print barcode labels in c# Estimated lesson time: 40 minutes in Visual Basic .NET

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Estimated lesson time: 40 minutes
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Designing a Server Cluster Deployment
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As you learned in Lesson 1 of this chapter, 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 serv ers 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 7-10.
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Figure 7-10 Server cluster nodes share application data
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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, you must decide whether the impact of having your applica tions offline justifies the expense of building the required hardware infrastructure.
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 sys tems 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 connec tion to the shared storage device. Windows Server 2003 supports three types of storage
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7
Clustering Servers
connections: Small Computer System Interface (SCSI) and two types of Fibre Channel, as discussed in the following sections.
Planning Microsoft strongly recommends that all the hardware components you use in your
cluster servers for Windows Server 2003, and particularly those that make up the shared storage solution, be properly tested and listed in the Windows Server Catalog.
Using SCSI
SCSI is a bus architecture used to connect storage devices and other peripherals to per sonal 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 mul tiple 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 7-11.
Server Server Storage Storage
Figure 7-11 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.
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