=head1 DESCRIPTION
If you have ever used a traditional HP calculator you already know
-B<RPN>. The idea behind B<RPN> is that you have a stack and push
+B<RPN> (Reverse Polish Notation).
+The idea behind B<RPN> is that you have a stack and push
your data onto this stack. Whenever you execute an operation, it
takes as many elements from the stack as needed. Pushing is done
implicitly, so whenever you specify a number or a variable, it gets
Pop two elements from the stack, compare them for the selected condition
and return 1 for true or 0 for false. Comparing an I<unknown> or an
-I<infinite> value will always result in 0 (false).
+I<infinite> value will result in I<unknown> returned ... which will also be
+treated as false by the B<IF> call.
B<UN, ISINF>
Add, subtract, multiply, divide, modulo
+B<ADDNAN>
+
+NAN-safe addition. If one parameter is NAN/UNKNOWN it'll be treated as
+zero. If both parameters are NAN/UNKNOWN, NAN/UNKNOWN will be returned.
+
B<SIN, COS, LOG, EXP, SQRT>
Sine and cosine (input in radians), log and exp (natural logarithm),
operation ignores all NAN-values in a sliding window and computes the
average of the remaining values.
+B<PREDICT, PREDICTSIGMA>
+
+Create a "sliding window" average/sigma of another data series, that also
+shifts the data series by given amounts of of time as well
+
+Usage - explicit stating shifts:
+CDEF:predict=<shift n>,...,<shift 1>,n,<window>,x,PREDICT
+CDEF:sigma=<shift n>,...,<shift 1>,n,<window>,x,PREDICTSIGMA
+
+Usage - shifts defined as a base shift and a number of time this is applied
+CDEF:predict=<shift multiplier>,-n,<window>,x,PREDICT
+CDEF:sigma=<shift multiplier>,-n,<window>,x,PREDICTSIGMA
+
+Example:
+CDEF:predict=172800,86400,2,1800,x,PREDICT
+
+This will create a half-hour (1800 second) sliding window average/sigma of x, that
+average is essentially computed as shown here:
+
+ +---!---!---!---!---!---!---!---!---!---!---!---!---!---!---!---!---!--->
+ now
+ shift 1 t0
+ <----------------------->
+ window
+ <--------------->
+ shift 2
+ <----------------------------------------------->
+ window
+ <--------------->
+ shift 1 t1
+ <----------------------->
+ window
+ <--------------->
+ shift 2
+ <----------------------------------------------->
+ window
+ <--------------->
+
+ Value at sample (t0) will be the average between (t0-shift1-window) and (t0-shift1)
+ and between (t0-shift2-window) and (t0-shift2)
+ Value at sample (t1) will be the average between (t1-shift1-window) and (t1-shift1)
+ and between (t1-shift2-window) and (t1-shift2)
+
+
+The function is by design NAN-safe.
+This also allows for extrapolation into the future (say a few days)
+- you may need to define the data series whit the optional start= parameter, so that
+the source data series has enough data to provide prediction also at the beginning of a graph...
+
+Here an example, that will create a 10 day graph that also shows the
+prediction 3 days into the future with its uncertainty value (as defined by avg+-4*sigma)
+This also shows if the prediction is exceeded at a certain point.
+
+rrdtool graph image.png --imgformat=PNG \
+ --start=-7days --end=+3days --width=1000 --height=200 --alt-autoscale-max \
+ DEF:value=value.rrd:value:AVERAGE:start=-14days \
+ LINE1:value#ff0000:value \
+ CDEF:predict=86400,-7,1800,value,PREDICT \
+ CDEF:sigma=86400,-7,1800,value,PREDICTSIGMA \
+ CDEF:upper=predict,sigma,3,*,+ \
+ CDEF:lower=predict,sigma,3,*,- \
+ LINE1:predict#00ff00:prediction \
+ LINE1:upper#0000ff:upper\ certainty\ limit \
+ LINE1:lower#0000ff:lower\ certainty\ limit \
+ CDEF:exceeds=value,UN,0,value,lower,upper,LIMIT,UN,IF \
+ TICK:exceeds#aa000080:1
+
+Note: Experience has shown that a factor between 3 and 5 to scale sigma is a good
+discriminator to detect abnormal behavior. This obviously depends also on the type
+of data and how "noisy" the data series is.
+
+This prediction can only be used for short term extrapolations - say a few days into the future-
=item Special values
Example: C<VDEF:avg=mydata,AVERAGE>
+=item STDEV
+
+Returns the standard deviation of the values.
+
+Example: C<VDEF:stdev=mydata,STDEV>
+
=item LAST, FIRST
Return the last/first value including its time. The time for
=item TOTAL
Returns the rate from each defined time slot multiplied with the
-step size. This can, for instance, return total bytes transfered
+step size. This can, for instance, return total bytes transferred
when you have logged bytes per second. The time component returns
the number of seconds.
Example: C<VDEF:total=mydata,TOTAL>
-=item PERCENT
+=item PERCENT, PERCENTNAN
This should follow a B<DEF> or B<CDEF> I<vname>. The I<vname> is popped,
another number is popped which is a certain percentage (0..100). The
data set is then sorted and the value returned is chosen such that
I<percentage> percent of the values is lower or equal than the result.
+For PERCENTNAN I<Unknown> values are ignored, but for PERCENT
I<Unknown> values are considered lower than any finite number for this
purpose so if this operator returns an I<unknown> you have quite a lot
of them in your data. B<Inf>inite numbers are lesser, or more, than the
(NaN E<lt> -INF E<lt> finite values E<lt> INF)
Example: C<VDEF:perc95=mydata,95,PERCENT>
+ C<VDEF:percnan95=mydata,95,PERCENTNAN>
=item LSLSLOPE, LSLINT, LSLCORREL
Program by Tobias Oetiker E<lt>tobi@oetiker.chE<gt>
-This manual page by Alex van den Bogaerdt E<lt>alex@ergens.op.het.netE<gt>
+This manual page by Alex van den Bogaerdt E<lt>alex@vandenbogaerdt.nlE<gt>
+with corrections and/or additions by several people
projects / rrdtool.git / blobdiff