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Arbitration For Data Integrity in Serviceguard Clusters: > Chapter 1 Arbitration for Data Integrity in Serviceguard Clusters

Use of a Quorum Server as the Cluster Lock


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A quorum server can be used in clusters of any size. The quorum server is an alternate form of cluster lock that uses a server program running on a separate system for tie-breaking rather than a lock disk. Should two equal sized groups of nodes (exactly 50% of the cluster in each group) become separated from each other, the quorum server allows one group to achieve quorum and form the cluster, while the other group is denied quorum and cannot start a cluster.

The quorum server process runs on a machine outside of the cluster for which it is providing quorum services. The quorum server listens to connection requests from the Serviceguard nodes on a known port. The server maintains a special area in memory for each cluster, and when a node obtains the cluster lock, this area is marked so that other nodes will recognize the lock as “taken.” The cluster will maintain this mark during an off-on power cycle of the quorum server.

The operation of the quorum server is shown in Figure 7. When there is a loss of communication between node 1 and node 2, the quorum server chooses one node (in this example, node 2) to continue running in the cluster. The other node halts.

Figure 1-7 Quorum Server Operation

Quorum Server Operation

The quorum server runs on a Linux or HP-UX system.You can configure the quorum server as a Serviceguard package in another cluster; it must be a cluster other than the one for which it is providing quorum services. A quorum server, whether or not it is running as a package, can provide quorum services for multiple clusters.

NOTE: Consult your Configuration Guide for up-to-the-minute information on types of supported quorum server configuration, as well as on support of quorum server as a package.

Setting up the Quorum Server

If you are using a quorum server (QS), the QS software, which has to be running during cluster configuration, must be installed on a system other than the nodes on which your cluster will be running. This can be either a single HP-UX or Linux system.

NOTE: On HP-UX, the use of Auto Port Aggregation in LAN_MONITOR mode is recommended for the quorum server system. In Linux, channel bonding can be employed.

Ensure that the connection with each node is independent of the heartbeat connection, so that both are not likely to fail at the same time.

Running the Quorum Server

The quorum server must be running during the following cluster operations:

  • when the cmquerycl command is issued.

  • when the cmapplyconf command is issued.

  • when there is a cluster re-formation.

Specifying a Quorum Server

If you will be using a quorum server, be sure to specify the -q qshost option with the cmquerycl command. Example:

# cmquerycl -v -n lp1 -n lp2 -q lp-qs  -C clus-lp.config

Quorum Server Status and State

The status of the quorum server can be one of the following:

  • Up. The quorum server is active.

  • Down. The quorum server is not active.

The state of the quorum server can be one of the following:

  • Running. Quorum services are active and available.

  • Unsupported Version. There is a version mismatch between Serviceguard and Quorum Server.

  • Access Denied. The Serviceguard node is not authorized to access the quorum server system.

  • Unknown. Serviceguard cannot determine whether the quorum server is up or down. This may happen when the quorum server cannot be reached from the current node.

Viewing Quorum Server Status and State

If the cluster is using a quorum server for tie-breaking services, you can use the cmviewcl command to display the server name, state, and status following the entry for each node, as in the following excerpt from the output of cmviewcl -v:

clus-lp      up

  NODE         STATUS       STATE
  lp1          up           running

Quorum Server Status:
lp-qs               up           running


  NODE         STATUS       STATE
  lp2          up           running

Quorum Server Status:
lp-qs                up           running

Viewing Quorum Server System Data

You can also log on to the quorum server system itself and use the ps command to confirm that the qs process is running. In addition, you can view data about connections from cluster nodes in the quorum server log file.

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