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Disk Scheduling & Hardware: Improving Disk Performance & Understanding Disk Types

This chapter covers disk hardware, disk formatting, disk arm scheduling algorithms, error handling, and clocks in computer systems. It provides insights into improving disk performance and understanding different disk types.

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Disk Scheduling & Hardware: Improving Disk Performance & Understanding Disk Types

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  1. ITEC 502 컴퓨터 시스템 및 실습 Chapter 9-2: Disk Scheduling Mi-Jung Choi mjchoi@postech.ac.kr DPNM Lab. Dept. of CSE, POSTECH

  2. Contents • Disk Hardware • Disk Formatting • Disk Arm Scheduling Algorithms • Error Handling • Clocks

  3. Moving-head Disk Machanism

  4. Disk Hardware (1) Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disk

  5. Disk Hardware (2) • Physical geometry of a disk with two zones • A possible virtual geometry for this disk

  6. Disk Hardware (3) • Redundant Array of Inexpensive Disks (RAID) • Raid levels 0 through 2 • Backup and parity drives are shaded

  7. Disk Hardware (4) • Raid levels 3 through 5 • Backup and parity drives are shaded

  8. Disk Hardware (5) Recording structure of a CD or CD-ROM

  9. Disk Hardware (6) Logical data layout on a CD-ROM

  10. Disk Hardware (7) • Cross section of a CD-R disk and laser • not to scale • Silver CD-ROM has similar structure • without dye layer • with pitted aluminum layer instead of gold

  11. Disk Hardware (8) A double sided, dual layer DVD disk

  12. Disk Formatting (1) A disk sector

  13. Disk Formatting (2) An illustration of cylinder skew

  14. Disk Formatting (3) • No interleaving • Single interleaving • Double interleaving

  15. Disk Arm Scheduling Algorithms (1) • Time required to read or write a disk block determined by 3 factors • Seek time • Rotational delay • Actual transfer time • Seek time dominates • Error checking is done by controllers

  16. Disk Arm Scheduling Algorithms (2) Shortest Seek First (SSF) disk scheduling algorithm Pending requests Initial position

  17. Disk Arm Scheduling Algorithms (3) The elevator algorithm for scheduling disk requests

  18. Error Handling • A disk track with a bad sector • Substituting a spare for the bad sector • Shifting all the sectors to bypass the bad one

  19. Stable Storage Analysis of the influence of crashes on stable writes

  20. Clocks: Clock Hardware A programmable clock

  21. Clock Software (1) Three ways to maintain the time of day

  22. Clock Software (2) Simulating multiple timers with a single clock

  23. Soft Timers • A second clock available for timer interrupts • specified by applications • no problems if interrupt frequency is low • Soft timers avoid interrupts • kernel checks for soft timer expiration before it exits to user mode • how well this works depends on rate of kernel entries

  24. Summary • Variety of disk types • Magnetic disks, RAIRs, optical disks, etc. • Disk arm scheduling algorithms • Can be used to improve disk performance • But, the presence of virtual geometries complicates matters • By paring two disks, a stable storage medium with certain useful properties can be constructed • Clocks • Used for keeping track of the real time • Limiting how long processes can run • Handling watchdog timers • Doing accounting

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