rrdtool quickly runs into disk-I/O problems.

rrdcached provide one solution for this problem.

To setup rrdcached with munin 2.0 and Ubuntu :

Install rrdcached from packages.

Update rrdcached init script :

remove OPTS= lines from /etc/init.d/rrdcached add to /etc/default/rrdcached

OPTS="-s munin -l unix:/var/run/rrdcached.sock"
OPTS="$OPTS -j /var/lib/rrdcached/journal/ -F"

In munin.conf set :

rrdcached on
rrdcached_socket /var/run/rrdcached.sock

See rrdcached man page for detailed options