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