Approaches To Fencing
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.
The STONITH method stands for "Shoot The Other Node In The Head", meaning that the suspected node is disabled or powered off. For instance, power fencing uses a power controller to turn off an inoperable node. The node may then restart itself and join the cluster later. However, there are approaches in which an operator is informed of the need for a manual restart for the node.
The resources fencing approach disallows access to resources without powering off the node. This may include:
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- Persistent reservation fencing uses the SCSI3 persistent reservations to block access to shared storage.
- Fibre Channel fencing disables the fibre channel port
- Global network block device (GNBD) fencing which disables access to the GNBD server
When the cluster has only two nodes, the reserve/release method may be used as a two node STONITH whereby upon detecting that the other node has 'failed', the other node will issue the reserve and obtain all resources (e.g. shared disk) for itself. The other node will be disabled if it tries to do I/O (in case it was temporarily hung). The I/O failure triggers some code to kill the node.
Persistent reservation is essentially a match on a key, so the node which has the right key can do I/O, otherwise its I/O fails. Therefore, it is sufficient to change the key on a failure to ensure the right behavior during failure.
STONITH is an easier and simpler method to implement on multiple clusters, while the various approaches to resources fencing require specific implementation approaches for each cluster implementation.
Read more about this topic: Fencing (computing)
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