RRDtool is designed to store time series of data. With every data update, an associated time stamp is stored. Time is always expressed in seconds passed since epoch (01-01-1970). RRDtool can be installed on Unix as well as Windows. It comes with a command set to carry out various operations on RRD databases. This command set can be accessed from the command line, as well as from Shell or Perl scripts. The scripts act as wrappers for accessing data stored in RRDtool databases.
The structure of a database and the terminology associated with it can be best explained with an example.
rrdtool create target.rrd \
--start 1023654125 \
--step 300 \
DS:mem:GAUGE:600:0:671744 \
RRA:AVERAGE:0.5:12:24 \
RRA:AVERAGE:0.5:288:31
This example creates a database named target.rrd. Start time (1'023'654'125) is specified in total number of seconds since epoch (time in seconds since 01-01-1970). While updating the database, the update time is also specified. This update time MUST be larger (later) than start time and MUST be in seconds since epoch.
The step of 300 seconds indicates that the database expects new values every 300 seconds. The wrapper script should be scheduled to run every step seconds so that it updates the database every step seconds.
DS (Data Source) is the actual variable which relates to the parameter on the device that is monitored. Its syntax is
DS:variable_name:DST:heartbeat:min:max
DS is a key word. "variable_name" is a name under which the parameter is saved in the database. There can be as many DSs in a database as needed. After every step interval, a new value of DS is supplied to update the database. This value is also called Primary Data Point (PDP). In our example mentioned above, a new PDP is generated every 300 seconds.
Note, that if you do NOT supply new data points exactly every 300 seconds, this is not a problem, RRDtool will interpolate the data accordingly.
DST (Data Source Type) defines the type of the DS. It can be COUNTER, DERIVE, ABSOLUTE, GAUGE. A DS declared as COUNTER will save the rate of change of the value over a step period. This assumes that the value is always increasing (the difference between the current and the previous value is greater than 0). Traffic counters on a router are an ideal candidate for using COUNTER as DST. DERIVE is the same as COUNTER, but it allows negative values as well. If you want to see the rate of change in free disk space on your server, then you might want to use the DERIVE data type. ABSOLUTE also saves the rate of change, but it assumes that the previous value is set to 0. The difference between the current and the previous value is always equal to the current value. Thus it just stores the current value divided by the step interval (300 seconds in our example). GAUGE does not save the rate of change. It saves the actual value itself. There are no divisions or calculations. Memory consumption in a server is a typical example of gauge. The difference between the different types DSTs can be explained better with the following example:
Values = 300, 600, 900, 1200 Step = 300 seconds COUNTER DS = 1, 1, 1, 1 DERIVE DS = 1, 1, 1, 1 ABSOLUTE DS = 1, 2, 3, 4 GAUGE DS = 300, 600, 900, 1200
The next parameter is heartbeat. In our example, heartbeat is 600 seconds. If the database does not get a new PDP within 300 seconds, it will wait for another 300 seconds (total 600 seconds). If it doesn't receive any PDP within 600 seconds, it will save an UNKNOWN value into the database. This UNKNOWN value is a special feature of RRDtool - it is much better than to assume a missing value was 0 (zero) or any other number which might also be a valid data value. For example, the traffic flow counter on a router keeps increasing. Lets say, a value is missed for an interval and 0 is stored instead of UNKNOWN. Now when the next value becomes available, it will calculate the difference between the current value and the previous value (0) which is not correct. So, inserting the value UNKNOWN makes much more sense here.
The next two parameters are the minimum and maximum value, respectively. If the variable to be stored has predictable maximum and minimum values, this should be specified here. Any update value falling out of this range will be stored as UNKNOWN.
The next line declares a round robin archive (RRA). The syntax for declaring an RRA is
RRA:CF:xff:step:rows
RRA is the keyword to declare RRAs. The consolidation function (CF) can be AVERAGE, MINIMUM, MAXIMUM, and LAST. The concept of the consolidated data point (CDP) comes into the picture here. A CDP is CFed (averaged, maximum/minimum value or last value) from step number of PDPs. This RRA will hold rows CDPs.
Lets have a look at the example above. For the first RRA, 12 (steps) PDPs (DS variables) are AVERAGEed (CF) to form one CDP. 24 (rows) of theses CDPs are archived. Each PDP occurs at 300 seconds. 12 PDPs represent 12 times 300 seconds which is 1 hour. It means 1 CDP (which is equal to 12 PDPs) represents data worth 1 hour. 24 such CDPs represent 1 day (1 hour times 24 CDPs). This means, this RRA is an archive for one day. After 24 CDPs, CDP number 25 will replace the 1st CDP. The second RRA saves 31 CDPs; each CPD represents an AVERAGE value for a day (288 PDPs, each covering 300 seconds = 24 hours). Therefore this RRA is an archive for one month. A single database can have many RRAs. If there are multiple DSs, each individual RRA will save data for all the DSs in the database. For example, if a database has 3 DSs and daily, weekly, monthly, and yearly RRAs are declared, then each RRA will hold data from all 3 data sources.
Values of different variables can be presented in 5 different shapes in a graph - AREA, LINE1, LINE2, LINE3, and STACK. AREA is represented by a solid colored area with values as the boundary of this area. LINE1/2/3 (increasing width) are just plain lines representing the values. STACK is also an area but it is ``stack''ed on top AREA or LINE1/2/3. Another important thing to note is that variables are plotted in the order they are defined in the graph command. Therefore care must be taken to define STACK only after defining AREA/LINE. It is also possible to put formatted comments within the graph. Detailed instructions can be found in the graph manual.
Shell script (collects data, updates database)
#!/bin/sh
a=0
while [ "$a" == 0 ]; do
snmpwalk -c public 192.168.1.250 hrSWRunPerfMem > snmp_reply
total_mem=`awk 'BEGIN {tot_mem=0}
{ if ($NF == "KBytes")
{tot_mem=tot_mem+$(NF-1)}
}
END {print tot_mem}' snmp_reply`
# I can use N as a replacement for the current time
rrdtool update target.rrd N:$total_mem
# sleep until the next 300 seconds are full
perl -e 'sleep 300 - time % 300'
done # end of while loop
Perl script (retrieves data from database and generates graphs and statistics)
#!/usr/bin/perl -w
# This script fetches data from target.rrd, creates a graph of memory
# consumption on the target (Dual P3 Processor 1 GHz, 656 MB RAM)
# call the RRD perl module
use lib qw( /usr/local/rrdtool-1.0.41/lib/perl ../lib/perl );
use RRDs;
my $cur_time = time(); # set current time
my $end_time = $cur_time - 86400; # set end time to 24 hours ago
my $start_time = $end_time - 2592000; # set start 30 days in the past
# fetch average values from the RRD database between start and end time
my ($start,$step,$ds_names,$data) =
RRDs::fetch("target.rrd", "AVERAGE",
"-r", "600", "-s", "$start_time", "-e", "$end_time");
# save fetched values in a 2-dimensional array
my $rows = 0;
my $columns = 0;
my $time_variable = $start;
foreach $line (@$data) {
$vals[$rows][$columns] = $time_variable;
$time_variable = $time_variable + $step;
foreach $val (@$line) {
$vals[$rows][++$columns] = $val;}
$rows++;
$columns = 0;
}
my $tot_time = 0;
my $count = 0;
# save the values from the 2-dimensional into a 1-dimensional array
for $i ( 0 .. $#vals ) {
$tot_mem[$count] = $vals[$i][1];
$count++;
}
my $tot_mem_sum = 0;
# calculate the total of all values
for $i ( 0 .. ($count-1) ) {
$tot_mem_sum = $tot_mem_sum + $tot_mem[$i];
}
# calculate the average of the array
my $tot_mem_ave = $tot_mem_sum/($count);
# create the graph
RRDs::graph ("/images/mem_$count.png",
"--title= Memory Usage",
"--vertical-label=Memory Consumption (MB)",
"--start=$start_time",
"--end=$end_time",
"--color=BACK#CCCCCC",
"--color=CANVAS#CCFFFF",
"--color=SHADEB#9999CC",
"--height=125",
"--upper-limit=656",
"--lower-limit=0",
"--rigid",
"--base=1024",
"DEF:tot_mem=target.rrd:mem:AVERAGE",
"CDEF:tot_mem_cor=tot_mem,0,671744,LIMIT,UN,0,tot_mem,IF,1024,/",
"CDEF:machine_mem=tot_mem,656,+,tot_mem,-",
"COMMENT:Memory Consumption between $start_time",
"COMMENT: and $end_time ",
"HRULE:656#000000:Maximum Available Memory - 656 MB",
"AREA:machine_mem#CCFFFF:Memory Unused",
"AREA:tot_mem_cor#6699CC:Total memory consumed in MB");
my $err=RRDs::error;
if ($err) {print "problem generating the graph: $err\n";}
# print the output
print "Average memory consumption is ";
printf "%5.2f",$tot_mem_ave/1024;
print " MB. Graphical representation can be found at /images/mem_$count.png.";