Overview of Windows Clustering

A cluster is a group of independent computers that work together to run a common set of applications or services and provide the image of a single system to the client and application. Clustered computers are physically connected by cables and logically  connected by cluster software. These connections allow computers to use problem-solving features, such as load balancing and fail-over that are not available for use with stand-alone computers.

Network Load Balancing: Network Load Balancing clusters enable you to manage a group of independent servers as a single system for greater scalability, increased availability, and easier manageability. You can use Network Load Balancing to implement enterprise-wide scalable solutions for the delivery of Transmission Control Protocol/Internet Protocol (TCP/IP) based services and applications.

Network Load Balancing works by distributing client requests across a set of servers. It is particularly useful for ensuring that stateless applications, such as Web pages from a server running Internet Information Services (IIS), are highly available and can be scaled out by adding additional servers as the load increases. The ease with which Network Load Balancing allows you to replace a malfunctioning server or add a new server provides scalability.

Failover Cluster :  Using Advanced Server, Cluster service lets you combine two servers to work together as a server cluster to ensure that mission-critical applications and resources remain available to clients. Server clusters enable users and administrators to access certain resources of the servers, or nodes, as a single system rather than as separate computers.

Quorum(Witness Disk) Overview

You can choose from among four possible quorum configuration.

Node Majority (recommended for clusters with an odd number of nodes)
Can sustain failures of half the nodes (rounding up) minus one. For example, a seven node cluster can sustain three node failures.

Node and Disk Majority (recommended for clusters with an even number of nodes)
Can sustain failures of half the nodes (rounding up) if the disk witness remains online. For example, a six node cluster in which the disk witness is online could sustain three node failures.

Can sustain failures of half the nodes (rounding up) minus one if the disk witness goes offline or fails. For example, a six node cluster with a failed disk witness could sustain two (3-1=2) node failures.

Node and File Share Majority (for clusters with special configurations)
Works in a similar way to Node and Disk Majority, but instead of a disk witness, this cluster uses a file share witness.

Note that if you use Node and File Share Majority, at least one of the available cluster nodes must contain a current copy of the cluster configuration before you can start the cluster. Otherwise, you must force the starting of the cluster through a particular node.

No Majority: Disk Only (not recommended)
Can sustain failures of all nodes except one (if the disk is online). However, this configuration is not recommended because the disk might be a single point of failure.

Why quorum is necessary

When network problems occur, quorum can interfere with communication between cluster nodes. A small set of nodes might be able to communicate together across a functioning part of a network but not be able to communicate with a different set of nodes in another part of the network. This can cause serious issues. In this "split" situation, at least one of the sets of nodes must stop running as a cluster.

To prevent the issues that are caused by a split in the cluster, the cluster software requires that any set of nodes running as a cluster must use a voting algorithm to determine whether, at a given time, that set has quorum. Because a given cluster has a specific set of nodes and a specific quorum configuration, the cluster will know how many "votes" constitutes a majority (that is, a quorum). If the number drops below the majority, the cluster stops running. Nodes will still listen for the presence of other nodes, in case another node appears again on the network, but the nodes will not begin to function as a cluster until the quorum exists again.

For example, in a five node cluster that is using a node majority, consider what happens if nodes 1, 2, and 3 can communicate with each other but not with nodes 4 and 5. Nodes 1, 2, and 3 constitute a majority, and they continue running as a cluster. Nodes 4 and 5, being a minority, stop running as a cluster. If node 3 loses communication with other nodes, all nodes stop running as a cluster. However, all functioning nodes will continue to listen for communication, so that when the network begins working again, the cluster can form and begin to run.

Quorum improvements in Windows 2012

Dynamic witness

In Windows Server 2012 R2, if the cluster is configured to use dynamic quorum (the default), the witness vote is also dynamically adjusted based on the number of voting nodes in current cluster membership. If there are an odd number of votes, the quorum witness does not have a vote. If there is an even number of votes, the quorum witness has a vote.

The quorum witness vote is also dynamically adjusted based on the state of the witness resource. If the witness resource is offline or failed, the cluster sets the witness vote to "0."