rrdtool CDEFTUTORIAL(1)
NAME
cdeftutorial - Alex van den Bogaerdt's CDEF tutorial
DESCRIPTION
Intention of this document: to provide some examples of the
commonly used parts of RDtool'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
Why this tutorial?
One of the powerful parts of RDtool is its ability to do
all sorts of calculations on the data retrieved from its
databases. However, RDtool'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 RDtool 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 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 RD, 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.
Syntax
DEF:varname1=some.rrd:dsname:CF
CDEF:varname2=RPNexpression
You first define "varname1" to be data collected from data
source "dsname" found in RD "some.rrd" with consolidation
function "CF".
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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:
DEF:inbytes=mrtg.rrd:in:AVERAGE
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 original:
CDEF:inbits=inbytes,8,*
This tells RDtool 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-expressions
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, RDtool 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 variable
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
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3. 108
4. 108*
5. 80
6.
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 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 position of
"f", but you do not reverse the order of the variables.
Converting your wishes to RPN
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 RDs and would like to add up some
counters in them. These could be, for instance, the counters
for every WAN link you are monitoring.
You have:
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 RD router1.rrd)
router1.rrd:link1in
router1.rrd:link2in
router2.rrd:link1in
router3.rrd:link1in
router3.rrd:link2in
-------------------- ]
(outcome of the sum)
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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 RDtool and RPN, first, define the inputs:
DEF:a=router1.rrd:link1in:AVERAGE
DEF:b=router1.rrd:link2in:AVERAGE
DEF:c=router2.rrd:link1in:AVERAGE
DEF:d=router3.rrd:link1in:AVERAGE
DEF:e=router3.rrd:link2in:AVERAGE
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)
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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 RDtool 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 RDtool. 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. RDtool
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 RD to contain an RA 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:
1. The updates are too far apart. This is tuned using the
"heartbeat" setting.
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2. The update was set to unknown on purpose by inserting no
value (using the template option) or by using "U" as the
value to insert.
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 minute:
counter value resulting rate
10000
10060 1; (10060-10000)/60 == 1
10120 1; (10120-10060)/60 == 1
unknown unknown; you dont know the last value
10240 unknown; you dont 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 RA.
Working with unknown data in your database
As you have read in the previous chapter, entries in an RA
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
RD 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)
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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.
Infinity
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.
RDtool 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 RDtool 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 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.
Some examples
Example: using a recently created RD
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:
]o While creating the new database, fill it with zeros from
the start to now. You have to make the database start
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at or before the least recent time in the other
database.
]o Alternatively, 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 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:
"CDEF:result=value,UN,0,value,IF"
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 RDtool
behavior:
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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 RD,
you may want to remove this rule so that unknown data is
properly displayed.
Example: better handling of unknown data, by using time
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:
]o If there is unknown data, look at the time that this
sample was taken.
]o If the unknown value is before time xxx, make it zero.
]o 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 counters).
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.
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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 937'521'357,
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
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 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:
"CDEF:result=TIME,937521357,GT,value,value,UN,0,value,IF,IF"
This looks very complex, however, as you can see, it was not
too hard to come up with.
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Example: Pretending weird data isn't there
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 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 RDtool 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:
CDEF:result=number,100000,GT,UNKN,number,IF
CDEF:result=number,100000,GT,100000,number,IF
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:
TIME,begintime,GE
TIME,endtime,LE
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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:
DEF:ds0=router1.rrd:AVERAGE
CDEF:ds0modified=TIME,begintime,GT,TIME,endtime,LE,*,ds0,UNKN,IF
This will return the value of ds0 if both comparisons return
true. You could also do it the other way around:
DEF:ds0=router1.rrd:AVERAGE
CDEF:ds0modified=TIME,begintime,LT,TIME,endtime,GT,],UNKN,ds0,IF
This will return an UNKNOWN if either comparison returns
true.
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:
DEF:users1=location1.rrd:onlineTS1:LAST
DEF:users2=location1.rrd:onlineTS2:LAST
DEF:users3=location2.rrd:onlineTS1:LAST
DEF:users4=location2.rrd:onlineTS2:LAST
CDEF:allusers=users1,users2,users3,users4,],],]
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.
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Define an extra CDEF that is unknown if all is okay and is
infinite if there is an unknown value:
CDEF:wrongdata=allusers,UN,INF,UNKN,IF
"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#0000F:combined user count
AREA:wrongdata#F0000:unknown data
Same example useful with STACKed data:
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" graph:
DEF:users1=location1.rrd:onlineTS1:LAST
DEF:users2=location1.rrd:onlineTS2:LAST
DEF:users3=location2.rrd:onlineTS1:LAST
DEF:users4=location2.rrd:onlineTS2:LAST
CDEF:allusers=users1,users2,users3,users4,],],]
CDEF:wrongdata=allusers,UN,INF,UNKN,IF
AREA:users1#0000F:users at ts1
STACK:users2#00F00:users at ts2
STACK:users3#00F:users at ts3
STACK:users4#F00:users at ts4
AREA:wrongdata#F0000: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.
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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 CDEFS:
]o It improves the readability of the script.
]o It can be used inside GPRINT to display the total number
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:
CDEF:wrongdata=users1,users2,users3,users4,],],],UN,INF,UNKN,IF
If you do so, you won't be able to use these next GPRINTs:
COMENT:"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 RD graph manual page
Degrees Celsius vs. Degrees Fahrenheit
To convert Celsius into Fahrenheit use the formula
F=9/5*C]32
rrdtool graph demo.png --title="Demo Graph" \
DEF:cel=demo.rrd:exhaust:AVERAGE \
CDEF:far=9,5,/,cel,*,32,] \
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:
CDEF:far=9,5,/,cel,*,32,]
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
1.3.5 Last change: 2008-10-30 14
rrdtool CDEFTUTORIAL(1)
Changing unknown into zero
rrdtool graph demo.png --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#0000F: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 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.
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#0000F:Value1 \
STACK:val2#00C000:Value2 \
STACK:val3#F00:Value3 \
STACK:val4#FC000:Value4 \
AREA:whipeout#F0000:Unknown
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rrdtool CDEFTUTORIAL(1)
This demo demonstrates two ways to use infinity. It is a bit
tricky to see what happens in the "background" CDEF.
"val4,POP,TIME,7200,%,3600,LE,INF,UNKN,IF"
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 "background".
The second CDEF has been discussed earlier in this document
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 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:
"CDEF:wipeout2=wipeout,-1,*"
1.3.5 Last change: 2008-10-30 16
rrdtool CDEFTUTORIAL(1)
Filtering data
You may do some complex data filtering:
MEDIAN FILTER: filters shot noise
DEF:var=database.rrd:traffic:AVERAGE
CDEF:prev1=PREV(var)
CDEF:prev2=PREV(prev1)
CDEF:prev3=PREV(prev2)
CDEF:median=prev1,prev2,prev3,],],3,/
LINE3:median#000077:filtered
LINE1:prev2#007700:raw data
DERIVATE:
DEF:var=database.rrd:traffic:AVERAGE
CDEF:prev1=PREV(var)
CDEF:time=TIME
CDEF:prevtime=PREV(time)
CDEF:derivate=var,prev1,-,time,prevtime,-,/
LINE3:derivate#000077:derivate
LINE1:var#007700:raw data
Out of ideas for now
This document was created from questions asked by either
myself or by other people on the RDtool 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:
Remember: No feedback equals no changes!
SEE ALSO
The RDtool manpages
AUTHOR
Alex van den Bogaerdt
1.3.5 Last change: 2008-10-30 17
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