From: oetiker Date: Wed, 31 Jul 2002 05:42:48 +0000 (+0000) Subject: improved fetch explanation -- Nenad.Antic@era.ericsson.se X-Git-Url: https://git.verplant.org/?a=commitdiff_plain;h=d53241d45f61489bd1bd2fb1aab5e6183db6009d;p=rrdtool.git improved fetch explanation -- Nenad.Antic@era.ericsson.se git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@160 a5681a0c-68f1-0310-ab6d-d61299d08faa --- diff --git a/doc/rrdfetch.pod b/doc/rrdfetch.pod index c938bb5..0b57340 100644 --- a/doc/rrdfetch.pod +++ b/doc/rrdfetch.pod @@ -35,7 +35,7 @@ want to fetch? (AVERAGE,MIN,MAX,LAST) what interval should the values have (seconds per value). B will try to match your request, but it will return data even if no absolute -match is possible. +match is possible. B See note below. =item B<--start>|B<-s> I (default end-1day) @@ -52,6 +52,62 @@ end time. =back +=head2 RESOLUTION INTERVAL + +In order to get rrdtool to fetch anything other than the finest resolution RRA +B the start and end time must be specified on boundaries that are +multiples of the wanted resolution. Consider the following example: + + rrdtool create subdata.rrd -s 10 DS:ds0:GAUGE:300:0:U \ + RRA:AVERAGE:0.5:30:3600 \ + RRA:AVERAGE:0.5:90:1200 \ + RRA:AVERAGE:0.5:360:1200 \ + RRA:MAX:0.5:360:1200 \ + RRA:AVERAGE:0.5:8640:600 \ + RRA:MAX:0.5:8640:600 + +This RRD collects data every 10 seconds and stores its averages over 5 minutes, +15 minutes, 1 hour and 1 day as well as the maxima for 1 hour and 1 day. + +Consider now that you want too fetch the 15 minute average data for last hour. +So you might try + + rrdtool fetch subdata.rrd AVERAGE -r 900 -s -1h + +However, this will almost always result in a time series that is B in the 15 +minute RRA. Therefore the highest resolution RRA, i.e. 5 minute averages, will +be chosen which, in this case, is not what you want. + +Hence, make sure that + +=over 3 + +=item 1. + +both start and end time are a multiple of 900 + +=item 2. + +both start and end time are within the wanted RRA + +=back + +So, if time now is called "t", + + do end time == int(t/900)*900, + start time == end time -1hour, resolution == 900. + +In e.g. bash this could look as: + + TIME=$(date +%s); RRDRES=900; rrdtool fetch subdata.rrd AVERAGE -r $RRDRES \ + -e $(echo $(($TIME/$RRDRES*$RRDRES))) -s e-1h + +Or in perl: + + perl -e '$ctime = time; $rrdres = 900; system "rrdtool fetch subdata.rrd AVERAGE \ + -r $rrdres -e @{[int($ctime/$rrdres)*$rrdres]} -s e-1h"' + + =head2 AT-STYLE TIME SPECIFICATION Apart from the traditional I, rrdtool does also