Windows Server 2003 : Designing a Server Cluster (part 3) - Creating a Server Cluster

1/9/2014 3:25:28 AM

4. Selecting a Quorum Model

Every node in a server cluster maintains a copy of the cluster database in its registry. The cluster database contains the properties of all the cluster’s elements, including physical components such as servers, network adapters, and shared storage devices, and cluster objects such as applications and other logical resources. When a cluster node goes offline for any reason, its cluster database is no longer updated as the cluster’s status changes. When the node comes back online, it must have a current copy of the database to rejoin the cluster, and it obtains that copy from the cluster’s quorum resource.

A cluster’s quorum contains all the configuration data needed for the recovery of the cluster, and the quorum resource is the drive where the quorum is stored. To create a cluster, the first node must be able to take control of the quorum resource so that it can save the quorum data there. Only one system can have control of the quorum resource at any one time. Additional nodes must be able to access the quorum resource so that they can create the cluster database in their registries.

Selecting the location for the quorum is a crucial part of creating a cluster. Server clusters running Windows Server 2003 support the following three types of quorum models:

  • Single-node cluster A cluster that consists of only one server. Because there is no need for a shared storage solution, the application data store and the quorum resource are located on the computer’s local drive. The primary reason for creating single-node clusters is for testing and development.

  • Single-quorum device cluster The cluster uses a single quorum resource, which is one of the shared storage devices accessible by all the nodes in the cluster. This is the quorum model that most server cluster installations use.

  • Majority node set cluster A separate copy of the quorum is stored in each cluster node, with the quorum resource responsible for keeping all copies of the quorum consistent. Majority node set clusters are particularly well suited to geographically dispersed server clusters and clusters that do not have shared data storage devices.

5. Creating a Server Cluster

Before you actually create the cluster, you must select, evaluate, and install a shared storage resource and install the critical applications on the computers running Windows Server 2003. All the computers that are to become cluster nodes must have access to the shared storage solution you have selected; you should know your applications’ capabilities with regard to partitioning; and you should have decided how to deploy them. Once you have completed these tasks, you will use the Cluster Administrator tool to create and manage server clusters. (See Figure 6.)

Figure 6. Cluster Administrator

To create a new cluster, you must have the following information available:

  • The name of the domain in which the cluster will be located

  • The host name to assign to the cluster

  • The static IP address to assign to the cluster

  • The name and password for a cluster service account

With this information in hand, you can proceed to deploy the cluster, taking the following basic steps:

Start up the computer running Windows Server 2003 that will be the first node in the cluster.

At this time, the other servers you will later add to the cluster should not be running.

Use the Cluster Administrator application on the first server to create a new cluster.

During this process, the New Server Cluster Wizard detects the storage devices and network interfaces on the computer and determines whether they are suitable for use by the cluster. You also supply the name and IP address for the cluster and the name and password for the cluster service account.

Verify that the cluster is operational and that you can access the cluster disks.

At this point, you have created a single-node cluster.

Start up the computers running Windows Server 2003 that will become the other nodes in the cluster.

Use the Add Nodes Wizard in Cluster Administrator to make the other servers part of the cluster.

Test the cluster by using Cluster Administrator to stop the cluster service on each node in turn, verifying that the cluster disks are still available after each stoppage.

Once you have added all the nodes to the cluster, you can view information about the nodes in Cluster Administrator as well as manage the nodes and their resources from a central location. In addition, there are many clustering features you can use to configure how the cluster behaves under various conditions.

Understanding Cluster Resources

When managing a cluster, you frequently work with cluster resources. A cluster resource is any physical or logical element the cluster service can manage by bringing it online or offline and moving it to a different node. By default, the cluster resources supported by server clusters running Windows Server 2003 include storage devices, configuration parameters, scripts, and applications. When you deploy a third-party application on a server cluster, the application developer typically includes resource types that are specific to that application.

Some configuration tasks you can perform in Cluster Administrator are as follows:

  • Create resource groups A resource group is a collection of cluster resources that functions as a single failover unit. When one resource in the group malfunctions, the cluster service fails the entire group over to another node. You use the New Group Wizard to create resource groups, after which you can create new resources or move existing resources into the group.

  • Define resource dependencies You can configure a specific cluster resource to be dependent on other resources in the same resource group. The cluster service uses these dependencies to determine the order in which it starts and stops the resources on a node in the event of a failover. For example, when an application is dependent on a particular shared disk where the application is stored, the cluster service always brings down the application on a node before bringing down the disk. Conversely, when launching the application on a new node, the service will always start the disk before the application so that the disk is available to the application when it starts.

  • Configure the cluster network role For each network to which a cluster is connected, you can specify whether the cluster should use that network for client access only, for internal cluster communications only, or for both.

  • Configure failover relationships For each resource the cluster manages, you can specify a list of nodes that are permitted to run that resource. With this capability, you can configure a wide variety of failover policies for your applications.

Configuring Failover Policies

By configuring the failover relationships of your cluster applications and other resources, you can implement a number of different failover policies that control which cluster nodes an application uses and when. With small server clusters, failover is usually a simple affair because you don’t have that many nodes to choose from. As server clusters grow larger, however, their failover capabilities become more flexible. Some failover policies you might consider using are as follows:

  • Failover pairs In a large server cluster running several applications, each application is running on one node and has one designated standby node. This makes server capacity planning simple, as the servers are never running more than one application. However, half of the cluster’s processing capacity is not in use, and, in the event of multiple node failures, some applications could go offline unless an administrator intervenes.

  • Hot-standby server A single node functions as the designated standby server for two or more applications. This option uses the cluster’s processing capacity more efficiently (fewer servers are idle), but might not handle multiple node failures well. For capacity planning, the standby server has to support only the most resource-intensive application it might run, unless you want to plan for multiple node failures, in which case the standby must be capable of running multiple applications at once.

  • N+I An expanded form of the hot-standby server policy, in which you configure a number of active nodes running different applications (N) to fail over to any one of a number of idle servers (I). As an example, you can create a six-node server cluster with four applications running on four separate nodes, plus two standby nodes that are idle. When one of the active nodes malfunctions, its application fails over to one of the standby servers. This policy is better at handling multiple server failures than failover pairs or hot-standby servers.

  • Failover ring Each node in a server cluster runs an application, and you configure each application to fail over to the next node. This policy is suitable for relatively small applications because, in the event of a failure, a server might have to run two or more applications at once. In the event of multiple node failures, the application load could be unbalanced across the active nodes. For example, in a four-node cluster, if Server 1 fails, Server 2 must run its own application and that of Server 1. If Server 2 then fails, Server 3 must take on the Server 2 and Server 1 applications in addition to its own, while Server 4 continues to run only one application. This makes server capacity planning difficult.

  • Random In some cases, the best policy is for the administrator not to define any specific failover relationships at all, and let the cluster service be responsible for failing over resources to other nodes in the cluster. This policy is usually preferable for smaller applications so that a single node can conceivably run multiple applications if necessary. Random failovers also place less of a burden on the cluster administrator.

  •  Windows Server 2003 : Clustering Servers - Using Network Load Balancing (part 3) - Monitoring Network Load Balancing
  •  Windows Server 2003 : Clustering Servers - Using Network Load Balancing (part 2) - Deploying a Network Load Balancing Cluster
  •  Windows Server 2003 : Clustering Servers - Using Network Load Balancing (part 1) - Planning a Network Load Balancing Deployment
  •  Windows Server 2003 : Clustering Servers - Understanding Clustering (part 2) - Designing a Clustering Solution
  •  Windows Server 2003 : Clustering Servers - Understanding Clustering (part 1) - Clustering Types
  •  Windows Server 2003 : Administering Software Update Services (part 6) - SUS Backup and Recovery,Designing a Network Security Update Infrastructure
  •  Windows Server 2003 : Administering Software Update Services (part 5) - Configuring Automatic Updates Through Group Policy , SUS Troubleshooting
  •  Windows Server 2003 : Administering Software Update Services (part 4) - The Automatic Updates Client
  •  Windows Server 2003 : Administering Software Update Services (part 3) - Synchronizing SUS, Approving Updates
  •  Windows Server 2003 : Administering Software Update Services (part 2) - Configuring and Administering SUS - Configuring Software Update Services
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