HP Cisco MDS 9140 Cisco MDS 9000 Family Storage Media Encryption Configuration - Page 93
Cluster Quorum, Master Switch Election, Two-Switch Cluster Scenarios
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Chapter 4 Cisco SME Cluster Management Cluster Quorum and Master Switch Election Overview Send documentation comments to [email protected] Cluster Quorum For a cluster to be operational, it must include more than half the number of configured switches in the cluster view. In an N-node cluster, N/2 + 1 nodes form a cluster quorum. If N is even, the cluster quorum requires N/2 nodes and also, the presence of the switch with the lowest node ID. The quorum logic ensures that in the event of cluster partitions, at most one partition can be operational. All other switches are nonoperational. This guarantees the consistency of the cluster. Master Switch Election When a cluster is created, the switch on which the cluster is created becomes the cluster master switch. When the master switch fails or is rebooted, another switch takes over as the master switch. The master election logic uses the node ID and the latest cluster configuration to determine which switch in the cluster will become the master switch. The master election logic is describe as follows: • If the master switch fails in an operational cluster, the switch with the next lowest node ID takes over as the master switch. Note that in an operational cluster, all the switches run the same cluster configuration. - When the previous master switch comes back online and joins the cluster, it does not immediately become the master. • When all the switches of a cluster are coming up, the switch that has the latest cluster configuration becomes the master switch. If there are multiple switches with the same configuration, the switch with the lowest node ID is chosen to be the master switch. - Once a master switch is chosen and the cluster is operational (there is a quorum), even if a switch with a lower node ID joins the cluster at a later time, the master switch does not change. For example, there are three switches S1, S2, and S3 with node IDs 1, 2, and 3, respectively. If switches S2 and S3 form a quorum then switch S2 becomes the master switch. Even if switch S1 with the node ID of 1 comes up and joins the cluster at a later time, switch S2 continues to be the master. However, if switch S2 goes down for any reason, switch S1 will become the master switch. Two-Switch Cluster Scenarios According to the cluster quorum logic (see the "Cluster Quorum" section on page 4-31), a cluster with two configured switches can be operational if both switches are operational or the switch with the lowest node ID is operational. In the latter case, the switch with the lowest node ID is the master of the one-switch cluster. The other switch could have failed or simply lost connectivity to the operational switch. In either case, the switch with the higher node ID would become nonoperational. If the node with the lower node ID failed, the other switch cannot form an operational cluster. The examples that follow describe these scenarios. The first three examples consider single switch failures. 1. Assume that in a two-switch cluster with switches S1 (node ID 1) and S2 (node ID 2), S1 is the master (the master has the lower node ID). OL-18091-01, Cisco MDS NX-OS Release 4.x Cisco MDS 9000 Family Storage Media Encryption Configuration Guide 4-31