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Controlling server access to storage
Because a user request is going to a virtual IP address associated with an application rather than a server, the actual server that's "actively" running the application needs to have access and control of the underlying storage. And because there are multiple servers that could be running the application and possibly answering the request, the servers' access to the storage must be controlled.

The storage administrator has to leverage all of the spindles and array controllers to ensure database performance, as discussed in the first article of this series (see "

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Configuring storage for ERP," Storage, December 2006). Once that's complete, the physical disks that house the tablespace, archive files and log files that make up the ERP application's data must be presented to every node in the cluster. SANs, of course, connect the physical data to multiple servers in the cluster. How the storage is accessed, and which server is actively in control of the storage (and when), is typically a function of the server's operating system and its clustering technique, rather than a function of the storage array software.

Active-passive/active-active clusters
Methods of controlling access to the storage vary greatly. In an active-passive cluster, one server answers all user requests and the second (passive) server waits to take over if required. The second server can do other things, but it must be prepared to be repurposed immediately to service user requests if the need arises. The clustering software enables the passive server to constantly check on the active server. This is typically done through multiple network connections and is referred to as the heartbeat between the two servers. Certain cluster-based configuration parameters can dramatically change the timing of a cluster failover.

In an active-active cluster, more than one server can respond to user requests and more than one server can access the storage to retrieve or write the data. In an active-active cluster, either the database application or the operating system must determine who has control of the storage at that moment, allowing access to storage to move back and forth between servers with every request. Oracle Real Application Clusters (RAC) is a form of active-active clustering in which the Oracle application manages all storage access from multiple servers. RAC enables every database server in the cluster to process user requests and allows every server to access the underlying storage. The servers must be able to communicate quickly; in many cases, high-speed server interconnects like InfiniBand are used to keep the independent servers and their respective operating systems acting in unison.

A clustered database server architecture basically has an IP address that can move between servers, and the ability to control access to the data. This means that an entire layer of the production model can fail and the application will still be available. However, the business unit that depends on the application needs to know how long the application will be down if the database server fails.

A significant benefit of clustering an ERP app is a reduction in planned downtime. While this is never the reason companies initiate a clustering project, it's the most frequently realized benefit IT organizations enjoy. Clustering allows maintenance activities to occur outside the critical path of ERP app downtime.

This was first published in February 2007

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