Seven tiers of disaster recovery

Tier levels

When the SHARE Technical Steering Committee, working with IBM, described the levels of service for disaster recovery in the late 1980s, it produced a model that used Tiers 0 through 6. As a result of the introduction by IBM of their Geographically Dispersed Parallel Sysplex (GDPS) system, which allowed for an organization to manage end-to-end application and data availability across multiple geographically separate sites, an additional seventh tier was added to represent the industry's highest possible level of service availability. The seven tiers of business continuity solutions offer a simple method to define current service levels and associated risks.

Tier 0: No off-site data – Possibly no recovery

Businesses with a Tier 0 business continuity solution have no business continuity plan. There is no saved information, no documentation, no backup hardware, and no contingency plan. The time necessary to recover in this instance is unpredictable. In fact, it may not be possible to recover at all.

Tier 1: Data backup with no hot site

Businesses that use Tier 1 continuity solutions back up their data and send these backups to an off-site storage facility. The method of transporting these backups is often referred to as "PTAM" - the "Pick-up Truck Access Method." Depending on how often backups are created and shipped, these organisations must be prepared to accept several days to weeks of data loss, but their backups are secure off-site. However, this tier lacks the systems on which to restore data.

Tier 2: Data backup with a hot site

Businesses using Tier 2 business continuity solutions make regular backups on tape. This is combined with an off-site facility and infrastructure (known as a hot site) in which to restore systems from those tapes in the event of a disaster. This solution will still result in the need to recreate several hours or even days' worth of data, but the recovery time is more predictable.

Tier 3: Electronic vaulting

Tier 3 solutions build on the components of Tier 2. Additionally, some mission critical data is electronically vaulted. This electronically vaulted data is typically more current than that which is shipped via PTAM. As a result there is less data recreation or loss after a disaster occurs. The facilities for providing Electronic Remote Vaulting consists of high-speed communication circuits, some form of channel extension equipment and either physical or a virtual tape library and an automated tape library at the remote site. IBM's Peer-to-Peer VTS and Oracle StorageTek Virtual Storage Manager (VSM) Clustering are two examples of this type implementation.

Tier 4: Point-in-time copies

Tier 4 solutions are used by businesses that require both greater data currency and faster recovery than users of lower tiers. Rather than relying largely on shipping tape, as is common on the lower tiers, Tier 4 solutions begin to incorporate more disk based solutions. Several hours of data loss is still possible, but it is easier to make such point-in-time (PiT) copies with greater frequency than tape backups even when electronically vaulted.

Tier 5: Transaction integrity

Tier 5 solutions are used by businesses with a requirement for consistency of data between the production data center and the recovery data centers. There is little to no data loss in such solutions; however, the presence of this functionality is entirely dependent on the application in use.

Tier 6: Zero or near-zero data loss

Tier 6 business continuity solutions maintain the highest levels of data currency. They are used by businesses with little or no tolerance for data loss and who need to restore data to applications rapidly. These solutions have no dependence on the applications or applications staffs to provide data consistency. Tier 6 solutions often require some form of Disk mirroring. There are various synchronous and asynchronous solutions available from the mainframe storage vendors. Each solution is somewhat different, offering different capabilities and providing different Recovery Point and Recovery Time objectives. Often some form of automated tape solution is also required. However, this can vary somewhat depending on the amount and type of data residing on tape.

Tier 7: Highly automated, business integrated solution

Tier 7 solutions include all the major components being used for a Tier 6 solution with the additional integration of automation. This allows a Tier 7 solution to ensure consistency of data above that which is granted by Tier 6 solutions. Additionally, recovery of the applications is automated, allowing for restoration of systems and applications much faster and more reliably than would be possible through manual business continuity procedures.

Other schemes

Some authorities have developed alternate DR tier schemes, also called Recovery Classes. For example, Computer Network Technologies (CNT) has developed a scheme by categorizing RTOs and RPOs into different classes. Class 1 is the lowest level, where acceptable recovery times range from 72 hours to one week, and the most up-to-date data can be from a weekly backup. A class 4 recovery environment contains the most stringent requirements. With Class 4, the recovery time must be immediate and the data recovered must be less than one second old. The following table illustrates this Recovery Classes 4-tier scheme: