Fencing (computing)

Fencing is the process of isolating a node of a computer cluster or protecting shared resources when a node appears to be malfunctioning.

As the number of nodes in a cluster increases, so does the likelihood that one of them may fail at some point. The failed node may have control over shared resources that need to be reclaimed and if the node is acting erratically, the rest of the system needs to be protected. Fencing may thus either disable the node, or disallow shared storage access, thus ensuring data integrity.

A node fence (or I/O fence) is a virtual "fence" that separates nodes which may have access to a shared resource from nodes which must not. It may separate an active node from its backup. If the backup crosses the fence and, for example, tries to control the same disk array as the primary, a data hazard may occur. Mechanisms such as STONITH are designed to prevent this condition.

Isolating a node means ensuring that I/O can no longer be done from it. Fencing is typically done automatically, by cluster infrastructure such as shared disk file systems, in order to protect processes from other active nodes modifying the resources during node failures. Mechanisms to support fencing, such as the reserve/release mechanism of SCSI, have existed since at least 1985.

Fencing is required because it is impossible to distinguish between a real failure and a temporary hang. If the malfunctioning node is really down, then it cannot do any damage, so theoretically no action would be required (it could simply be brought back into the cluster with the usual join process). However, because there is a possibility that a malfunctioning node could itself consider the rest of the cluster to be the one that is malfunctioning, a split brain condition could ensue, and cause data corruption. Instead, the system has to assume the worst scenario and always fence in case of problems.

There are two classes of fencing methods, one which disables a node itself, the other disallows access to resources such as shared disks. In some cases, it is assumed that if a node does not respond after a given time-threshold it may be assumed as non-operational, although there are counterexamples, e.g. a long paging rampage.

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