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