Windows Server 2003 : Designing a Server Cluster (part 1) - Planning a Server Cluster Hardware Configuration

1/9/2014 3:23:27 AM
Server clusters are, by definition, more complicated than Network Load Balancing clusters, both in the way they handle applications and in the way they handle the application data. When designing a server cluster implementation, you still must evaluate your organization’s high-availability needs, but you must do so in light of a server cluster’s greater deployment cost and greater capabilities.

1. Designing a Server Cluster Deployment

Server clusters are intended to provide advanced failover capabilities for stateful applications, particularly database and e-mail servers. Because the data files maintained by these applications change frequently, it is not practical for individual servers in a cluster to maintain their own individual copies of the data files. If this were the case, the servers would have to immediately propagate changes that clients make to their data files to the other servers, so that the server could present a unified data set to all clients at all times.

As a result, server clusters are based on a shared data storage solution. The cluster stores the files containing the databases or e-mail stores on a drive array (typically using RAID or some other data availability technique) that is connected to all the servers in the cluster. Therefore, all the application’s clients, no matter which server in the cluster they connect to, are working with the same data files, as shown in Figure 1.

Figure 1. Server cluster nodes share application data

The shared data store adds significantly to the cost of building a server cluster, especially if you plan to create a geographically dispersed cluster. Unlike geographically dispersed NLB clusters, which are usually separate clusters unified by an external technology, such as round robin DNS, the hosts in server clusters must be connected to the central data store, even when the servers are in different cities. This means that you must construct a SAN connecting the various sites, as well as a standard WAN. When considering a deployment of this type of cluster, you must decide whether the impact of having your applications offline justifies the expense of building the required hardware infrastructure.

2. Planning a Server Cluster Hardware Configuration

The computers running Windows Server 2003 that you use to build a server cluster must all use the same processor architecture, meaning that you cannot mix 32-bit and 64-bit systems in the same cluster. Each server in the cluster must have at least one standard network connection giving it access to the other cluster servers and to the client computers that use the cluster’s services. For maximum availability, having two network interface adapters in each computer is preferable, one providing the connection to the client network, and one connecting to a network dedicated to communications between the servers in the cluster.

In addition to standard network connections, each server must have a separate connection to the shared storage device. Windows Server 2003 supports three types of storage connections: Small Computer System Interface (SCSI) and two types of Fibre Channel, as discussed in the following sections.

Using SCSI

SCSI is a bus architecture used to connect storage devices and other peripherals to personal computers. SCSI implementations typically take the form of a host adapter in the computer and a number of internal or external devices that you connect to the card, using appropriate SCSI cables. In a shared SCSI configuration, however, you use multiple host adapters, one for each server in the cluster, and connect the adapters and the storage devices to a single bus, as shown in Figure 2.

Figure 2. A cluster using a SCSI bus

Understanding SCSI

The SCSI host adapter is the component responsible for receiving device access requests from the computer and feeding them to the appropriate devices on the SCSI bus. Although you can use SCSI devices on any personal computer by installing a host adapter card, SCSI is usually associated with servers because it can handle requests for multiple devices more efficiently than other interfaces.

When the Integrated Drive Electronics (IDE) devices used in most PC workstations receive an access request from the computer’s host adapter, the device processes the request and sends a response to the adapter. The adapter remains idle until it receives the response from that device. Only when that response arrives can the adapter send the next request. SCSI host adapters, by contrast, can send requests to many different devices in succession, without having to wait for the results of each one. Therefore, SCSI is better for servers that must handle large numbers of disk access requests.

Many personal computers marketed as servers have an integrated SCSI host adapter. If the computers you use for your cluster servers do not already have SCSI adapters, you must purchase and install a SCSI host adapter card for each one.

Because of the limitations of the SCSI architecture, Windows Server 2003 supports only two-node clusters using SCSI, and only with the 32-bit version of Windows Server 2003, Enterprise Edition. SCSI is not supported on Windows Server 2003, Datacenter Edition. SCSI hubs are also not supported. In addition, you cannot use SCSI for a geographically dispersed cluster, as the maximum length for a SCSI bus is 25 meters.

SCSI Clustering

SCSI is designed to support multiple devices and multiple device types on a single bus. The original SCSI standard supported up to eight devices (including the SCSI host adapter), while some newer versions of the standard can support up to 16. For the SCSI adapter to communicate with each device individually, you must configure each device on the bus with a unique SCSI ID. SCSI IDs range from 0 to 7 on the standard bus, and SCSI host adapters traditionally use ID 7. When you create a shared SCSI bus for your server cluster, you must modify the SCSI ID of one of the host adapters on the bus so that both are not using the same ID.

The other requirement for all SCSI buses is that both ends of the bus be terminated so that the signals generated by the SCSI devices do not reflect back in the other direction and interfere with new signals. A terminator uses resistors to remove the electrical signals from the cable. You must have appropriate terminators installed at the ends of your shared SCSI bus, and Microsoft recommends physical terminating devices rather than the termination circuits built into many SCSI devices.

Using Fibre Channel

Fibre Channel is a high-speed serial networking technology that was originally conceived as a general purpose networking solution, but which has instead been adopted primarily for connections between computers and storage devices. Unlike SCSI, which is a parallel signaling technology, Fibre Channel uses serial signaling, which enables it to transmit over much longer distances. Fibre Channel devices can transmit data at speeds up to 100 megabytes per second using full duplex communications, which means that the devices can transmit at full speed in both directions simultaneously.

Off the Record

The nonstandard spelling of the word “fibre” in Fibre Channel is deliberate. The designers of the technology want to avoid confusion with the term “fiber optic,” because Fibre Channel connections can use copper-based as well as fiber-optic cable as a network medium.

The most common method for implementing a Fibre Channel storage solution on a server cluster is to install a Fibre Channel host adapter in each cluster server and then use them to connect the computers to one or more external storage devices. The storage devices are typically self-contained drive arrays or NAS devices, using RAID to provide high data availability.

Windows Server 2003 supports two types of Fibre Channel topologies for connecting cluster servers to storage devices: Fibre Channel arbitrated loop (FC-AL) and Fibre Channel switched fabric (FC-SW).

Fibre Channel Arbitrated Loop

In the context of Windows Server 2003 server clusters, a Fibre Channel arbitrated loop is a ring topology that connects cluster servers with a collection of storage devices, as shown in Figure 3. The total number of devices in an arbitrated loop is limited to 126, but Windows Server 2003 limits the number of servers in an arbitrated loop cluster to two.

Figure 3. A cluster using a Fibre Channel arbitrated loop network

A Fibre Channel arbitrated loop is a shared network medium, which is one reason for the two-server limit. Data packets transmitted by one device on the loop might have to pass through other devices to reach their destinations, which lowers the overall bandwidth available to the individual devices. Compared to switched fabric, arbitrated loop is a relatively inexpensive clustering hardware technology that enables administrators to easily expand their storage capacity (although not the number of cluster nodes).

Fibre Channel Switched Fabric

The only shared storage solution supported by Windows Server 2003 that is suitable for server clusters of more than two nodes is the Fibre Channel switched fabric network. FC-SW is similar in configuration to a switched Ethernet network, in which each device is connected to a switch, as shown in Figure 4. Switching enables any device on the network to establish a direct, dedicated connection to any other device. There is no shared network medium as in FC-AL; the full bandwidth of the network is available to all communications.

Figure 4. A cluster using a Fibre Channel switched fabric network

An FC-SW network that is wholly dedicated to giving servers access to data storage devices is a type of SAN. Building a SAN to service your server cluster provides the greatest possible amount of flexibility and scalability. You can add nodes to the cluster by installing additional servers and connecting them to the SAN, or you can expand the cluster’s shared storage capacity by installing additional drives or drive arrays. You can also build a geographically dispersed server cluster by extending the SAN to locations in other cities.

  •  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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