Scaling the munin master with rrdcached

When the master grows big, and has a lot of nodes, there is a risk of disk IO becoming a bottleneck.

To reduce this disk IO, you can use the RRD Cache Daemon.

This will spool RRD changes in a queue, and flush changes on demand, and periodically. This will replace lots of random writes with a much smaller amount of sequential writes.

The effects on disk IO can be quite dramatic.

This example is a graph of a munin master with 400 nodes. Even with storage on mirrored SSDs, the effect of adding rrdcached is an immediate reduction in IO, and especially on wait times.


Configuring rrdcached


RRDCached writes the spool data every 5 minutes by default. This is the same as the munin master. To have an effect, change the flushing intervals to allow more data to be spooled. Use the following parameters, and tune to your liking:

-w 1800 Wait 30 minutes before writing data
-z 1800 Delay writes by a random factor of up to 30 minutes (this should be equal to, or lower than, “-w”)
-f 3600 Flush all data every hour


Create a directory for the rrdcached journal, and have the “munin” user own it. (in this example: /var/lib/munin/rrdcached-journal).

Set up a separate RRDCached instance, run by the munin user. The following command starts an RRDCached instance, and can be added to /etc/rc.local.

sudo -u munin /usr/bin/rrdcached \
  -p /run/munin/ \
  -B -b /var/lib/munin/ \
  -F -j /var/lib/munin/rrdcached-journal/ \
  -m 0660 -l unix:/run/munin/rrdcached.sock \
  -w 1800 -z 1800 -f 3600

Note: While testing, add “-g” to the command line to prevent rrdcached from forking into the background.

The munin grapher also needs write access to this socket, in order for it to tell the RRDCached to flush data needed for graphing. If you run munin with CGI graphing, you will need to give the web server access. For a common setup, run the following command, as root, after starting rrdcached:

chgrp www-data /run/munin/rrdcached.sock

Recommended: If you have systemd or upstart installed, use the examples below.

Configuring munin to use rrdcached

To enable rrdcached on the munin master, you will need to set the “rrdcached_socket” line in /etc/munin/munin.conf

rrdcached_socket /run/munin/rrdcached.sock

Is it working?

If all goes well, you should see the following:

Munin logging

There should be no messages regarding rrdcached in /var/log/munin/munin-update.log.

On failure to connect, there will be log lines like:

2012/06/26 18:56:12 [WARN] RRDCached feature ignored: rrdcached socket not writable

…and you should then check for permissions problems.

RRDCached spool

The rrdcached spool file should be in /var/lib/munin/rrdcached-journal/, and it should grow for each run of munin-update until it hits the flush time. The file looks like:


For a munin master with 200 nodes, this could well grow to 100MiB, depending on the number of plugins, and the spool file time parameters.

Monitoring rrdcached

Munin plugins to monitor rrdcached are in the distribution. You can download them also from our repository plugin for stable 2.0 and plugin from master branch You need to configure your munin-node to run this plugin as group rrdcached.