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Windows Server 2008 R2 high-availability and recovery features : Introduction to High Availability

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2/11/2014 8:27:39 PM

Many large organizations rely heavily on computer systems for mission critical business processes. In fact, some companies rely on these systems so much that a single hour of downtime for a critical system can end up costing a company thousands of dollars. Most administrators are well aware of all the things that can cause downtime to a server, including hardware failures, operating system issues, and regular maintenance. Microsoft has developed and continued to evolve HA solutions over the years to meet the needs of organizations that require very high uptimes even in the event of a server failure.

Notes From the Field

High availability versus disaster recovery

It is important to understand the differences between HA and disaster recovery when administering a Windows network. Though the two are very different, their purposes can overlap in some instances. Traditionally, HA is used to ensure that an application remains online in the event of server failure. Disaster recovery is usually defined as a process used to recover a business process including systems in the event that the system has been completely lost or in the worst-case scenario, the geographic location that hosted the business process and supporting systems has been destroyed or taken offline by a natural disaster such as fire, flood, hurricane, or tornado. With that understanding, there are situations where overlap occurs.


Windows Server 2008 R2 offers several features to ensure that applications and network services can sustain a failure of a primary server without the application or service experiencing significant downtime. The two primary Windows features that offer HA are

  • Failover Clusters

  • Network Load Balancing Clusters

Let us take a look at both of these features to understand their differences and how they can be used to provide HA to applications and services on your network.

Failover Clusters

Failover Clusters provide HA by implementing a “failover” process from a primary server (active) to a standby server (passive). Each server in the cluster is referred to as a node. Using the active/passive technology reserves one or more servers that sit idle in the event of a failure on the active server. In the event of a failure of the active node, a passive node will become active and carry on all activities the previous active node performed. Failover Clusters are typically used for back-end applications such as Microsoft Exchange Server or SQL Server. Figure 1 depicts a typical Windows Server 2008 R2 two-node Failover Cluster.

Figure 1. Two-node Failover Cluster.


Network Load Balancing Clusters

NLB is the second type of HA service offered by Windows Server 2008 R2. Load balancing clusters can offer HA features to traditional front-end services such as IIS-based Web sites. NLB Clusters work in a very similar fashion as hardware-based network load balancers in that they distribute traffic between multiple servers that are members of the cluster. In the event that a server fails, NLB will direct traffic only to online servers, skipping the failed server. NLB Clusters not only provide HA services but also balance traffic between all servers in the cluster ensuring that the load is evenly distributed. This also provides an easy way to scale front-end systems. For example, you might have two web servers in a NLB Cluster. Suppose the load on those servers becomes heavy and the Web site performance decreases dramatically. You notice that load is using all of the CPU resources on both web servers. You could simply add a third node to the cluster providing additional resources. The network traffic would then be distributed across three servers opposed to two. Figure 2 depicts a typical network load balanced cluster.

Figure 2. Three-node Network Load-Balanced Cluster.


Best Practices

Change management

Even with HA and recovery technologies, you should always follow good change management processes when making changes to any, and especially mission critical systems. Be sure that you document any configuration changes to production systems, and if your organization requires, get proper approval before making changes. Even with HA technologies, misconfigurations can easily bring business services offline and cause helpdesk phones to start ringing.

 
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