[rrd-users] Strange activity after a DST change

Smith, Kyle A. (Security) kasmith at tgslc.org
Tue Mar 11 20:00:52 CET 2008


Yes, I noticed that the timestamp really didn't change, but the CST->CDT
did change. So while RRD may not use the system time, that is the point
at which things went strange.. So it appears a likely area of interest.

The sending machines (remember, there are 2 identical systems sending to
the receiver) are still sending information in the same format as
The receiving machine is still parsing the data into the same upload
line format to go into the various rrdb's as needed.
When I look at any rrdb associated with the 2nd machine using 'rrdtool
dump', since the DST time change, all the data elements are NaN.
When I look at any rrdb associated with the 1st machine using 'rrdtool
dump', the data is correct.

If I manually enter data (appropriately I think), I still see NaN as the
data points.

[]$ rrdtool update rrd/server3.load.rrd 1205260300:0.1
[]$ rrdtool update rrd/server3.load.rrd 1205260320:0.1
[]$ rrdtool update rrd/server3.load.rrd 1205260340:0.1
[]$ rrdtool update rrd/server3.load.rrd 1205260360:0.1
[]$ rrdtool update rrd/server3.load.rrd 1205260380:0.1
[]$ rrdtool update rrd/server3.load.rrd 1205260400:0.1

[]$ rrdtool dump rrd/server3.load.rrd | more
	<!-- 2008-03-11 13:31:00 CDT / 1205260260 --> <row><v> NaN
  	<!-- 2008-03-11 13:32:00 CDT / 1205260320 --> <row><v> NaN
	<!-- 2008-03-11 13:33:00 CDT / 1205260380 --> <row><v> NaN

So far, no errors directly from rrdtool appear either from an update
script or from a manual update to the rrdb.  Another reason I am
particularly stumped.

Kyle A Smith

-----Original Message-----
From: rrd-users-bounces at lists.oetiker.ch
[mailto:rrd-users-bounces at lists.oetiker.ch] On Behalf Of Alex van den
Sent: Tuesday, March 11, 2008 11:57 AM
To: rrd-users at lists.oetiker.ch
Subject: Re: [rrd-users] Strange activity after a DST change

On Tue, Mar 11, 2008 at 09:59:46AM -0500, Smith, Kyle A. (Security)
> Howdy all,
> 	I am a relatively new to rrdtool, but have setup a few
> re-occuring charts for system metrics for 2 systems.  The process
> (gathering the metrics from the systems, sending them to the 'local'
> system housing rrdtool, updating the rrd db's, and generating the
> graphs) has been working for a few months now without a hitch.. until
> this past weekend when the system 'corrected' its time for local
> daylight-savings.

Time isn't corrected, only its representation to the user.  Internally
time just continues, as can be seen here:

>             <!-- 2008-03-09 00:00:00 CST / 1205042400 --> <row><v>
> 1.4726296296e-01 </v></row>
> 		<!-- 2008-03-09 01:00:00 CST / 1205046000 --> <row><v>
> 1.0902040816e-01 </v></row>
>             <!-- 2008-03-09 03:00:00 CDT / 1205049600 --> <row><v> NaN
> </v></row>

Timestamp 1205042400
Timestamp 1205046000 = 1205042400 + 3600
Timestamp 1205049600 = 1205046000 + 3600, one hour difference, not two.
And it should be that way!

You aren't indicating that errors occur, but from past experience I
learnt the hard way I have to ask: do you get any errors, most notably
such as "ERROR: illegal attempt to update using ti"... ?
Or are you perhaps sending all those messages to /dev/null ?

Things to look at:
1: is the device measured still sending data, or do you get unknowns
   to begin with, in which cases it's quite normal for the RRD to show

2: has someone or something fiddled with the computer's clock to
   'correct' for DST, and has the database been filled with a bunch
   of unknowns as a result. Check rrdtool last on that file against
   the current system time as seen by date +%s. If last updated is
   in the future, someone set the clock forward, e.g. to 2008-09-03
   instead of 2008-03-09.

Alex van den Bogaerdt

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