cdeftutorial - Alex van den Bogaerdt's CDEF tutorial
Intention of this document: to provide some examples of the commonly used parts
of RRDtool's CDEF language.
If you think some important feature is not explained properly, and if adding it
to this document would benefit most users, please do ask me to add it. I will
then try to provide an answer in the next release of this tutorial. No
feedback equals no changes! Additions to this document are also welcome. --
Alex van den Bogaerdt <firstname.lastname@example.org>
One of the powerful parts of RRDtool is its ability to do all sorts of
calculations on the data retrieved from its databases. However, RRDtool's many
options and syntax make it difficult for the average user to understand. The
manuals 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 documentation, you may find
this document too simple or even boring. If you do choose to read this
tutorial, I also expect you to have read and fully understand my other
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.
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 consolidation
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:
Say you want to display bits per second (instead of bytes per second as stored
in the database.) You have to define a calculation (hence "CDEF") on
variable "inbytes" and use that variable (inbits) instead of the
This tells RRDtool to multiply inbytes by eight to get inbits. I'll explain
later how this works. In the graphing or printing functions, you can now use
inbits where you would use inbytes otherwise.
Note that the variable name used in the CDEF (inbits) must not be the same as
the variable named in the DEF (inbytes)!
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
Above multiplication by eight will look like:
- Start with an empty stack
- Put the content of variable inbytes on the stack
- Put the number eight on the stack
- Put the operation multiply on the stack
- Process the stack
- Retrieve the value from the stack and put it in variable
We will now do an example with real numbers. Suppose the variable inbytes would
have value 10, the stack would be:
Processing the stack (step 5) will retrieve one value from the stack (from the
right at step 4). This is the operation 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
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 position of "f", but you do not
reverse the order of the variables.
First, get a clear picture of what you want to do. Break down the problem in
smaller portions until they cannot be split anymore. Then it is rather simple
to convert your ideas into RPN.
Suppose you have several RRDs and would like to add up some counters in them.
These could be, for instance, the counters for every WAN link you are
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 mathematical function, this could be written:
"add(router1.rrd:link1in , router1.rrd:link2in , router2.rrd:link1in ,
router3.rrd:link1in , router3.rrd:link2.in)"
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 result.
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,+"
This is correct but it can be made more clear to humans. It does not matter if
you add a to b and then add c to the result or first add b to c and then add a
to the result. This makes it possible to rewrite the RPN into
"CDEF:result=a,b,c,d,e,+,+,+,+" which is evaluated differently:
push value of variable a on the stack: a
push value of variable b on the stack: a b
push value of variable c on the stack: a b c
push value of variable d on the stack: a b c d
push value of variable e on the stack: a b c d e
push operator + on the stack: a b c d e +
and process it: a b c P (where P == 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,+". This is called the commutative law of addition,
but you may forget this right away, as long as you remember what it means.
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 must not alter the
position 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 complaining, 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
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(s) 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 defined your RRD to contain an RRA
that stores 5-minute values, you need five of those PDPs to create one CDP
(consolidated data point). These PDPs can become unknown in two cases:
- The updates are too far apart. This is tuned using the
- The update was set to unknown on purpose by inserting no
value (using the template option) or by using "U" as the value
When a CDP is calculated, another mechanism determines if this CDP is valid or
not. If there are too many PDPs 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 unknown!
Suppose the counter increments with one per second and you retrieve it every
counter value resulting rate
10'060 1; (10'060-10'000)/60 == 1
10'120 1; (10'120-10'060)/60 == 1
unknown unknown; you don't know the last value
10'240 unknown; you don't know the previous value
10'300 1; (10'300-10'240)/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.
As you have read in the previous chapter, entries in an RRA can be set to the
unknown value. If you do calculations 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 were unknown (after all, it wasn't
installed yet!) but you know that the data rate through the device had to be
zero (because of the same reason: it was not installed).
There are some examples below that make this change.
Infinite data is another form of a special number. It cannot be graphed because
by definition you would never reach the infinite value. You can think of
positive and negative infinity 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 knowing 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.
Sometimes you would like to discard unknown data and pretend 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 examples available that show
unknown data by using infinity.
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 devices.
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 database.
- Alternatively, you could use CDEF and alter unknown data to
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 manually.
Implementing the second method is described next:
What we want is: "if the value is unknown, replace it with zero". This
could be written in pseudo-code as: if (value is unknown) then (zero) else
(value). When reading the rrdgraph manual you notice the "UN"
function that returns zero or one. You also notice the "IF" function
that takes zero or one as input.
First look at the "IF" function. It takes three values from the stack,
the first value is the decision point, the second value is returned to the
stack if the evaluation is "true" and if not, the third value is
returned to the stack. We want the "UN" function to decide what
happens so we combine those two functions in one CDEF.
Lets write down the two possible paths for the "IF" function:
if true return a
if false return b
In RPN: "result=x,a,b,IF" where "x" is either true or false.
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 finished:
You may want to read Steve Rader's RPN guide if you have difficulties with the
way I explained this last example.
If you want to check this RPN expression, just mimic RRDtool 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.
The 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
- If the unknown value is before time xxx, make it zero.
- If it is after time xxx, leave it as unknown data.
This is doable: you can compare the time that the sample was taken to some known
time. Assuming you started to monitor your device on Friday September 17,
1999, 00:35:57 MET DST. Translate this time in seconds since 1970-01-01 and it
becomes 937'521'357. If you process unknown values that were received after
this time, you want to leave them unknown and if they were
"received" before this time, you want to translate them into zero
(so you can effectively ignore them while adding them to your other routers
Translating Friday September 17, 1999, 00:35:57 MET DST into 937'521'357 can be
done by, for instance, using gnu date:
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
- If the timestamp of the value is after 937'521'357, leave
it as is.
- If the value is a known value, leave it as is.
- 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 current 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 thoroughly in the previous chapter so lets do
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:
This looks very complex, however, as you can see, it was not too hard to come up
Suppose you have a problem that shows up as huge spikes in your graph. You know
this happens and why, so you decide to work around the problem. Perhaps you're
using your network to do a backup at night and by doing so you get almost
10mb/s while the rest of your network activity does not produce numbers higher
There are two options:
- If the number exceeds 100kb/s it is wrong and you want it
masked out by changing it into unknown.
- 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 1'000 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 100kb/s.
The two CDEF expressions would be:
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
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
logical AND and logical OR.
For "*", the result will be zero (false) if either one of the two
operators is zero. For "+", the result will only be false (0) when
two false (0) operators will be added. Warning: *any* number not equal to 0
will be considered "true". This means that, for instance,
"-1,1,+" (which should be "true or true") will become
FALSE ... In other words, use "+" only if you know for sure that you
have positive numbers (or zero) only.
Let's compile the complete CDEF:
This will return the value of ds0 if both comparisons return true. You could
also do it the other way around:
This will return an UNKNOWN if either comparison returns true.
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
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
If you use stack in the previous example (as I would do) then you don't add up
the values. Therefore, there is no relationship between the four values and
you don't get a single value to test. Suppose users3 would be unknown at one
point in time: users1 is plotted, users2 is stacked on top of users1, users3
is unknown and therefore nothing happens, users4 is stacked on top of users2.
Add the extra CDEFs anyway and use them to overlay the "normal"
AREA:users1#0000FF:users at ts1
STACK:users2#00FF00:users at ts2
STACK:users3#00FFFF:users at ts3
STACK:users4#FFFF00:users at ts4
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 writing two
- It improves the readability of the script.
- It can be used inside GPRINT to display the total number of
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"
To convert Celsius into Fahrenheit use the formula F=9/5*C+32
rrdtool graph demo.png --title="Demo Graph" \
LINE2:cel#00a000:"D. Celsius" \
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:
1. push 9, push 5
2. push function "divide" and process it
the stack now contains 9/5
3. push variable "cel"
4. push function "multiply" and process it
the stack now contains 9/5*cel
5. push 32
6. push function "plus" and process it
the stack contains now the temperature in Fahrenheit
rrdtool graph demo.png --title="Demo Graph" \
AREA:agginput#00cc00:Input 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 original 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.
rrdtool graph example.png --title="INF demo" \
This demo demonstrates two ways to use infinity. It is a bit tricky to see what
happens in the "background" CDEF.
This RPN takes the value of "val4" as input and then immediately
removes it from the stack using "POP". The stack is now empty but as
a side effect we now know the time that this sample was taken. This time is
put on the stack by the "TIME" function.
"TIME,7200,%" takes the modulo of time and 7'200 (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
The second CDEF has been discussed earlier in this document so we won't do that
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
Next you draw the data on top of this background, 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:
You may do some complex data filtering:
MEDIAN FILTER: filters shot noise
This document was created from questions asked by either myself or by other
people on the RRDtool mailing list. Please let me know if you find errors in
it or if you have trouble understanding it. If you think there should be an
addition, mail me: <email@example.com>
Remember: No feedback equals no changes!
The RRDtool manpages
Alex van den Bogaerdt <firstname.lastname@example.org>