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2.2. Fencing Overview
In a cluster system, there can be many nodes working on several pieces of vital production data. Nodes in a busy, multi-node cluster could begin to act erratically or become unavailable, prompting action by administrators. The problems caused by errant cluster nodes can be mitigated by establishing a fencing policy.
Fencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O from shared storage, thus ensuring data integrity. The cluster infrastructure performs fencing through the STONITH facility.
When Pacemaker determines that a node has failed, it communicates to other cluster-infrastructure components that the node has failed. STONITH fences the failed node when notified of the failure. Other cluster-infrastructure components determine what actions to take, which includes performing any recovery that needs to done. For example, DLM and GFS2, when notified of a node failure, suspend activity until they detect that STONITH has completed fencing the failed node. Upon confirmation that the failed node is fenced, DLM and GFS2 perform recovery. DLM releases locks of the failed node; GFS2 recovers the journal of the failed node.
Node-level fencing through STONITH can be configured with a variety of supported fence devices, including:
- Uninterruptible Power Supply (UPS) — a device containing a battery that can be used to fence devices in event of a power failure
- Power Distribution Unit (PDU) — a device with multiple power outlets used in data centers for clean power distribution as well as fencing and power isolation services
- Blade power control devices — dedicated systems installed in a data center configured to fence cluster nodes in the event of failure
- Lights-out devices — Network-connected devices that manage cluster node availability and can perform fencing, power on/off, and other services by administrators locally or remotely