disk access. seek algorithms fcfs sstf scan – r/w in (current) preferred direction only –so...
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Disk Access
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Seek Algorithms
• FCFS• SSTF• SCAN – r/w in (current) preferred direction only
– So inward, then outward, etc
• CSCAN – one way, r/w, then jump back to start• FSCAN - scan for only those at start of scan• nSCAN - only first n requests• LOOK - lookahead & change direction• C-LOOK – LOOK with jump back to start point
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Rotation Algorithms
• SLTF - latency, by sector position
• SPTF - positioning-time
• SATF - access-time (next in appearance)
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Other considerations
• Cache
• Fragmentation
• Partitioning
• Compression
• Record blocking
• Hot-spot detection (speeds up access)
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problems
• Write-through– Coherency is high
• Lazy-write– Low coherency
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RAID
• 0 - striping - no FT• 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