \
             DEF:idat1=interface1.rrd:ds0:AVERAGE \
             DEF:idat2=interface2.rrd:ds0:AVERAGE \
             DEF:odat1=interface1.rrd:ds1:AVERAGE \
             DEF:odat2=interface2.rrd:ds1:AVERAGE \
             CDEF:agginput=idat1,UN,0,idat1,IF,idat2,UN,0,idat2,IF,+,8,* \
             CDEF:aggoutput=odat1,UN,0,odat1,IF,odat2,UN,0,odat2,IF,+,8,* \
             AREA:agginput#00cc00:Input Aggregate \
             LINE1:aggoutput#0000FF:Output Aggregate

       These two CDEFs are built from several functions. It helps
       to split them when viewing what they do.  Starting with
       the first CDEF we would get:
             idat1,UN --> a
             0        --> b
             idat1    --> c
             if (a) then (b) else (c) The result is therefore "0"
       if it is true that "idat1" equals "UN".  If not, the orig­
       inal value of "idat1" is put back on the stack.  Lets call
       this answer "d". The process is repeated for the next five
       items on the stack, it is done the same and will return
       answer "h". The resulting stack is therefore "d,h".  The
       expression has been simplified to "d,h,+,8,*" and it will
       now be easy to see that we add "d" and "h", and multiply
       the result with eight.

       The end result is that we have added "idat1" and "idat2"
       and in the process we effectively ignored unknown values.
       The result is multiplied by eight, most likely to convert
       bytes/s to bits/s.

       Infinity demo

          rrdtool graph example.png --title="INF demo" \
             DEF:val1=some.rrd:ds0:AVERAGE \
             DEF:val2=some.rrd:ds1:AVERAGE \
             DEF:val3=some.rrd:ds2:AVERAGE \
             DEF:val4=other.rrd:ds0:AVERAGE \
             CDEF:background=val4,POP,TIME,7200,%,3600,LE,INF,UNKN,IF \
             CDEF:wipeout=val1,val2,val3,val4,+,+,+,UN,INF,UNKN,IF \
             AREA:background#F0F0F0 \
             AREA:val1#0000FF:Value1 \
             STACK:val2#00C000:Value2 \

       "TIME,7200,%" takes the modulo of time and 7200 (which is
       two hours).  The resulting value on the stack will be a
       number in the range from 0 to 7199.

       For people who don't know the modulo function: it is the
       remainder after an integer division. If you divide 16 by
       3, the answer would be 5 and the remainder would be 1. So,
       "16,3,%" returns 1.

       We have the result of "TIME,7200,%" on the stack, lets
       call this "a". The start of the RPN has become "a,3600,LE"
       and this checks if "a" is less or equal than "3600". It is
       true half of the time.  We now have to process the rest of
       the RPN and this is only a simple "IF" function that
       returns either "INF" or "UNKN" depending on the time. This
       is returned to variable "background".

       The second CDEF has been discussed earlyer in this docu­
       ment so we won't do that here.

       Now you can draw the different layers. Start with the
       background that is either unknown (nothing to see) or
       infinite (the whole positive part of the graph gets
       filled).  Next you draw the data on top of this back­
       ground. It will overlay the background. Suppose one of
       val1..val4 would be unknown, in that case you end up with
       only three bars stacked on top of each other.  You don't
       want to see this because the data is only valid when all
       four variables are valid. This is why you use the second
       CDEF, it will overlay the data with an AREA so the data
       cannot be seen anymore.

       If your data can also have negative values you also need
       to overwrite the other half of your graph. This can be
       done in a relatively simple way: what you need is the
       "wipeout" variable and place a negative sign before it:

       Filtering data

       You may do some complex data filtering:

         MEDIAN FILTER: filters shot noise

           LINE1:prev2#007700:'raw data'

Out of ideas for now

       This document was created from questions asked by either
       myself or by other people on the list. Please let me know
       if you find errors in it or if you have trouble under­
       standing it. If you think there should be an addition,
       mail me: <alex@ergens.op.het.net>

       Remember: No feedback equals no changes!


       The RRDtool manpages


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

1.0.45                      2002-06-20            CDEFTUTORIAL(1)
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