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Disk Access.

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Presentation on theme: "Disk Access."— Presentation transcript:

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 striped disks + 2 mirrors 10 - mirrored disks + a striped pair

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