Disaster recovery planning on a budget


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RPO/RTO impact of potential DR solutions

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In this example, introducing replication and increasing communications bandwidth to support higher volumes of data could further reduce recovery point objective (RPO) to 12 hours. By upgrading from a warm site to a hot site, it would be possible to decrease recovery time objective (RTO) from under two days to about six hours.

Source: GlassHouse Technologies

Recently, I received a call from an IT manager responsible for disaster recovery (DR) for a large organization. He was reasonably certain that the DR test that was scheduled in a few months would fail. He was hoping to identify and resolve some of the problems.

More than any other category of IT activity, DR strains the planning and operational capabilities of an IT organization. A DR infrastructure typically is expensive to implement, has low utilization and requires tasks to be accomplished in a time-critical manner. A careful analysis of the trade-offs that you can tolerate--with an understanding of what resources you can commit--should help you get closer to an optimal set up.

The perfect DR infrastructure
Based on today's technological capabilities, experts agree on a standard model that's regarded as the state of the art. The foundation of this model involves disk-based replication. Ideally, you would start with a storage area network (SAN) infrastructure with anywhere from two to six times the primary storage capacity to enable remote replication--preferably via synchronous means--to ensure a current copy of data is available in at least two geographically distinct locations.

To accomplish this, you'd have to standardize on a tier-one storage system such as EMC Symmetrix, HDS Lightning or IBM ESS, with appropriate split mirror and remote replication software. Another critical requirement is the appropriate bandwidth. Dense wavelength division multiplexing (DWDM) is the premiere technology available (see "Linking SANs for disaster recovery, Parts 1 and 2" in the September and October 2003 issues of Storage).

To incorporate this technology, there's a complex integration effort that includes planning and implementing the right combinations of split mirrors and replicated volumes to meet the environment's requirements. You have to balance business requirements, latency considerations, application characteristics, bandwidth constraints and organizational capabilities. The good news is that once the effort has been expended and the management processes are refined, disk-based replication can be highly effective.

The downside--cost
By now, you're probably asking yourself if you can afford this and if you really need it. Disk replication is expensive. Not only is the initial investment in the equipment, software and integration services costly, but for many companies the monthly expense for the additional bandwidth is simply too much.

Lower-cost solutions such as host-based software replication or appliance-based solutions using an enhanced WAN infrastructure are available and can be effective in some situations. One significant benefit is the ability of these products to replicate independently of the storage platform, allowing you to replicate from a high-priced storage system to a lower priced system.

Another expensive consideration is the recurring cost of maintaining a hot or warm site. Large organizations with multiple data centers can often leverage existing space within these sites for DR purposes or can justify the cost of a centralized DR facility shared by multiple divisions. For others, a remote hosting facility with systems made available on an as-needed basis is the only cost-justifiable solution. However, such an environment increases the time to recover and complicates recovery.

This was first published in January 2004

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