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Disk scheduling (Policies (SCAN (C-SCAN (Circular scan), N-STEP-SCAN…
Disk scheduling
Policies
FIFO
Fair
Low performance
Priority
Linux
Linus elevator
Linux anticipatory I/O
SCAN
C-SCAN
Circular scan
N-STEP-SCAN
FSCAN
Move arm in one direction and service requests until all done, turn
Shortest service time first
Service request w/ least arm movement from current position
LIFO
Starvation
Little arm movement
Buffering
Single buffer
Double buffer
Buffer-swapping
Block-oriented
Stream-oriented
Circular buffer
Double buffer but w/ more buffers
Disk cache
Smoother interaction between varying speed systems
Replacement policy
Least recently used
Least frequently used
RAID
Levels
0
Striping
No redundancy
1
Mirroring
Duplicate all data
2
Parallel access
Hamming code
Striping
3
Parity bit
Parallel access
4
Independent access
Parity strip
5
Parity bits across all disks
High data recovery cost
6
Two different parity calculations
High data recovery cost
Seek time + rotational delay = access time
Reduce seek time by scheduling