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

• 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

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 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