[rrd-users] Graphing question: Phanton drops

Michael Van Camp mcamp at alchemy.net
Fri Oct 27 01:53:32 MEST 2000

I create graphs using a perl CGI(RRDs.pm) script that returns them when
called via an <img src="/cgi-bin/blah.cgi"> tag.  The problem is that I get
a "phantom drop" at the far edge of the graph.  Extrapolating from what I
see when I do an rrdtool fetch, it seems a query against an RRDtool datafile
includes as the last datapoint the NEXT point in time...

I store data on a five minute interval, and so say I query with a start
of -34 hours and and end of now, now being 12:31pm, my last datapoint is
12:35pm, and is NAN.  Unfortunately this, I believe, is the source of my
"phantom drop"... a drop to zero at the most current right edge of the
graph, which makes people think the port has just spiked downwards, when in
fact it is fine.

I use RRDtool 1.0.25, with a step of 300 and a heartbeat of 600.  I use
counters for data sources.  I am polling switches via SNMP.  If I change the
endtime to "now-5minutes" it gets rid of the phantom drop... half of the

My graph command is as follows:

my @command = ();
push( @command, "$full_file", "--title", "$title", "--imgformat",
"$format" );
push( @command, "--lazy" ) if $lazy;
push( @command, "--height", "$h" ) if $h;
push( @command, "--width", "$w" ) if $w;
push( @command, "--vertical-label", "$label" ) if $label;
push( @command, "--start", "$start" ) if $start;
push( @command, "--end", "$end" ) if $end;
push( @command, "--base", "1024" );
push( @command, "--color", "BACK#0000ff", "--color", "FONT#ffff00" );
push( @command, qq/DEF:inoctets=$rrd_file:input:$gtype/ );
push( @command, qq/DEF:outoctets=$rrd_file:output:$gtype/ );
push( @command, qq/CDEF:inbits=inoctets,100000000,GT,0,inoctets,IF,8,*/ );
push( @command,
qq/CDEF:outbits=outoctets,100000000,GT,0,outoctets,IF,8,*/ );
push( @command, qq/$gtypeout:outbits#9A556D:Outbound/ );
push( @command, qq/$gtypein:inbits#22556D:Inbound/ );


Michael Van Camp
Alchemy Communications, Inc.
Email:  mcamp at alchemy.net
Voice:  213-596-3000 x107
Yahoo:  mikevc_la
ICQ:    8412705

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