Reliability, Availability and Serviceability
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Reliability, Availability, and Serviceability are computer hardware engineering terms. It originated from IBM to advertise the robustness of their mainframe computers. The concept is often known by the acronym RAS.
Mainframe computers have a multitude of features that help them stay up (stay operational) for long periods of time without crashing (stop working) due to system faults. These long uptimes (periods between system crashes or downtime) are a key selling feature of mainframes and even more so for fault-tolerant systems. Such computer vendors pride themselves in uptimes on the order of years without a system crash.
• Reliability means features that help avoid and detect such faults. A reliable system does not silently continue and deliver results that include uncorrected corrupted data, instead it corrects the corruption when possible or else stops and reports the corruption.
• Availability is the amount of time a device is actually operating as the percentage of total time it should be operating. Availability may be shown as minutes or hours of downtime per year. Availability features allow the system to stay operational even when faults do occur. A highly available system would disable the malfunctioning portion and continue operating at a reduced capacity. In contrast, a less capable system might crash and become totally nonoperational.
• Serviceability takes the form of various methods of easily diagnosing the system when problems arise. Early detection of faults can decrease or avoid system downtime. For example, some of IBM's systems could automatically call an IBM service center (without human intervention) when the system experiences a system fault. Traditional focus has been on making the correct repairs with as little disruption to normal operations as possible.
RAS features might include:
• Parity or ECC protection of memory components as well as buses.
• Cyclic redundancy check checksums for data transmission and data storage.
• RAID configurations for magnetic disk storage.
• Journaling file systems for file repair after crashes.
• Over-designing the system for the specified operating ranges of clock frequency, temperature, voltage, vibration.
• Duplication of computing components running in lock-step to perform master-checker or voting schemes.
• Duplication of components to avoid single point of failures (for example power-supplies).
• Hot swapping of components.
• Partitioning/domaining of computer components to allow one large system to act as several smaller systems.
• Computer clustering capability.
• Virtual machines to decrease the severity of operating system software faults.
• Temperature sensors to throttle operating frequency when temperature goes out of specification.
• Surge protector, uninterruptible power supply, auxiliary power.
• Failover capability.
Fault-tolerant designs from vendors like Tandem Computers extended the idea by making RAS to be the defining feature of their computers for applications like stock market exchanges or air traffic control where system crashes would be catastrophic. Fault-tolerant computers, which tend to have duplicate components running in lock-step for reliability, have become less popular due to their high cost. High availability systems, using distributed computing techniques like computer clusters, are often used as cheaper alternatives.
这些RAS特性的最终实现需要系统厂商和软件厂商的支持。目前,在操作系统和虚拟化方面,Novell、Redhat、微软和VMware都已经做好准备,为至强7500中的高级RAS特性提供支持,以保证可以将这些功能提供给上层的应用软件使用。另一方面,虽然至强7500提供了20多个RAS功能,但并不是所有服务器厂商都需要或者有能力去实现这些功能,比如硬件分区、CPU热添加/热删除、内存板的热添加/热移除,不同厂商会有各自的考虑。这些都需要用户在方案选型时加以考虑。
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send_linux 发表于 2010-04-29 11:26
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