High-availability computer systems

Abstract

Today's highly available systems deliver four years of uninterrupted service.The challenge is to build systems with 100-year mean time to failure and one-minute repair times. September 1991aradoxically, the larger a system is, the more critical -but less likelyit is to be highly available. We can build small ultra-available modules, but building large systems involving thousands of modules and millions of lines of code is a poorly understood art, even though such large systems are a core technology of modern society.Three decades ago, hardware components were the major source of faults and outages. Today, hardware faults are a relatively minor cause of system outages when compared with operations, environment, and software faults. Techniques and designs that tolerate these broader classes of faults are still in their infancy.This article sketches the techniques used to build highly available computer systems. Historical perspectiveComputers built in the late 1950s offered a 12-hour mean time to failure. A maintenance staff of a dozen full-time computer engineers could repair the machine in about eight hours. This failure-repair cycle provided 60 percent availability. The vacuum tube and relay components of these computers were the major sources of failures; they had lifetimes of a few months. So the machines rarely operated for more than a day without interruption.'Many fault-detection and fault-masking techniques used today were first used on these early computers. Diagnostics tested the machine. Self-checking computational techniques detected faults while the computation progressed. The program occasionally saved (checkpointed) its state on stable media. After a failure and repair, the program read the most recent checkpoint and continued the computation from that point. This checkpoint-restart technique let computers that failed every few hours perform long-running computations.Device improvements have increased computer system availability. By 1980, typical well-run computer systems offered 99 percent availability.2 This sounds good, but 99 percent availability is 100 minutes of downtime per week. Such

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