System-Level Fault Tolerance (Clustering/Network Load Balancing) - Active and Passive Clustering Modes
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Active/Passive Clustering Mode
Active/passive clustering occurs when one node in the cluster provides clustered services while the other available node or nodes remain online but do not provide services or applications to end users. When the active node fails, the cluster groups previously running on that node are failed over to the passive node, causing the node's participation in the cluster to go from passive to active state to begin servicing client requests.
This configuration is usually implemented with database servers that provide access to data that is stored in only one location and is too large to replicate throughout the day. One advantage of Active/Passive mode is that if each node in the cluster has similar hardware specifications, there is no performance loss when a failover occurs. The only real disadvantage of this mode is that the passive node's hardware resources cannot be leveraged during regular daily cluster operation.
Note - Active/passive configurations are a great choice for keeping cluster administration and maintenance as low as possible. For example, the passive node can be used to test updates and other patches without directly impacting production. However, it is nonetheless important to test in an isolated lab environment or, at a minimum, during after hours or predefined maintenance windows.
Active/Active Clustering Mode
Active/active clustering occurs when one instance of an application runs on each node of the cluster. When a failure occurs, two or more instances of the application can run on one cluster node. The advantage of Active/Active mode over Active/Passive mode is that the physical hardware resources on each node are used simultaneously. The major disadvantage of this configuration is that if you are running each node of the cluster at 100% capacity, in the event of a node failure, the remaining active node assumes 100% of the failed node's load, greatly reducing performance. As a result, it is critical to monitor server resources at all times and ensure that each node has enough resources to take over the other node's responsibilities if the other should failover.
 This chapter is from Microsoft Windows Server 2003 Unleashed, by Rand Morimoto, et al. (Sams Publishing, 2004, ISBN: 0672326671). Check it out at your favorite bookstore today.
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