Dell EMC Metro node 7.1 Product Guide

Quorum

Quorum refers to the minimum number of directors required for the cluster to service and maintain operations.

There are different quorum rules for a cluster to become operational and start servicing I/Os when it is booting up, also called “gaining quorum.” Different rules for an operational cluster seeing director failures to either continue servicing operations and I/O after failure handling is called “maintaining quorum.” Stopping servicing operations and I/O is called “losing quorum.” These rules are described below:

• Gaining quorum - A non-operational metro node cluster gains quorum and becomes operational when more than half of the configured directors restart and come in contact with each other. In a single engine cluster, it refers to all the directors.
• Maintaining quorum - An operational metro node cluster seeing failures will continue operating in the following scenarios:
  • Director failures
    • If less than half of the operational directors with quorum fail.
    • If half of the operational directors with quorum fail, then the remaining directors will check the operational status of the failed directors over the management network and remain alive.

    After recovering from this failure, a cluster can tolerate further similar director failures until only one director is remaining. In a single engine cluster, a maximum of one director failure can be tolerated.

  • Intra-cluster communication failure
    • If there is a split in the middle, that is, half of the operational directors with quorum lose communication with the other half of the directors, and both halves are running, then the directors detect the operational status over the management network and instruct half with the director with the lowest UUID to keep running and the directors without the lowest UUID to operationally stop.
• Quorum loss - An operational metro node cluster seeing failures stops operating in the following scenarios:
  • If more than half of the operational directors with quorum fail at the same time.
  • If half of the operational directors with quorum fail, and the directors are unable to determine the operation status of the other half of the directors (whose membership includes a low UUID).
  • In a dual or quad engine cluster, if all of the directors loose contact with each other.