In Part 1 and Part 2 of this series, Is a Server Cluster Right for Your Organization? and Choosing the Cluster Type that's Right For You , I explained about the two different types of server clusters in Windows 2000. In this article, I'll discuss the issues involved in planning a network load balancing (NLB) server cluster in more depth. In the final article of this series, I'll present an in-depth discussion of server clusters.
|If you just need a refresher, an NLB cluster refers to an environment in which each server contains its own hard disks, and therefore, its own copy of the application you're clustering. Each server in an NLB environment functions as an independent entity. The cluster can contain between 2 and 32 servers. The extent of the actual clustering process is that each server communicates its present workload and other status information to other servers in the cluster. This is done so that as more clients try to use the clustered application, they are sent to the server with the least current workload.|
The NLB models
There are four basic models of NLB clusters for single or multiple network adapters running in either multicast or unicast mode. Multicasting refers to sending packets to a group of recipients. For example, when you send an e-mail message to a mailing list, you're multicasting. Unicasting, on the other hand, refers to sending packets to a single recipient. Each model has its own specific hardware requirements as well as a unique set of advantages and disadvantages.
The four models are as follows:
- Each server contains a single network adapter running in unicast mode.
- Each server contains a single network adapter running in multicast mode.
- Each server contains multiple network adapters running in unicast mode.
- Each server contains multiple network adapters running in multicast mode.
Each Server Contains a Single Network Adapter Running in Unicast Mode
NLB clusters that use a single network adapter and run in unicast mode are among the easiest types of cluster environments to configure. They are also perhaps the cheapest, because minimal hardware is needed. However, as you might expect with a starter system, you need to be aware of some rather significant limitations.
Most of the limitations are caused by having a single network card. The network card must maintain the normal network traffic flow along with carrying the backbone traffic that flows between servers in the cluster. Forcing so much data to flow across a single network card per server means that overall network performance will usually be compromised. Another limitation is that with the NLB cluster implementation, NetBIOS can't be run in single network card environments. These details are trivial, though, when compared to the fact that ordinary communications between servers within the cluster are impossible in this model.
Each Server Contains a Single Network Adapter Running in Multicast Mode
Even with a single network adapter present, running your clustered servers in multicast mode is a much better alternative to running in unicast mode. For starters, this model makes ordinary communications between clustered servers possible. The downside to this model is that not all routers support multicast Media Access Control (MAC) addresses. As with the other single network card model, this model also suffers from the performance issues related to channeling all of the network traffic through a single network card. Likewise, this model doesn't offer any NetBIOS support within the cluster.