Archive for the ‘Server 2008’ Category
Quorum Modes in Server 2008 and 2008 R2
The most vital concept needed for a cluster to function is that of a quorum. A quorum guarantees consistency of the services offered by the cluster by ensuring that specific services and applications are offered only by a single
active node, and that services and applications are offered only if the resources needed are available.
In Windows Server 2003, there were two basic quorum models that defined how the quorum was achieved and used. The most used model was the Single Quorum Device cluster, which consisted of a number of nodes connected to some kind of shared storage (SAN commonly), with a volume specified as the quorum disk that helped the cluster configuration database.
As long as the quorum was available with at least one node, the cluster was available, no matter how many nodes in total there were. The problem with this model was if the cluster quorum disk was unavailable, the entire cluster became unavailable; even though SANs are highly available with RAID to protect volumes, there were problems in production,
and the loss of the quorum disk was a major threat and a single point of failure. The other model supported in Windows Server 2003 was the Majority Node Set Cluster, which did not use a shared quorum disk, but instead each node in the cluster held its own copy of the cluster database.
To maintain the quorum, over half the nodes had to be available, which meant you had to have a minimum of three nodes in the cluster, which was not commonly the case—in most production environments, a cluster consisted of two nodes. Read the rest of this entry »
