+=head2 RESOLUTION INTERVAL
+
+In order to get RRDtool to fetch anything other than the finest resolution RRA
+B<both> the start and end time must be specified on boundaries that are
+multiples of the desired 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 to fetch the 15 minute average data for the
+last hour. 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<NOT> 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 desired RRA
+
+=back
+
+So, if time now is called "t", do
+
+ end time == int(t/900)*900,
+ start time == end time - 1hour,
+ resolution == 900.
+
+Using the bash shell, this could look be:
+
+ TIME=$(date +%s)
+ RRDRES=900
+ rrdtool fetch subdata.rrd AVERAGE -r $RRDRES \
+ -e $(($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"'
+
+