[OS]I/O System and Disk Management

I/O Mechanisms

• Processor controlled memory access
  • Polling (Programmed I/O)
  • Interrupt
• Direct Memory Access (DMA)

Pooling(Programmed I/O)

• The processor periodically checks the status of the I/O devices
  • Simple
  • Pooling overhead

Interrupt

• After I/O devices completes the task, communicate its status to the processor
  • lower overhead than pooling
  • Interrupt handling overhead

I/O Servies of OS

• I/O Scheduling
  • E.g., Disk I/O scheduling
• Error handling
  • E.g., disk access fail, network communication error
• I/O device information managements
• Buffering
• Caching
• Spooling

Disk Scheduling

• Optimizing seek time
  • FCFS (First Come First SErvice)
  • SSTF (Shortest Seek Time First)
  • Scan
  • C-Scan (Circular Scan)
  • Look
• Optimizing rotational delay
  • Sector queueing (SLTF, Shortest Latency Time First)
• SPTF (Shortest Positioning Time First)

FCFS

• Advantage
  • Simple
    • Low scheduling overhead
• Problem
  • Does not consider optimization

SSTF

• Advantage
  • High throughput
  • Low average response time
• Problem
  • Low predictability
  • Starvation

Scan Scheduling

• Advantage
  • Fix starvation issue in SSTF
  • Excellent throughput and average response time
• Problem
  • High response time on the other side of the process

C-Scan Scheduling

• Similar to Scan Scheduling
• Advantage
  • Equal opportunity than Scan

Look Scheduling

• Elevator algorithm
• Advantage
  • Remove unnecessary head movement of Scan

SLTF

• Use Fixde head disk system
  • E.g., drum disk
• Sector queuing algorithm

SPTF

• Positioning time = Seek time + rotational delay
• Advantage
  • High throughput
  • Low average response time
• Problem
  • Starvation occurs for innermost and outer cylinder requests

RAID Architecture

• Redundant Array of Inexpensive Disks(RAID)
  • OS support, RAID controller
• Use to improve disk system performance
  • Performance (access speed)
  • Reliability

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Reference

I/O System and Disk Management