What does the ganglia directory __SummaryInfo__ contain?

Question

Ganglia writes cluster matrices in rrd file and keeps under the location "/var/lib/ganglia/rrds/ClusterName/" (in default configuration). In the above location there are directories for different hosts in the cluster and "__SummaryInfo__" directory

My question is What does the directory "__SummaryInfo__" contain?
and How can I collect the avg matrices of all machines in the cluster?

Answer 1

__SummaryInfo__ contains the information you need to calculate the averages of all metric values in the cluster. The rrd files there have two data sources, sum for the sum of the metric values over all hosts and num for the number of hosts that reported that metric.

You can calculate the average by dividing sum by num in an rrdtool CDEF statement:

DEF:sum=nameOfYour.rrd:sum:AVERAGE  
DEF:num=nameOfYour.rrd:num:AVERAGE 
CDEF:average=sum,num,/ 

Note that data resolution will deteriorate as the data gets older (more data is added).