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Disk Access

Disk Access. Seek Algorithms. FCFS SSTF SCAN - ssd in preferred direction CSCAN - outer to inner, ssd, jump to outer FSCAN - scan for only those at start of scan nSCAN - only first n requests LOOK - lookahead & change direction C-LOOK - outer to inner & jump. Rotation Algorithms.

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Disk Access

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  1. Disk Access

  2. Seek Algorithms • FCFS • SSTF • SCAN - ssd in preferred direction • CSCAN - outer to inner, ssd, jump to outer • FSCAN - scan for only those at start of scan • nSCAN - only first n requests • LOOK - lookahead & change direction • C-LOOK - outer to inner & jump

  3. Rotation Algorithms • SLTF – Shortest Latency Time First • latency, by sector position • SPTF - Shortest Positioning Time First • Time to position the access-head • SATF – Shortest Access Time First • close coupling of CPU and disk • processor knows rotational position of the disk at any time

  4. Other considerations • Cache • Fragmentation • Partitioning • Compression • Record blocking • Hot-spot detection (speeds up access)

  5. problems • Write-through • Coherency is high • Lazy-write • Low coherency

  6. RAID • 0 - striping - no Fault Tolerance • 1 - mirroring (see my patent) • 2 - bit-striping with Hamming parity stripes • 3 - like 2 but XOR ECC stripes • 4 - large block stripes + XOR ECC stripes • 5 - like 4 but parity with data • 0+1 - 2 striped disks + 2 mirrors • 10 - mirrored disks + a striped pair

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