SHARE originally defined seven tiers of disaster recovery to help identify
various methods of recovering mission-critical computer-systems as
required to support business continuity. IBM also aided in the development of the
model. Although the original published concept
dates back to the 1990s, specialists in business continuity planning and in
disaster recovery as of 2015 continue to use the seven tiers to illustrate
continuity capabilities and costs at a very high level. The definitions for the
various tiers have been updated as technology has evolved in support of
business requirements and their associated Recovery Time Objectives and
Recovery Point Objectives. 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 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 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 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 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 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: References
External links The tiers of Disaster Recovery and TSM.
Charlotte Brooks, Matthew Bedernjak, Igor Juran, and John Merryman. In,
Disaster Recovery Strategies with Tivoli Storage Management. Chapter 2. Pages
21–36. Red Books Series. IBM. Tivoli Software. 2002.