Quorum device in a cluster configuration

Server and Cluster Administration Guide for Hitachi NAS Platform

Version
15.1.x
Audience
anonymous
Part Number
MK-92HNAS010-35

The quorum device (QD) enables a cluster to maintain operations following a communications failure between nodes and also to restore the cluster registry (containing the cluster configuration), as follows:

  • Surviving a communication failure between nodes. Clustering preserves data integrity through a quorum voting algorithm that ensures only one node can access a given file system at any time. Under this algorithm, each of the cluster nodes may “vote” regarding file access. When a cluster contains an even number of nodes, the QD also votes. When a cluster node has obtained a quorum (a simple majority of the votes available in the entire cluster) it receives exclusive access to the file system. Under certain failure scenarios, cluster nodes can lose communication with each other and may attempt to access the same file system; in this situation, the QD alone “votes” for one of the nodes, establishing the quorum and granting one node exclusive access to the file system.
  • Preserving a copy of the cluster registry. Although the registry is replicated across cluster nodes, some failure scenarios could result in the loss of recent configuration changes, a condition called amnesia. Anticipating the possibility of such a condition, the QD preserves a copy of the registry, ensuring that configuration changes can always be replicated.

An external quorum device runs on the same system as the NAS Manager and can provide QD services for up to eight clusters (or up to eight servers in a server farm).

Note: The cluster created in a VSP N series or VSP Gx00 or VSP Fx00 with NAS modules has a maximum of two nodes. The NAS module hardware introduced with version 12.6 contains two nodes that are automatically clustered, and quorum device management is internal.

The number of entities (devices) required to form an acceptable quorum depends on the number of nodes in the cluster. The formula for establishing the acceptable number of entities is simple: (the number of nodes in the cluster / 2) + 1. For example:

Number of nodes in the cluster Number of entities (devices) required for a functioning quorum Entities (devices) that may be in the quorum
2 2 1 node + 1 NAS Manager
3 2 2 nodes (the NAS Manager is not used as a quorum device in a cluster with an odd number of nodes)
4 3 2 nodes + 1 NAS Manager, or 3 nodes and no NAS Manager
5 3 3 nodes (the NAS Manager is not used as a quorum device in a cluster with an odd number of nodes)
6 4 3 nodes + 1 NAS Manager, or 4 nodes and no NAS Manager
7 4 4 nodes (the NAS Manager is not used as a quorum device in a cluster with an odd number of nodes)
8 5 4 nodes + 1 NAS Manager, or 5 nodes and no NAS Manager.

If enough of the entities (nodes and the NAS Manager) in the cluster are not functioning, and an acceptable quorum cannot be formed, and the cluster will fail, and stop providing file serving functions. Cluster nodes may go into a boot loop cycle, trying to establish a quorum, and cluster nodes may even go off line in certain circumstances.