multiple master data aggregation

This example describes a way to have multiple master collecting different information, and show all the data in a single presentation.

When you reach some size (probably several hundreds of nodes, several tousands plugins), 5 minutes is not enough for your single master to connect and gather data from all hosts, and you end up having holes in your graph.

Requirements

This example requires a shared nfs space for the munin data between the nodes.

Before going that road, you should make sure to check other options first, like changing the number of update threads, and having rrdcached.

An other option you might consider, is using munin-async. It requires modifications on all nodes, so it might not be an option, but I felt compeled to mention it. If you can’t easily have shared nfs, or if you might have connectivity issues between master and some node, async would probably be a better approach.

Because there is some rrd path merge required, it is highly recommended to have all nodes in groups.

Overview

Munin-Master runs differents scripts via the cron script (munin-cron).

munin-update
is the only part actually connecting to the nodes. It gathers information and updates the rrd (you’ll probably need rrdcached, especially via nfs).
munin-limits
checks what was collected, compared to the limits and places warning and criticals.

The trick about having multiple master running to update is :

  • run munin-update on different masters (called update-masters there after), having dbdir on nfs
  • run munin-limits on either each of the update-masters, or the html-master (see next line)

Of course, all hosts must have access to the shared nfs directory.

Exemples will consider the shared folder /nfs/munin.

Running munin-update

Cange the munin-cron to only run munin-update (and munin-limits, if you have alerts you want to be managed directly on those masters).

Change your munin.conf to use a dbdir within the shared nfs, (ie: /nfs/munin/db/<hostname>).

To make it easier to see the configuration, you can also update the configuration with an includedir on nfs, and declare all your nodes there (ie: /nfs/munin/etc/<hostname>.d/).

If you configured at least one node, you should have /nfs/munin/db/<hostname> that starts getting populated with subdirectories (groups), and a few files, including datafile, and datafile.storable (and limits if you also have munin-limits running here).

Merging data

All our update-masters generate update their dbdir including:

  • datafile and datafile.storable which contain information about the collected plugins, and graphs to generate.
  • directory tree with the rrd files

Merging files

datafile is just plain text with lines of key value, so concatenating all the files is enough.

datafile.storable is a binary representation of the data as loaded by munin. It requires some munin internal structures knowledge to merge them.

If you have munin-limits also running on update-masters, it generate a limits files, those are also plain text.

In order to make that part easier, a munin-mergedb.pl is provided in contrib.

Merging rrd tree

The main trick is about rrd. As we are using a shared nfs, we can use symlinks to get them to point to one an other, and not have to duplicate them. (Would be hell to keep in sync, that’s why we really need shared nfs storage.)

As we deal with groups, we could just link top level groups to a common rrd tree.

Example, if you have two updaters (update1 and update2), and 4 groups (customer1, customer2, customer3, customer4), you could make something like that:

/nfs/munin/db/shared-rrd/customer1/
/nfs/munin/db/shared-rrd/customer2/
/nfs/munin/db/shared-rrd/customer3/
/nfs/munin/db/shared-rrd/customer4/
/nfs/munin/db/update1/customer1 -> ../shared-rrd/customer1
/nfs/munin/db/update1/customer2 -> ../shared-rrd/customer2
/nfs/munin/db/update1/customer3 -> ../shared-rrd/customer3
/nfs/munin/db/update1/customer4 -> ../shared-rrd/customer4
/nfs/munin/db/update2/customer1 -> ../shared-rrd/customer1
/nfs/munin/db/update2/customer2 -> ../shared-rrd/customer2
/nfs/munin/db/update2/customer3 -> ../shared-rrd/customer3
/nfs/munin/db/update2/customer4 -> ../shared-rrd/customer4
/nfs/munin/db/html/customer1 -> ../shared-rrd/customer1
/nfs/munin/db/html/customer2 -> ../shared-rrd/customer2
/nfs/munin/db/html/customer3 -> ../shared-rrd/customer3
/nfs/munin/db/html/customer4 -> ../shared-rrd/customer4

At some point, an option to get the rrd tree separated from the dbdir, and should avoid the need of such links.