Maintenance in active Pacemaker clusters

Posted on Mon 24 September 2012 in hints-and-kinks • 3 min read

In a Pacemaker cluster, as in a standalone system, operators must complete maintenance tasks such as software upgrades and configuration changes. Here’s what you need to keep Pacemaker’s built-in monitoring features from creating unwanted side effects.

Maintenance mode

This is quite possibly Pacemaker’s single most useful feature for cluster maintenance. In maintenance mode, Pacemaker essentially takes a “hands-off” approach to your cluster. Enabling Pacemaker maintenance mode is very easy using the Pacemaker crm shell:

crm configure property maintenance-mode=true

In maintenance mode, you can stop or restart cluster resources at will. Pacemaker will not attempt to restart them. All resources automatically become unmanaged, that is, Pacemaker will cease monitoring them and hence be oblivious about their status. You can even stop all Pacemaker services on a node, and all the daemons and processes originally started as Pacemaker managed cluster resources will continue to run.

You should know that when you start Pacemaker services on a node while the cluster in maintenance mode, Pacemaker will initiate a single one-shot monitor operation (a “probe”) for every resource just so it has an understanding of what resources are currently running on that node. It will, however, take no further action other than determining the resources’ status.

You disable maintenance mode with the crm shell, as well:

crm configure property maintenance-mode=false

Maintenance mode is something you enable before running other maintenance actions, not when you’re already half-way through them. And unless you’re very well versed in the interdependencies of resources running on the cluster you’re working on, it’s usually the very safest option.

In short: when doing maintenance on your Pacemaker cluster, by default, enable maintenance mode before you start, and disable it after you’re done.

Disabling monitoring and error recovery on specific resources

For any configuration changes that take no more than a few minutes, involving an admin that is potentially watching a console window the whole time, maintenance mode is highly recommended. However, enabling maintenance mode can be a bit hard to argue for large configuration changes lasting, say, several hours. Think of a massive database rebuild, for example. In such a case, you may want to put only your database resource in something like maintenance mode, and have Pacemaker continue to monitor other resources like normal.

You can do so by switching the resource to unmanaged mode and disable its monitor operation:

crm configure edit p_database

Then change the is-managed meta attribute and disable the monitor operation:

meta is-managed=false
op monitor interval=<interval> enabled=false

Once you’ve done that, you’ll effectively have enabled something akin to maintenance mode for a single resource. You can reverse this as you would expect:

crm configure edit p_database

Then change the is-managed meta attribute and re-enable the monitor operation:

meta is-managed=true
op monitor interval=<interval> enabled=true

When using this approach, all other resources will be monitored and automatically recovered as they normally would. Thus, you’ll have to be acutely aware of any side effects your maintenance activities have on other resources. If you’re unsure, you should use the global maintenance mode instead.

This article originally appeared on the website (now defunct).