Within an HA cluster, critical services are constantly monitored and restarted automatically on other nodes in the case of failure. This automation can introduce problems, however, if some communications problem occurs with the primary node; although the service running on that node appears to be down, it in fact continues to run and write data to the shared storage. In this case, starting a new set of services on a backup node could easily cause data corruption.
Clusters use a variety of techniques collectively called fencing to prevent this from happening, including Split Brain Detection (SBD) and Shoot The Other Node In The Head (STONITH). The primary goal is to prevent data corruption on the shared storage.