[rrd-users] Re: Consolidation over Weeks

[Tobias Oetiker]

Today you sent me mail regarding [rrd-users] Re: Consolidation over Weeks:

AvdB> The number of seconds since the epoch is zero for the epoch itself,
AvdB> this is the number 0.  All boundaries, no matter what size the interval
AvdB> is, will be calculated from that number 0.
AvdB> 
AvdB> Time to start perl:
AvdB> 
AvdB> [alex at home ~]$ perl -e 'use POSIX; print ctime(0)'
AvdB> Thu Jan  1 01:00:00 1970
AvdB> 
AvdB> Yup, it's a thursday.  If your interval is a week, all boundaries will
AvdB> be on thursday, 01:00 local time.
AvdB> 
AvdB> > Is there a workaround to my problem ?
AvdB> 
AvdB> The problem has been discussed in the past, I think it was on the
AvdB> developers list but I'm not sure.  That discussion was about the time
AvdB> offset, but the problem is the same.  Look it up if you like.
AvdB> 
AvdB> The outcome was (IIRC) that you shouldn't use these large intervals if
AvdB> you don't like the current behaviour.  If you can live with the fact that
AvdB> the boundaries are at 01:00 local time, use an interval of 86400 seconds
AvdB> (which is a day).  If you can't live with that, use an interval of 3600
AvdB> as this gives you the desired boundary on monday, 00:00 local time.
AvdB> You will also have other boundaries that you would prefer not to have,
AvdB> it will cost you disk space and processing time.
AvdB> 
AvdB> There's an additional problem with intervals of a week: does the week
AvdB> start on sunday or monday?  This has always been a point of discussion.
AvdB> You say it should start on monday.  Others say it is sunday.  RRDtool
AvdB> would have to support both.

and what about a monthly interval ... or even a 1 year interval ...
you run into all sorts of interesting problems there ... that is
why rrdtool intervals are based strictly on unix time ...
everything else is just asking for trouble ... as alex pointed out,
the trick is to use an interval which is one 'time magnitude'
smaller than what you are looking for and thus allows you to
combine at will.

cheers
tobi

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