1CDEFTUTORIAL(1) rrdtool CDEFTUTORIAL(1)
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6 cdeftutorial - Alex van den Bogaerdt's CDEF tutorial
7
9 Intention of this document: to provide some examples of the commonly
10 used parts of RRDtool's CDEF language.
11 If you think some important feature is not explained properly, and if
13 adding it to this document would benefit most users, please do ask me
14 to add it. I will then try to provide an answer in the next release of
15 this tutorial. No feedback equals no changes! Additions to this docu‐
16 ment are also welcome. -- Alex van den Bogaerdt
17 <alex@ergens.op.het.net>
18 Why this tutorial?
20 One of the powerful parts of RRDtool is its ability to do all sorts of
22 calculations on the data retrieved from its databases. However, RRD‐
23 tool's many options and syntax make it difficult for the average user
24 to understand. The manuals are good at explaining what these options
25 do; however they do not (and should not) explain in detail why they are
26 useful. As with my RRDtool tutorial: if you want a simple document in
27 simple language you should read this tutorial. If you are happy with
28 the official documentation, you may find this document too simple or
29 even boring. If you do choose to read this tutorial, I also expect you
30 to have read and fully understand my other tutorial.
31 More reading
33 If you have difficulties with the way I try to explain it please read
35 Steve Rader's rpntutorial. It may help you understand how this all
36 works.
37
39 When retrieving data from an RRD, you are using a "DEF" to work with
40 that data. Think of it as a variable that changes over time (where time
41 is the x-axis). The value of this variable is what is found in the
42 database at that particular time and you can't do any modifications on
43 it. This is what CDEFs are for: they takes values from DEFs and perform
44 calculations on them.
45
47 DEF:var_name_1=some.rrd:ds_name:CF
48 CDEF:var_name_2=RPN_expression
49 You first define "var_name_1" to be data collected from data source
51 "ds_name" found in RRD "some.rrd" with consolidation function "CF".
52 Assume the ifInOctets SNMP counter is saved in mrtg.rrd as the DS "in".
54 Then the following DEF defines a variable for the average of that data
55 source:
56 DEF:inbytes=mrtg.rrd:in:AVERAGE
58 Say you want to display bits per second (instead of bytes per second as
60 stored in the database.) You have to define a calculation (hence
61 "CDEF") on variable "inbytes" and use that variable (inbits) instead of
62 the original:
63 CDEF:inbits=inbytes,8,*
65 This tells RRDtool to multiply inbytes by eight to get inbits. I'll
67 explain later how this works. In the graphing or printing functions,
68 you can now use inbits where you would use inbytes otherwise.
69 Note that the variable name used in the CDEF (inbits) must not be the
71 same as the variable named in the DEF (inbytes)!
72
74 RPN is short-hand for Reverse Polish Notation. It works as follows.
75 You put the variables or numbers on a stack. You also put operations
76 (things-to-do) on the stack and this stack is then processed. The
77 result will be placed on the stack. At the end, there should be exactly
78 one number left: the outcome of the series of operations. If there is
79 not exactly one number left, RRDtool will complain loudly.
80 Above multiplication by eight will look like:
82 1. Start with an empty stack
84 2. Put the content of variable inbytes on the stack
86 3. Put the number eight on the stack
88 4. Put the operation multiply on the stack
90 5. Process the stack
92 6. Retrieve the value from the stack and put it in variable inbits
94 We will now do an example with real numbers. Suppose the variable
96 inbytes would have value 10, the stack would be:
97 1. ||
99 2. |10|
101 3. |10|8|
103 4. |10|8|*|
105 5. |80|
107 6. ||
109 Processing the stack (step 5) will retrieve one value from the stack
111 (from the right at step 4). This is the operation multiply and this
112 takes two values off the stack as input. The result is put back on the
113 stack (the value 80 in this case). For multiplication the order doesn't
114 matter, but for other operations like subtraction and division it does.
115 Generally speaking you have the following order:
116 y = A - B --> y=minus(A,B) --> CDEF:y=A,B,-
118 This is not very intuitive (at least most people don't think so). For
120 the function f(A,B) you reverse the position of "f", but you do not
121 reverse the order of the variables.
122
124 First, get a clear picture of what you want to do. Break down the prob‐
125 lem in smaller portions until they cannot be split anymore. Then it is
126 rather simple to convert your ideas into RPN.
127 Suppose you have several RRDs and would like to add up some counters in
129 them. These could be, for instance, the counters for every WAN link you
130 are monitoring.
131 You have:
133 router1.rrd with link1in link2in
135 router2.rrd with link1in link2in
136 router3.rrd with link1in link2in
137 Suppose you would like to add up all these counters, except for link2in
139 inside router2.rrd. You need to do:
140 (in this example, "router1.rrd:link1in" means the DS link1in inside the
142 RRD router1.rrd)
143 router1.rrd:link1in
145 router1.rrd:link2in
146 router2.rrd:link1in
147 router3.rrd:link1in
148 router3.rrd:link2in
149 -------------------- +
150 (outcome of the sum)
151 As a mathematical function, this could be written:
153 "add(router1.rrd:link1in , router1.rrd:link2in , router2.rrd:link1in ,
155 router3.rrd:link1in , router3.rrd:link2.in)"
156 With RRDtool and RPN, first, define the inputs:
158 DEF:a=router1.rrd:link1in:AVERAGE
160 DEF:b=router1.rrd:link2in:AVERAGE
161 DEF:c=router2.rrd:link1in:AVERAGE
162 DEF:d=router3.rrd:link1in:AVERAGE
163 DEF:e=router3.rrd:link2in:AVERAGE
164 Now, the mathematical function becomes: "add(a,b,c,d,e)"
166 In RPN, there's no operator that sums more than two values so you need
168 to do several additions. You add a and b, add c to the result, add d to
169 the result and add e to the result.
170 push a: a stack contains the value of a
172 push b and add: b,+ stack contains the result of a+b
173 push c and add: c,+ stack contains the result of a+b+c
174 push d and add: d,+ stack contains the result of a+b+c+d
175 push e and add: e,+ stack contains the result of a+b+c+d+e
176 What was calculated here would be written down as:
178 ( ( ( (a+b) + c) + d) + e) >
180 This is in RPN: "CDEF:result=a,b,+,c,+,d,+,e,+"
182 This is correct but it can be made more clear to humans. It does not
184 matter if you add a to b and then add c to the result or first add b to
185 c and then add a to the result. This makes it possible to rewrite the
186 RPN into "CDEF:result=a,b,c,d,e,+,+,+,+" which is evaluated differ‐
187 ently:
188 push value of variable a on the stack: a
190 push value of variable b on the stack: a b
191 push value of variable c on the stack: a b c
192 push value of variable d on the stack: a b c d
193 push value of variable e on the stack: a b c d e
194 push operator + on the stack: a b c d e +
195 and process it: a b c P (where P == d+e)
196 push operator + on the stack: a b c P +
197 and process it: a b Q (where Q == c+P)
198 push operator + on the stack: a b Q +
199 and process it: a R (where R == b+Q)
200 push operator + on the stack: a R +
201 and process it: S (where S == a+R)
202 As you can see the RPN expression "a,b,c,d,e,+,+,+,+,+" will evaluate
204 in "((((d+e)+c)+b)+a)" and it has the same outcome as
205 "a,b,+,c,+,d,+,e,+". This is called the commutative law of addition,
206 but you may forget this right away, as long as you remember what it
207 means.
208 Now look at an expression that contains a multiplication:
210 First in normal math: "let result = a+b*c". In this case you can't
212 choose the order yourself, you have to start with the multiplication
213 and then add a to it. You may alter the position of b and c, you must
214 not alter the position of a and b.
215 You have to take this in consideration when converting this expression
217 into RPN. Read it as: "Add the outcome of b*c to a" and then it is easy
218 to write the RPN expression: "result=a,b,c,*,+" Another expression that
219 would return the same: "result=b,c,*,a,+"
220 In normal math, you may encounter something like "a*(b+c)" and this can
222 also be converted into RPN. The parenthesis just tell you to first add
223 b and c, and then multiply a with the result. Again, now it is easy to
224 write it in RPN: "result=a,b,c,+,*". Note that this is very similar to
225 one of the expressions in the previous paragraph, only the multiplica‐
226 tion and the addition changed places.
227 When you have problems with RPN or when RRDtool is complaining, it's
229 usually a good thing to write down the stack on a piece of paper and
230 see what happens. Have the manual ready and pretend to be RRDtool.
231 Just do all the math by hand to see what happens, I'm sure this will
232 solve most, if not all, problems you encounter.
233
235 The unknown value
236 Sometimes collecting your data will fail. This can be very common,
238 especially when querying over busy links. RRDtool can be configured to
239 allow for one (or even more) unknown value(s) and calculate the missing
240 update. You can, for instance, query your device every minute. This is
241 creating one so called PDP or primary data point per minute. If you
242 defined your RRD to contain an RRA that stores 5-minute values, you
243 need five of those PDPs to create one CDP (consolidated data point).
244 These PDPs can become unknown in two cases:
245 1. The updates are too far apart. This is tuned using the "heartbeat"
247 setting.
248 2. The update was set to unknown on purpose by inserting no value
250 (using the template option) or by using "U" as the value to insert.
251 When a CDP is calculated, another mechanism determines if this CDP is
253 valid or not. If there are too many PDPs unknown, the CDP is unknown as
254 well. This is determined by the xff factor. Please note that one
255 unknown counter update can result in two unknown PDPs! If you only
256 allow for one unknown PDP per CDP, this makes the CDP go unknown!
257 Suppose the counter increments with one per second and you retrieve it
259 every minute:
260 counter value resulting rate
262 10'000
263 10'060 1; (10'060-10'000)/60 == 1
264 10'120 1; (10'120-10'060)/60 == 1
265 unknown unknown; you don't know the last value
266 10'240 unknown; you don't know the previous value
267 10'300 1; (10'300-10'240)/60 == 1
268 If the CDP was to be calculated from the last five updates, it would
270 get two unknown PDPs and three known PDPs. If xff would have been set
271 to 0.5 which by the way is a commonly used factor, the CDP would have a
272 known value of 1. If xff would have been set to 0.2 then the resulting
273 CDP would be unknown.
274 You have to decide the proper values for heartbeat, number of PDPs per
276 CDP and the xff factor. As you can see from the previous text they
277 define the behavior of your RRA.
278 Working with unknown data in your database
280 As you have read in the previous chapter, entries in an RRA can be set
282 to the unknown value. If you do calculations with this type of value,
283 the result has to be unknown too. This means that an expression such as
284 "result=a,b,+" will be unknown if either a or b is unknown. It would
285 be wrong to just ignore the unknown value and return the value of the
286 other parameter. By doing so, you would assume "unknown" means "zero"
287 and this is not true.
288 There has been a case where somebody was collecting data for over a
290 year. A new piece of equipment was installed, a new RRD was created
291 and the scripts were changed to add a counter from the old database and
292 a counter from the new database. The result was disappointing, a large
293 part of the statistics seemed to have vanished mysteriously ... They
294 of course didn't, values from the old database (known values) were
295 added to values from the new database (unknown values) and the result
296 was unknown.
297 In this case, it is fairly reasonable to use a CDEF that alters unknown
299 data into zero. The counters of the device were unknown (after all, it
300 wasn't installed yet!) but you know that the data rate through the
301 device had to be zero (because of the same reason: it was not
302 installed).
303 There are some examples below that make this change.
305 Infinity
307 Infinite data is another form of a special number. It cannot be graphed
309 because by definition you would never reach the infinite value. You can
310 think of positive and negative infinity depending on the position rela‐
311 tive to zero.
312 RRDtool is capable of representing (-not- graphing!) infinity by stop‐
314 ping at its current maximum (for positive infinity) or minimum (for
315 negative infinity) without knowing this maximum (minimum).
316 Infinity in RRDtool is mostly used to draw an AREA without knowing its
318 vertical dimensions. You can think of it as drawing an AREA with an
319 infinite height and displaying only the part that is visible in the
320 current graph. This is probably a good way to approximate infinity and
321 it sure allows for some neat tricks. See below for examples.
322 Working with unknown data and infinity
324 Sometimes you would like to discard unknown data and pretend it is zero
326 (or any other value for that matter) and sometimes you would like to
327 pretend that known data is unknown (to discard known-to-be-wrong data).
328 This is why CDEFs have support for unknown data. There are also exam‐
329 ples available that show unknown data by using infinity.
330
332 Example: using a recently created RRD
333 You are keeping statistics on your router for over a year now. Recently
335 you installed an extra router and you would like to show the combined
336 throughput for these two devices.
337 If you just add up the counters from router.rrd and router2.rrd, you
339 will add known data (from router.rrd) to unknown data (from
340 router2.rrd) for the bigger part of your stats. You could solve this in
341 a few ways:
342 · While creating the new database, fill it with zeros from the start
344 to now. You have to make the database start at or before the least
345 recent time in the other database.
346 · Alternatively, you could use CDEF and alter unknown data to zero.
348 Both methods have their pros and cons. The first method is troublesome
350 and if you want to do that you have to figure it out yourself. It is
351 not possible to create a database filled with zeros, you have to put
352 them in manually. Implementing the second method is described next:
353 What we want is: "if the value is unknown, replace it with zero". This
355 could be written in pseudo-code as: if (value is unknown) then (zero)
356 else (value). When reading the rrdgraph manual you notice the "UN"
357 function that returns zero or one. You also notice the "IF" function
358 that takes zero or one as input.
359 First look at the "IF" function. It takes three values from the stack,
361 the first value is the decision point, the second value is returned to
362 the stack if the evaluation is "true" and if not, the third value is
363 returned to the stack. We want the "UN" function to decide what happens
364 so we combine those two functions in one CDEF.
365 Lets write down the two possible paths for the "IF" function:
367 if true return a
369 if false return b
370 In RPN: "result=x,a,b,IF" where "x" is either true or false.
372 Now we have to fill in "x", this should be the "(value is unknown)"
374 part and this is in RPN: "result=value,UN"
375 We now combine them: "result=value,UN,a,b,IF" and when we fill in the
377 appropriate things for "a" and "b" we're finished:
378 "CDEF:result=value,UN,0,value,IF"
380 You may want to read Steve Rader's RPN guide if you have difficulties
382 with the way I explained this last example.
383 If you want to check this RPN expression, just mimic RRDtool behavior:
385 For any known value, the expression evaluates as follows:
387 CDEF:result=value,UN,0,value,IF (value,UN) is not true so it becomes 0
388 CDEF:result=0,0,value,IF "IF" will return the 3rd value
389 CDEF:result=value The known value is returned
390 For the unknown value, this happens:
392 CDEF:result=value,UN,0,value,IF (value,UN) is true so it becomes 1
393 CDEF:result=1,0,value,IF "IF" sees 1 and returns the 2nd value
394 CDEF:result=0 Zero is returned
395 Of course, if you would like to see another value instead of zero, you
397 can use that other value.
398 Eventually, when all unknown data is removed from the RRD, you may want
400 to remove this rule so that unknown data is properly displayed.
401 Example: better handling of unknown data, by using time
403 The above example has one drawback. If you do log unknown data in your
405 database after installing your new equipment, it will also be trans‐
406 lated into zero and therefore you won't see that there was a problem.
407 This is not good and what you really want to do is:
408 · If there is unknown data, look at the time that this sample was
410 taken.
411 · If the unknown value is before time xxx, make it zero.
413 · If it is after time xxx, leave it as unknown data.
415 This is doable: you can compare the time that the sample was taken to
417 some known time. Assuming you started to monitor your device on Friday
418 September 17, 1999, 00:35:57 MET DST. Translate this time in seconds
419 since 1970-01-01 and it becomes 937'521'357. If you process unknown
420 values that were received after this time, you want to leave them
421 unknown and if they were "received" before this time, you want to
422 translate them into zero (so you can effectively ignore them while
423 adding them to your other routers counters).
424 Translating Friday September 17, 1999, 00:35:57 MET DST into
426 937'521'357 can be done by, for instance, using gnu date:
427 date -d "19990917 00:35:57" +%s
429 You could also dump the database and see where the data starts to be
431 known. There are several other ways of doing this, just pick one.
432 Now we have to create the magic that allows us to process unknown val‐
434 ues different depending on the time that the sample was taken. This is
435 a three step process:
436 1. If the timestamp of the value is after 937'521'357, leave it as is.
438 2. If the value is a known value, leave it as is.
440 3. Change the unknown value into zero.
442 Lets look at part one:
444 if (true) return the original value
446 We rewrite this:
448 if (true) return "a"
450 if (false) return "b"
451 We need to calculate true or false from step 1. There is a function
453 available that returns the timestamp for the current sample. It is
454 called, how surprisingly, "TIME". This time has to be compared to a
455 constant number, we need "GT". The output of "GT" is true or false and
456 this is good input to "IF". We want "if (time > 937521357) then (return
457 a) else (return b)".
458 This process was already described thoroughly in the previous chapter
460 so lets do it quick:
461 if (x) then a else b
463 where x represents "time>937521357"
464 where a represents the original value
465 where b represents the outcome of the previous example
466 time>937521357 --> TIME,937521357,GT
468 if (x) then a else b --> x,a,b,IF
470 substitute x --> TIME,937521357,GT,a,b,IF
471 substitute a --> TIME,937521357,GT,value,b,IF
472 substitute b --> TIME,937521357,GT,value,value,UN,0,value,IF,IF
473 We end up with:
475 "CDEF:result=TIME,937521357,GT,value,value,UN,0,value,IF,IF"
476 This looks very complex, however, as you can see, it was not too hard
478 to come up with.
479 Example: Pretending weird data isn't there
481 Suppose you have a problem that shows up as huge spikes in your graph.
483 You know this happens and why, so you decide to work around the prob‐
484 lem. Perhaps you're using your network to do a backup at night and by
485 doing so you get almost 10mb/s while the rest of your network activity
486 does not produce numbers higher than 100kb/s.
487 There are two options:
489 1. If the number exceeds 100kb/s it is wrong and you want it masked
491 out by changing it into unknown.
492 2. You don't want the graph to show more than 100kb/s.
494 Pseudo code: if (number > 100) then unknown else number or Pseudo code:
496 if (number > 100) then 100 else number.
497 The second "problem" may also be solved by using the rigid option of
499 RRDtool graph, however this has not the same result. In this example
500 you can end up with a graph that does autoscaling. Also, if you use the
501 numbers to display maxima they will be set to 100kb/s.
502 We use "IF" and "GT" again. "if (x) then (y) else (z)" is written down
504 as "CDEF:result=x,y,z,IF"; now fill in x, y and z. For x you fill in
505 "number greater than 100kb/s" becoming "number,100000,GT" (kilo is
506 1'000 and b/s is what we measure!). The "z" part is "number" in both
507 cases and the "y" part is either "UNKN" for unknown or "100000" for
508 100kb/s.
509 The two CDEF expressions would be:
511 CDEF:result=number,100000,GT,UNKN,number,IF
513 CDEF:result=number,100000,GT,100000,number,IF
514 Example: working on a certain time span
516 If you want a graph that spans a few weeks, but would only want to see
518 some routers' data for one week, you need to "hide" the rest of the
519 time frame. Don't ask me when this would be useful, it's just here for
520 the example :)
521 We need to compare the time stamp to a begin date and an end date.
523 Comparing isn't difficult:
524 TIME,begintime,GE
526 TIME,endtime,LE
527 These two parts of the CDEF produce either 0 for false or 1 for true.
529 We can now check if they are both 0 (or 1) using a few IF statements
530 but, as Wataru Satoh pointed out, we can use the "*" or "+" functions
531 as logical AND and logical OR.
532 For "*", the result will be zero (false) if either one of the two oper‐
534 ators is zero. For "+", the result will only be false (0) when two
535 false (0) operators will be added. Warning: *any* number not equal to
536 0 will be considered "true". This means that, for instance, "-1,1,+"
537 (which should be "true or true") will become FALSE ... In other words,
538 use "+" only if you know for sure that you have positive numbers (or
539 zero) only.
540 Let's compile the complete CDEF:
542 DEF:ds0=router1.rrd:AVERAGE
544 CDEF:ds0modified=TIME,begintime,GE,TIME,endtime,LE,*,UNKN,ds0,IF
545 This will return the value of ds0 if both comparisons return true. You
547 could also do it the other way around:
548 DEF:ds0=router1.rrd:AVERAGE
550 CDEF:ds0modified=TIME,begintime,LT,TIME,endtime,GT,+,UNKN,ds0,IF
551 This will return an UNKNOWN if either comparison returns true.
553 Example: You suspect to have problems and want to see unknown data.
555 Suppose you add up the number of active users on several terminal
557 servers. If one of them doesn't give an answer (or an incorrect one)
558 you get "NaN" in the database ("Not a Number") and NaN is evaluated as
559 Unknown.
560 In this case, you would like to be alerted to it and the sum of the
562 remaining values is of no value to you.
563 It would be something like:
565 DEF:users1=location1.rrd:onlineTS1:LAST
567 DEF:users2=location1.rrd:onlineTS2:LAST
568 DEF:users3=location2.rrd:onlineTS1:LAST
569 DEF:users4=location2.rrd:onlineTS2:LAST
570 CDEF:allusers=users1,users2,users3,users4,+,+,+
571 If you now plot allusers, unknown data in one of users1..users4 will
573 show up as a gap in your graph. You want to modify this to show a
574 bright red line, not a gap.
575 Define an extra CDEF that is unknown if all is okay and is infinite if
577 there is an unknown value:
578 CDEF:wrongdata=allusers,UN,INF,UNKN,IF
580 "allusers,UN" will evaluate to either true or false, it is the (x) part
582 of the "IF" function and it checks if allusers is unknown. The (y)
583 part of the "IF" function is set to "INF" (which means infinity) and
584 the (z) part of the function returns "UNKN".
585 The logic is: if (allusers == unknown) then return INF else return
587 UNKN.
588 You can now use AREA to display this "wrongdata" in bright red. If it
590 is unknown (because allusers is known) then the red AREA won't show up.
591 If the value is INF (because allusers is unknown) then the red AREA
592 will be filled in on the graph at that particular time.
593 AREA:allusers#0000FF:combined user count
595 AREA:wrongdata#FF0000:unknown data
596 Same example useful with STACKed data:
598 If you use stack in the previous example (as I would do) then you don't
600 add up the values. Therefore, there is no relationship between the four
601 values and you don't get a single value to test. Suppose users3 would
602 be unknown at one point in time: users1 is plotted, users2 is stacked
603 on top of users1, users3 is unknown and therefore nothing happens,
604 users4 is stacked on top of users2. Add the extra CDEFs anyway and use
605 them to overlay the "normal" graph:
606 DEF:users1=location1.rrd:onlineTS1:LAST
608 DEF:users2=location1.rrd:onlineTS2:LAST
609 DEF:users3=location2.rrd:onlineTS1:LAST
610 DEF:users4=location2.rrd:onlineTS2:LAST
611 CDEF:allusers=users1,users2,users3,users4,+,+,+
612 CDEF:wrongdata=allusers,UN,INF,UNKN,IF
613 AREA:users1#0000FF:users at ts1
614 STACK:users2#00FF00:users at ts2
615 STACK:users3#00FFFF:users at ts3
616 STACK:users4#FFFF00:users at ts4
617 AREA:wrongdata#FF0000:unknown data
618 If there is unknown data in one of users1..users4, the "wrongdata" AREA
620 will be drawn and because it starts at the X-axis and has infinite
621 height it will effectively overwrite the STACKed parts.
622 You could combine the two CDEF lines into one (we don't use "allusers")
624 if you like. But there are good reasons for writing two CDEFS:
625 · It improves the readability of the script.
627 · It can be used inside GPRINT to display the total number of users.
629 If you choose to combine them, you can substitute the "allusers" in the
631 second CDEF with the part after the equal sign from the first line:
632 CDEF:wrongdata=users1,users2,users3,users4,+,+,+,UN,INF,UNKN,IF
634 If you do so, you won't be able to use these next GPRINTs:
636 COMMENT:"Total number of users seen"
638 GPRINT:allusers:MAX:"Maximum: %6.0lf"
639 GPRINT:allusers:MIN:"Minimum: %6.0lf"
640 GPRINT:allusers:AVERAGE:"Average: %6.0lf"
641 GPRINT:allusers:LAST:"Current: %6.0lf\n"
642
644 Degrees Celsius vs. Degrees Fahrenheit
645 To convert Celsius into Fahrenheit use the formula F=9/5*C+32
647 rrdtool graph demo.png --title="Demo Graph" \
649 DEF:cel=demo.rrd:exhaust:AVERAGE \
650 CDEF:far=9,5,/,cel,*,32,+ \
651 LINE2:cel#00a000:"D. Celsius" \
652 LINE2:far#ff0000:"D. Fahrenheit\c"
653 This example gets the DS called "exhaust" from database "demo.rrd" and
655 puts the values in variable "cel". The CDEF used is evaluated as fol‐
656 lows:
657 CDEF:far=9,5,/,cel,*,32,+
659 1. push 9, push 5
660 2. push function "divide" and process it
661 the stack now contains 9/5
662 3. push variable "cel"
663 4. push function "multiply" and process it
664 the stack now contains 9/5*cel
665 5. push 32
666 6. push function "plus" and process it
667 the stack contains now the temperature in Fahrenheit
668 Changing unknown into zero
670 rrdtool graph demo.png --title="Demo Graph" \
672 DEF:idat1=interface1.rrd:ds0:AVERAGE \
673 DEF:idat2=interface2.rrd:ds0:AVERAGE \
674 DEF:odat1=interface1.rrd:ds1:AVERAGE \
675 DEF:odat2=interface2.rrd:ds1:AVERAGE \
676 CDEF:agginput=idat1,UN,0,idat1,IF,idat2,UN,0,idat2,IF,+,8,* \
677 CDEF:aggoutput=odat1,UN,0,odat1,IF,odat2,UN,0,odat2,IF,+,8,* \
678 AREA:agginput#00cc00:Input Aggregate \
679 LINE1:aggoutput#0000FF:Output Aggregate
680 These two CDEFs are built from several functions. It helps to split
682 them when viewing what they do. Starting with the first CDEF we would
683 get:
684 idat1,UN --> a
686 0 --> b
687 idat1 --> c
688 if (a) then (b) else (c)
689 The result is therefore "0" if it is true that "idat1" equals "UN". If
691 not, the original value of "idat1" is put back on the stack. Lets call
692 this answer "d". The process is repeated for the next five items on the
693 stack, it is done the same and will return answer "h". The resulting
694 stack is therefore "d,h". The expression has been simplified to
695 "d,h,+,8,*" and it will now be easy to see that we add "d" and "h", and
696 multiply the result with eight.
697 The end result is that we have added "idat1" and "idat2" and in the
699 process we effectively ignored unknown values. The result is multiplied
700 by eight, most likely to convert bytes/s to bits/s.
701 Infinity demo
703 rrdtool graph example.png --title="INF demo" \
705 DEF:val1=some.rrd:ds0:AVERAGE \
706 DEF:val2=some.rrd:ds1:AVERAGE \
707 DEF:val3=some.rrd:ds2:AVERAGE \
708 DEF:val4=other.rrd:ds0:AVERAGE \
709 CDEF:background=val4,POP,TIME,7200,%,3600,LE,INF,UNKN,IF \
710 CDEF:wipeout=val1,val2,val3,val4,+,+,+,UN,INF,UNKN,IF \
711 AREA:background#F0F0F0 \
712 AREA:val1#0000FF:Value1 \
713 STACK:val2#00C000:Value2 \
714 STACK:val3#FFFF00:Value3 \
715 STACK:val4#FFC000:Value4 \
716 AREA:whipeout#FF0000:Unknown
717 This demo demonstrates two ways to use infinity. It is a bit tricky to
719 see what happens in the "background" CDEF.
720 "val4,POP,TIME,7200,%,3600,LE,INF,UNKN,IF"
722 This RPN takes the value of "val4" as input and then immediately
724 removes it from the stack using "POP". The stack is now empty but as a
725 side effect we now know the time that this sample was taken. This time
726 is put on the stack by the "TIME" function.
727 "TIME,7200,%" takes the modulo of time and 7'200 (which is two hours).
729 The resulting value on the stack will be a number in the range from 0
730 to 7199.
731 For people who don't know the modulo function: it is the remainder
733 after an integer division. If you divide 16 by 3, the answer would be 5
734 and the remainder would be 1. So, "16,3,%" returns 1.
735 We have the result of "TIME,7200,%" on the stack, lets call this "a".
737 The start of the RPN has become "a,3600,LE" and this checks if "a" is
738 less or equal than "3600". It is true half of the time. We now have to
739 process the rest of the RPN and this is only a simple "IF" function
740 that returns either "INF" or "UNKN" depending on the time. This is
741 returned to variable "background".
742 The second CDEF has been discussed earlier in this document so we won't
744 do that here.
745 Now you can draw the different layers. Start with the background that
747 is either unknown (nothing to see) or infinite (the whole positive part
748 of the graph gets filled).
749 Next you draw the data on top of this background, it will overlay the
751 background. Suppose one of val1..val4 would be unknown, in that case
752 you end up with only three bars stacked on top of each other. You
753 don't want to see this because the data is only valid when all four
754 variables are valid. This is why you use the second CDEF, it will over‐
755 lay the data with an AREA so the data cannot be seen anymore.
756 If your data can also have negative values you also need to overwrite
758 the other half of your graph. This can be done in a relatively simple
759 way: what you need is the "wipeout" variable and place a negative sign
760 before it: "CDEF:wipeout2=wipeout,-1,*"
761 Filtering data
763 You may do some complex data filtering:
765 MEDIAN FILTER: filters shot noise
767 DEF:var=database.rrd:traffic:AVERAGE
769 CDEF:prev1=PREV(var)
770 CDEF:prev2=PREV(prev1)
771 CDEF:prev3=PREV(prev2)
772 CDEF:median=prev1,prev2,prev3,+,+,3,/
773 LINE3:median#000077:filtered
774 LINE1:prev2#007700:'raw data'
775 DERIVATE:
777 DEF:var=database.rrd:traffic:AVERAGE
779 CDEF:prev1=PREV(var)
780 CDEF:time=TIME
781 CDEF:prevtime=PREV(time)
782 CDEF:derivate=var,prev1,-,time,prevtime,-,/
783 LINE3:derivate#000077:derivate
784 LINE1:var#007700:'raw data'
785
787 This document was created from questions asked by either myself or by
788 other people on the RRDtool mailing list. Please let me know if you
789 find errors in it or if you have trouble understanding it. If you think
790 there should be an addition, mail me: <alex@ergens.op.het.net>
791 Remember: No feedback equals no changes!
793
795 The RRDtool manpages
796
798 Alex van den Bogaerdt <alex@ergens.op.het.net>
7991.2.27 2008-02-17 CDEFTUTORIAL(1)