HP Integrity rx5670 Windows Integrity Cluster Installation and Configuration G - Page 14

Introduction Cluster terminology NOTE There are two main reasons why the quorum resource is important in a cluster: • Consistency-Because the basic idea of a cluster is multiple physical servers acting as a single virtual server, it is critical that each physical server have a consistent view of how the cluster is configured. The quorum acts as the definitive repository for all configuration information relating to the cluster. In the event that the Cluster Service is unable to read the quorum log, it will not start, since it is unable to guarantee that the cluster is in a consistent state, which is one of the primary requirements for a cluster. • Arbitration-The quorum is used as the tie-breaker to avoid "split-brain" scenarios. A split-brain scenario occurs when all of the network communication links between two or more cluster nodes fail. In these cases, the cluster may split into two or more partitions that cannot communicate with each other. The quorum then guarantees that any cluster resource is only brought online on one node only. It does this by allowing the partition that "owns" the quorum to continue, while the other partitions are evicted from the cluster. There are two types quorums: the single quorum, and the Majority Node Set (MNS) quorum. Given the differences in the way that single quorum clusters behave compared to MNS quorum clusters, care must be taken when deciding which model to choose. For example, if you only have two nodes in your cluster, the MNS model is not recommended, as failure of one node will lead to failure of the entire cluster (since a majority of nodes is impossible). Also, once the cluster is created, you cannot change the quorum from one type to another. Your decision about the quorum type must be made during creation of the cluster, and cannot be changed later by simply switching the cluster quorum resource. A description of the two quorum types follows. Single quorum As mentioned previously, a quorum is basically a configuration database for the cluster, and is stored in the quorum log file. A single quorum uses a quorum log file that is located on a single disk hosted on a shared storage interconnect that is accessible by all nodes in the cluster. Single quorums are available in Windows Server 2003 Enterprise Edition and Windows Server 2003 Datacenter Edition. It is possible to configure clusters to use the local hard disk on one node to store the quorum, but this is only supported for testing and development purposes, and should not be used in a production environment. Each node connects to the shared storage through some type of interconnect, with the storage consisting of either external hard disks (usually configured as RAID disks), or a storage area network (SAN), where logical slices of the SAN are presented as physical disks. Also, it is important that the quorum uses a physical disk resource, as opposed to a disk partition, as the entire physical disk resource is moved during failover. 14 Chapter 1