Disk Access

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

Rotation Algorithms

• SLTF - latency, by sector position

• SPTF - positioning-time

• SATF - access-time (next in appearance)

Other considerations

• Cache

• Fragmentation

• Partitioning

• Compression

• Record blocking

• Hot-spot detection (speeds up access)

problems

• Write-through– Coherency is high

• Lazy-write– Low coherency

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