The most common reason for extending storage networks over geographical distances is to safeguard critical business data and provide near-continuous access to applications and services in the event of a localized disaster. Designing a distance connectivity solution involves a number of considerations. The enterprise must classify stored data and determine how important it is for business operation, how often it must be backed up and how quickly it needs to be recovered in the event of failure.

This is the first of a two-part series on long-distance storage networking extension for business continuity and Disaster Recovery (DR). This article focuses on some of the basics of fiber optic technology and presents the alternative technologies available today for long-distance extension for business continuity. The second article will discuss network design for high availability, best practices and take a look at Fibre Channel over IP (FCIP) trunking.

Classifying Data and Applications 

From the business perspective, applications and their data need to be classified by how critical they are for business operation, how often data must be backed up and how quickly it needs to be recovered in the event of failure.

Two important concepts in the design process are:

• Recovery Point Objective (RPO) is the period of time between backup points that describes the acceptable age of the data that must be restored after a failure has occurred. For example, if a remote backup occurs every day at midnight and a site failure occurs at 11 p.m., changes to data made within the last 23 hours won’t be recoverable.
• Recovery Time Objective (RTO) is the time it takes to recover from the disaster; this determines the acceptable length of time a break in continuity can occur with minimal or no impact to business services.

Options for replication generally fall into one of several categories. A business continuity solution with strict RTO and RPO may require high-speed synchronous or near-synchronous replication between sites as well as application clustering for immediate service recovery. A medium-level DR solution may require high-speed replication that could be synchronous or asynchronous with an RTO from several minutes to a few hours. Backup of non-critical application data that doesn’t require immediate access after a failure can be accomplished via tape vaulting. Recovery from tape has the greatest RTO. Other technologies, such as Continuous Data Protection (CDP), can be used to find the appropriate RPO and RTO.

From a technology perspective, there are several choices for the optical transport network and configuration options for the Fibre Channel (FC) Storage Area Network (SAN) when it’s extended over distance. Applications with strict RTO and RPO require high-speed synchronous or near-synchronous replication between sites with application clustering over distance for immediate service recovery. Less critical applications may only require high-speed replication that could be asynchronous to meet the RPO/RTO metrics. Lower priority applications that don’t need immediate recovery after a failure can be restored from backup tapes from remote vaults.

Business Requirements

As more applications drive business value, and the associated data becomes key to competitive advantage, cost-effective protection of the applications and data from site disasters and extended outages has become the norm. Modern storage arrays provide synchronous as well as asynchronous array-to-array replication over extended distances.

When the array provides block-level storage for applications, FC is the primary network technology used to connect the storage arrays to servers, both physical and virtual. For this reason, cost-effective DR designs leverage FC to transport replicated data between arrays in different data centers over distances spanning a few to more than 100 kilometers. Therefore, SAN distance extension using FC is an important part of a comprehensive, cost-effective and effective DR design.

It’s important to understand the FC protocol and the optical transport technology and how they interact. In a discussion of long-distance configuration, it’s also useful to note the formal structure of the FC protocol and the specific standards that define the operation of the protocol: 

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