Google Paper:Design Project 3?PageRankRedundant Array of Disks6.033 Quiz 2 ReviewApril 18, 2007Why RAID?v Economicsv Aggregate I/O of multiple cheap disks can surpass individual SLEDv Lower powerv Belief in unreliable componentsRAID Level 0 v Interleave or stripe data (bit, bytes, sectors)q Block A is spread across multiple disksRAID Level 1v Mirroring; duplicate data on second diskv Advantages:q Could double read throughput by reading in parallelv Disadvantages:q Write to multiple disks; subject to slowdownq 50% of disk capacity wastedRAID Level 2v Reduce number of check disks using ECCq e.g. 10 data disks, need 4 check disks1234567P1P2D0P3D1D2D3P3 = D3 ⊕ D1 ⊕ D0P2 = D3 ⊕ D2 ⊕ D0P1 = D3 ⊕ D2 ⊕ D1RAID Level 3v Rely on hardware error-detection (fail-fast)v Bit/Byte-level interleaving + parityq Block A is broken up into individual bytesa1b1Disk 1S0a2b2Disk 2S0a3b3Disk 3S0a4b4Disk 4S0aPbPCheckS0c1d1c2d2c3d3c4d4cPdPRAID Level 4v Keep files together; group blocks into single sectora1a2Disk 1b1b2Disk 2c1c2Disk 3d1d2Disk 4P1P2Checka3a4b3b4c3c4d3d4P3P4RAID Level 5v Spread parity sector across diskss0s1Disk 1s0s1Disk 2s0s1Disk 3s0s1Disk 4s0s1Disk 5s2s3s2s3s2s3s2s3s2s3s4s4 s4
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