RAID - MIT OpenCourseWare

6.033 Spring 2018Lecture #14

• Reliability via Replication• General approach to building fault-tolerance systems• Single-disk failures: RAID

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How to Design Fault-tolerant Systems in Three Easy Steps

1. identify all possible faults

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How to Design Fault-tolerant Systems in Three Easy Steps

1. identify all possible faults

2. detect and contain the faults

3. handle the fault

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quantifying reliability

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dealing with disk failures

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dealing with disk failures

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RAID 1 (mirroring)

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can recover from single-disk failure requires 2N disks

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RAID 4 (dedicated parity disk)

xxx xxx xxx xxx xxx

sector i of the parity diskxor is the xor of sector i… from all data disks

parity data disks disk

! can recover from single-disk failure! requires N+1 disks (not 2N)! performance benefits if you stripe a

single file across multiple data disks" all writes hit the parity disk

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RAID 5 (spread out the parity)

xxx xxx … xxx

xxx xxx

! can recover from single-disk failure! requires N+1 disks (not 2N)! performance benefits if you stripe a

single file across multiple data disks! writes are spread across disks

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• Systems have faults. We have to take them into accountand build reliable, fault-tolerant systems. Reliabilityalways comes at a cost — there are tradeoffs betweenreliability and monetary cost, reliability and simplicity, etc.

• Our main tool for improving reliability is redundancy.One form of redundancy is replication, which can beused to combat many things including disk failures(important, because disk failures mean lost data).

• RAID replicates data across disks in a smart way: RAID 5protects against single-disk failures while maintaininggood performance.

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6.033 Computer System EngineeringSpring 2018

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