Windows Clustering

A cluster is a group of computers, called nodes that function as a single computer/system to provide high availability and high fault tolerance for applications or services.  Windows 2003 Servers can participate in a cluster configuration through the use of Cluster Services. If one member of the cluster (the node) is unavailable, the other computers carry the load so that applications or services are always (with a small interruption) available.

All nodes of the cluster use a Shared Disk – an external disk or disk subsystem which is accessible for all nodes through SCSI (2 Nodes) or Fibre Channel (more than 2 nodes). All data will be stored on the shared disk or an external disk subsystem (for example Exchange databases).

Every node has a local Exchange 2003 installation with a unique configuration for every cluster node. Each Cluster with Exchange 2003 has at a minimum one Exchange Virtual Server (EVS). An EVS is the logical node that will be used for all cluster operations. An EVS contains an IP address, network name, physical disk and an application.

A cluster can be …

• Active/Active or
• Active/Passive

Microsoft recommends only Active/Passive clusters – I will give you the reason later.

The number of cluster nodes supported by Windows 2003 Enterprise and Datacenter is 8 nodes. Windows Server 2003 Standard and Web Editon doesn’t support a Cluster.

In an Active/Passive cluster – If one node in the cluster fails, the active cluster failover to another node which becomes Active. This is called Failover. If the failed node is back online, a Failback can be manually initiated or automatically configured in the Cluster Group properties.

Every cluster node must have two network interfaces. One network interface for the cluster communication called the private LAN and one network interface called the public LAN. You can link a cluster with two nodes with a simple cross link cable. If more than two nodes exist in the cluster you have to use a dedicated switch / hub.

The private NIC is used for the Heartbeat communication (Cluster communication). A Heartbeat is much like a ping which can be used to test if the other cluster node is still available. If the heartbeat fails, the Failover process occurs.

Why Active/Active cluster is not recommended

Active / Active cluster is not recommended because of Virtual memory (VM) fragmentation.
When two instances of the information store is running in one memory space, it might be possible that sufficient VM does not exist to bring the resource online during a failover/failback

 

http://www.msexchange.org/tutorials/Implementing-Two-Node-Cluster-Windows-2003-Enterprise.html