Disaster recovery options
With a business impact analysis in hand and agreement on RTOs and RPOs, IT management can devise implementation options. Disaster recovery site terminology can be confusing -- terms like hot site, warm site and cold site are common in DR parlance, but they're used inconsistently. A hot site in the U.S. typically comprises shared equipment, while "in Europe the term hot site is predominantly used for dedicated equipment," says Ferguson. The following definitions match the prevailing U.S. interpretations of these terms:
- Hosted site. A site with dedicated equipment; required whenever RTO and RPO need to be close to zero.
- Hot site. Uses shared equipment with dedicated storage and real-time replication; a typical RTO of a few hours.
- Warm site. Uses shared equipment without dedicated storage, but depends on data backup for recovery; RTOs can range from a few hours to days depending on the backup method in use.
- Cold site. Typically, dedicated space in a data center fully loaded with cooling, power and connectivity ready to accept equipment; RTOs are usually a week or more.
It's quite common for a DR site to serve various roles for different applications. For instance, a DR site may serve as a hosted site with close to real-time failover for a mission-critical e-commerce
Click here for a comparison of disaster recovery tiering options for a 5 TB Microsoft Exchange 2007 environment using plans and pricing from Recovery Point Systems as an example.
A tier 1 DR offering provides the highest level of DR protection, and is typically used for applications that require close to zero RTOs and RPOs. A characteristic of tier 1 DR is the use of dedicated equipment in the DR site. As a result, it carries the highest price tag and is usually only for the most mission-critical applications. Because the equipment in the DR site is dedicated to a single client company, there are very few constraints on the equipment that can be used, even if the service is outsourced.
Among all DR options, it's best suited to be hosted in-house, where your company owns and maintains the site. Because of the need for specific DR equipment, it's typically less expensive to build tier 1 DR in-house than to outsource it. "Tier 1 DR can be done more cost-effectively in-house, especially if you have the facility and people," explains HP's Ferguson.
Because applications in the primary and DR site are closely coupled, production and DR equipment are commonly managed by one entity.
If the DR site is hosted by a third party, it's not unusual for both the primary and DR equipment to be managed by the DR services provider. As an example, Citrix Systems Inc. decided to outsource management of both its primary HP XP12000 SAN and its DR site. While the production SAN physically resides in Citrix's primary data center in Miami, the DR SAN is hosted by HP. "Our SAN storage in Miami is outsourced with and managed by HP," says Michael Emerson, director, IT security, governance and business continuity at Citrix. "They own the SAN and manage it, including the replication from the production to the DR SAN at HP, using HP Continuous Access replication [HP StorageWorks XP Continuous Access Software]."
This was first published in January 2009