The reliability, availability, and serviceability (or "RAS") of a computer system have always been important factors in data processing. When we say that a particular computer system "exhibits RAS characteristics," we mean that its design places a high priority on the system remaining in service at all times. Ideally, RAS is a central design feature of all aspects of a computer system, including the applications.

RAS has become accepted as a collective term for many characteristics of hardware and software that are prized by mainframe users. The terms are defined as follows:

Reliability

The system's hardware components have extensive self-checking and self-recovery capabilities. The system's software reliability is a result of extensive testing and the ability to make quick updates for detected problems.

Availability

The system can recover from a failed component without impacting the rest of the running system. This term applies to hardware recovery (the automatic replacing of failed elements with spares) and software recovery (the layers of error recovery that are provided by the operating system).

Serviceability

The system can determine why a failure occurred. This capability allows for the replacement of hardware and software elements while impacting as little of the operational system as possible. This term also implies well-defined units of replacement, either hardware or software.

A computer system is available when its applications are available. An available system is one that is reliable; that is, it rarely requires downtime for upgrades or repairs. And, if the system is brought down by an error condition, it must be serviceable; that is, easy to fix within a relatively short period of time.

Mean time between failure (MTBF) refers to the availability of a computer system. The New Mainframe and its associated software have evolved to the point that customers often experience months or even years of system availability between system downtimes. Moreover, when the system is unavailable because of an unplanned failure or a scheduled upgrade, this period is typically very short. The remarkable availability of the system in processing the organization's mission-critical applications is vital in today's 24-hour global economy. Along with the hardware, mainframe operating systems exhibit RAS through such features as storage protection and a controlled maintenance process.

Beyond RAS, a state-of-the-art mainframe system might be said to provide "high availability" and fault tolerance. Redundant hardware components in critical paths, enhanced storage protection, a controlled maintenance process, and system software designed for unlimited availability all help to ensure a consistent, highly available environment for business applications in the event that a system component fails. Such an approach allows the system designer to minimize the risk of having a single point of failure undermine the overall RAS of a computer system.