The need to address application and data availability to keep operations running at levels customers expect has long been understood by IT managers and executives. Each year this need becomes more critical as more of the business becomes dependent on IT operations, and as all the non-IT support services disappear. Several years ago, cost-of-downtime was used to justify expenditures on business continuity. Today, downtime could shut down the business.
It has become common practice among some marketing people, consultants, analysts, and vendors to use the term Highly Available with little real appreciation of what it actually means. In order to help reduce confusion among customers and to hold solution providers accountable in 1994 Harvard Research Group, Inc. (HRG) took a significant step in defining availability and what it means for a solution to be Highly Available. Now with the introduction of Cloud Computing and converged systems the need to hold Cloud providers and systems vendors accountable for their use of the term. For example, If your business is an on-line /real time business how much down time can you afford before your customers move to a competitor? How you define availability in term of your SLA agreement with your Cloud provider takes on new importance and it is critical that you share a common definition of what it means to be available. With regard to converged systems you need to know ahead of time how sensitive your applications and customers that will be supported on these systems are to down time. Once again a common definition of terms can be very helpful in making sure you and your vendor are on the same page.
It is for these reason and others not detailed above that HRG decided to define availability in terms of the impact of an outage on the activity of the business and consumer (end user) of the service. HRG's five Availability Environments (AE) define availability in terms of the impact on the both the business and the end user or consumer. Each successive level incorporates all the functionality of the previous level. When a failure and subsequent system recovery response has occurred the system is assumed to not be at its original Availability Environment Classification until the failure has been corrected and any failed components have been replaced or repaired. In some cases, there may be system redundancies that support fail over capabilities in the event of an additional failure. The minimum requirement for a system to be considered highly available is that there is a backup copy of data available on a redundant disk and a log-based or journal file system exists for identification and recovery of incomplete transactions -- this represents the AE 1 Availability Environment Classification.
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