[rrd-users] Re: RRDTool graph question: one data point a day, then it starts at 1

[Alex van den Bogaerdt]

Hsiao, Chang-Ping wrote:

>     So far this seems happens only when I plot one datum a day.  No matter
> how I try to manipulate the EPOCH used to create an RRD, update an RRD, the
> result by fetch the RRD will always show hour 16 for now and hour 17 when it
> becomes PDT.  (I thought it should become hour 15.)

All of the times in RRDtool are UTC based.  All of the intervals in RRDtool
are a whole number of seconds times a whole number of intervals.  The start
of time as far as RRDtool is concerned is januari 1st 1970, UTC.

There is no way you can "manipulate the EPOCH".  What you could do is
change the time zone on your computer to match UTC and forget about
local time although I don't think this is a good solution for you.

When daylight saving is applied (I assume PDT is exactly that) then time
is "moved to the east" by (usually) one hour.

>     Does anyone know how to fix this?  If so, may I ask for help?  Thanks.

What you're trying to do is find a way to make n*24h-16h match m*24h.
This cannot work if both n and m need to be whole numbers.  You could
however match n*1h-16h to m*1h.

In other words: do not use intervals of 24 hours.  Use intervals of 1 hour.

The RRA you create should have a step size of 3600 seconds.  This can
be 1*3600 (so: --step 3600  and  RRA:AVERAGE:0.5:1:8784) or 12*300
(so: --step 300  and  RRA:AVERAGE:0.5:12:8784) or whatever other
combination.

When you query this RRA and ask for midnight local time, you will get
the desired information.

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