[rrd-developers] thinking about localtime and graphing
tobi at oetiker.ch
Fri Nov 25 18:54:39 CET 2011
Today Alex van den Bogaerdt wrote:
> > DEF:weekly=test.rrd:ifHCInOctets:AVERAGE:step=3600:ctrigger=%V:tz=CET
> > (%V would be the ISO week number, but you could use any expression you want).
> > Whenever the value of the ctrigger changes, one 'grouping' comes to
> > an end. If the original data is not available at 'step' resolution
> > from rrd fetch, some interpolation might arificially enhance the
> > resolution prior the blocking it again.
> I'm sure you have already thought about daylight saving. Some
> questions which pop up in my brain are:
> I am not aware of such a time zone, but nevertheless you will
> need to have a 100% sure answer to the following question: Is
> there any time zone in the world which applies DST, and where the
> switch to/from DST is happening at midnight? If there is such a
> time zone, to which day does the extra hour (or whatever the
> offset is) belong to? Does strftime do it right? In every
that is the beauty of it ... its all out of my hands and since
strftime gets much wider testing than rrdtool, chances that it
works corectly are pretty high.
> Other questions, which may not be specific to the current case,
> but it may become more important now, and/or result in more cases
> where RRDtool does not do what the user expects it to do:
> Is 00:00-12:00 (or midnight to noon, or whatever range) invalid
> during summer time, when the user specifies time zone CET ? Or
> does CET also cover the last sunday in March, 1am GMT, until the
> last sunday in October, 1am GMT? If it does, is 2011-08-01 03:00
> CET equal to 2011-08-01 02:00 UTC or to 2011-08-01 01:00 UTC ?
all these questions do not matter, since all I do is, take a unix
time stamp and ask strftime to format it for me ... I then look at
the string that results and when it is different from the string
that resulted from the previous sample, then a new group starts,
otherwhise the same group continues ... this should be pretty
robuts in the face of any odd localtime behaviour.
> How to handle at-style time specifications, asking for example
> for 02:00-03:00 on october 30, 2011, european time? Will it be
> just one hour (because you ask CET, not both CET and CEST), or
> will it be two hours?
this has no influance on this task
> Does strftime behave consistently?
in my experiance it does ... we have been using it for formatting
the labels on the x axis all the time ...
> I don't have the answers, but I will look forward to the
> discussion about this topic. HTH Alex
this toppic should be especially interesting to those who do not
live as close to the GMT time zone as I do ...
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> rrd-developers at lists.oetiker.ch
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900
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