[rrd-users] rrdtool create --start parameter

[Holger.VanKoll at swisscom.com]

Hello,

thx to Tobias Oetiker for rrdtool and to Alex van den Bogaerdt (and
others) for his contributions to this list.

My question is: What --start parameter should I give to "rrdtool create"
when I dont know what the first update-time will be?
After reading the rrdtool manpage I assumed this parameter is just a
check to avoid insertion of too old data.
But thats wrong... I dont get the same .rrd-contents when setting this
parameter to different values, at least if those values differ more than
heartbeat-step of the DS.

I looked through the archives for anything regarding the --start
parameter and (only) found a hint that starttime divided by heartbeat
should give an integer.
But also when I care about this, I still see missing data or data where
I didnt enter anything.

Simple example:
root at 0-tvgicsc1 620# cat mkrrd2
#!/bin/sh
rrd="/usr/local/bin/rrdtool"
rrdfile=./test.rrd
STEP=600
DSF=GAUGE:6000:0:100
RRF1=AVERAGE:0.5:1:144

OFFSET=$1

stime="2003-01-12"
stime_u=`/usr/local/bin/gnudate -d "$stime 0:00" +%s`           # $stime
in "secs since 1.1.1970"; this is 1042326000
let starttime=$stime_u+$OFFSET
let utime=1042326600                                            # first
update time

echo creating $rrdfile with start $starttime
rm -f $rrdfile

$rrd create $rrdfile --start $starttime --step $STEP \
  DS:user:$DSF RRA:$RRF1 DS:system:$DSF RRA:$RRF1 \
  DS:idle:$DSF RRA:$RRF1 DS:wait:$DSF RRA:$RRF1

$rrd update $rrdfile -t user:system:idle:wait $utime:10:10:80:20 ; let
utime=$utime+$STEP
$rrd update $rrdfile -t user:system:idle:wait $utime:20:20:50:10 ; let
utime=$utime+$STEP
$rrd update $rrdfile -t user:system:idle:wait $utime:30:30:30:10 ; let
utime=$utime+$STEP

$rrd fetch $rrdfile AVERAGE -s 1042326000 -e $utime

root at 0-tvgicsc1 674# ./mkrrd2 0
creating ./test.rrd with start 1042326000
                     user        system          idle          wait

1042326000: NaNQ NaNQ NaNQ NaNQ
1042326600: 1.0000000000e+01 1.0000000000e+01 8.0000000000e+01
2.0000000000e+01
1042327200: 2.0000000000e+01 2.0000000000e+01 5.0000000000e+01
1.0000000000e+01
1042327800: 3.0000000000e+01 3.0000000000e+01 3.0000000000e+01
1.0000000000e+01
1042328400: NaNQ NaNQ NaNQ NaNQ

Great. Looks as expected.

root at 0-tvgicsc1 681# ./mkrrd2 -600
creating ./test.rrd with start 1042325400
                     user        system          idle          wait

1042326000: 1.0000000000e+01 1.0000000000e+01 8.0000000000e+01
2.0000000000e+01
1042326600: 1.0000000000e+01 1.0000000000e+01 8.0000000000e+01
2.0000000000e+01
1042327200: 2.0000000000e+01 2.0000000000e+01 5.0000000000e+01
1.0000000000e+01
1042327800: 3.0000000000e+01 3.0000000000e+01 3.0000000000e+01
1.0000000000e+01

Ok, I can live with that. But why do I have data at 1042326000 when I
didnt enter anything?

root at 0-tvgicsc1 685# ./mkrrd2 -6000		# or anything greater
than 5400 instead of 6000
creating ./test.rrd with start 1042320000
                     user        system          idle          wait

1042326000: NaNQ NaNQ NaNQ NaNQ
1042326600: NaNQ NaNQ NaNQ NaNQ
1042327200: 2.0000000000e+01 2.0000000000e+01 5.0000000000e+01
1.0000000000e+01
1042327800: 3.0000000000e+01 3.0000000000e+01 3.0000000000e+01
1.0000000000e+01
1042328400: NaNQ NaNQ NaNQ NaNQ

Here is my problem. I just changed the --start time to more than
(date_of_first_data_entered - [heartbeat-step]).
Now the (entered) data at 1042326600 went away.
What happens here? I think rrdtool tries to consolidate
data_of_1042326600, with the last update, and this is the --start
parameter in this case (concerning date, unknown data), so its more than
heartbeat secs older --> unknown data for the whole interval.

If my thinking is correct, the answer to my question "which
startparameter when first update-time is unknown" is obviously: "You
cannot create a rrd if you dont know the date of the first data you will
put in".

Hopefully I am missing something ;-)

Regards,

Holger



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