Chapter 40. Clustering

As a Technology Preview starting with Red Hat Enterprise Linux 7.4, the pcs tool supports bundle resources. You can now use the pcs resource bundle create and the pcs resource bundle update commands to create and modify a bundle. You can add a resource to an existing bundle with the pcs resource create command. For information on the parameters you can set for a bundle resource, run the pcs resource bundle --help command. (BZ#1433016)

As a Technology Preview, Pacemaker now supports the fence_heuristics_ping agent. This agent aims to open a class of experimental fence agents that do no actual fencing by themselves but instead exploit the behavior of fencing levels in a new way.

If the heuristics agent is configured on the same fencing level as the fence agent that does the actual fencing but is configured before that agent in sequence, fencing issues an off action on the heuristics agent before it attempts to do so on the agent that does the fencing. If the heuristics agent gives a negative result for the off action it is already clear that the fencing level is not going to succeed, causing Pacemaker fencing to skip the step of issuing the off action on the agent that does the fencing. A heuristics agent can exploit this behavior to prevent the agent that does the actual fencing from fencing a node under certain conditions.

A user might want to use this agent, especially in a two-node cluster, when it would not make sense for a node to fence the peer if it can know beforehand that it would not be able to take over the services properly. For example, it might not make sense for a node to take over services if it has problems reaching the networking uplink, making the services unreachable to clients, a situation which a ping to a router might detect in that case. (BZ#1476401)

Heuristics are a set of commands executed locally on startup, cluster membership change, successful connect to corosync-qnetd, and, optionally, on a periodic basis. When all commands finish successfully on time (their return error code is zero), heuristics have passed; otherwise, they have failed. The heuristics result is sent to corosync-qnetd where it is used in calculations to determine which partition should be quorate. (BZ#1413573, BZ#1389209)