Feature Spotlight - Controllable Resource Discovery

Coming in 1.1.13 is a new option for location constraints: resource-discovery

This new option controls whether or not Pacemaker performs resource discovery for the specified resource on nodes covered by the constraint. The default always, preserves the pre-1.1.13 behaviour.

The options are:

• always - (Default) Always perform resource discovery for the specified resource on this node.

• never - Never perform resource discovery for the specified resource on this node. This option should generally be used with a -INFINITY score. Although that is not strictly required.

• exclusive - Only perform resource discovery for the specified resource on this node. Multiple location constraints using exclusive discovery for the same resource across different nodes creates a subset of nodes resource-discovery is exclusive to. If a resource is marked for exclusive discovery on one or more nodes, that resource is only allowed to be placed within that subset of nodes.

Why would I want this?

Limiting resource discovery to a subset of nodes the resource is physically capable of running on can significantly boost performance when a large set of nodes are preset. When pacemaker_remote is in use to expand the node count into the 100s of nodes range, this option can have a dramatic affect on the speed of the cluster.

Is using this option ever a bad idea?

Absolutely!

Setting this option to never or exclusive allows the possibility for the resource to be active in those locations without the cluster’s knowledge. This can lead to the resource being active in more than one location!

There are primarily three ways for this to happen:

1. If the service is started outside the cluster’s control (ie. at boot time by init, systemd, etc; or by an admin)
2. If the resource-discovery property is changed while part of the cluster is down or suffering split-brain
3. If the resource-discovery property is changed for a resource/node while the resource is active on that node

When is it safe?

For the most part, it is only appropriate when:

1. you have more than 8 nodes (including bare metal nodes with pacemaker-remoted), and
2. there is a way to guarentee that the resource can only run in a particular location (eg. the required software is not installed anywhere else)

Want to know more?

Drop by IRC or ask us a question on the Pacemaker mailing list

There is also plenty of documentation available.