itec 502 컴퓨터 시스템 및 실습 chapter 9-2: disk scheduling mi-jung choi...

ITEC 502 컴퓨터 시스템 및 실습

Chapter 9-2:Disk Scheduling

Mi-Jung Choimjchoi@postech.ac.kr

DPNM Lab. Dept. of CSE, POSTECH

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Contents

Disk Hardware Disk Formatting Disk Arm Scheduling Algorithms Error Handling Clocks

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Moving-head Disk Machanism

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Disk Hardware (1)

Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disk

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Disk Hardware (2)

Physical geometry of a disk with two zones A possible virtual geometry for this disk

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Disk Hardware (3)

Redundant Array of Inexpensive Disks (RAID) Raid levels 0 through 2 Backup and parity drives are shaded

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Disk Hardware (4)

Raid levels 3 through 5 Backup and parity drives are shaded

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Disk Hardware (5)

Recording structure of a CD or CD-ROM

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Disk Hardware (6)

Logical data layout on a CD-ROM

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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

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Disk Hardware (8)

A double sided, dual layer DVD disk

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Disk Formatting (1)

A disk sector

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Disk Formatting (2)

An illustration of cylinder skew

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Disk Formatting (3)

No interleaving Single interleaving Double interleaving

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Disk Arm Scheduling Algorithms (1)

Time required to read or write a disk block determined by 3 factors

1. Seek time2. Rotational delay3. Actual transfer time

Seek time dominates Error checking is done by

controllers

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Disk Arm Scheduling Algorithms (2)

Shortest Seek First (SSF) disk scheduling algorithm

Initialposition

Pendingrequests

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Disk Arm Scheduling Algorithms (3)

The elevator algorithm for scheduling disk requests

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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

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Stable Storage

Analysis of the influence of crashes on stable writes

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Clocks: Clock Hardware

A programmable clock

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Clock Software (1)

Three ways to maintain the time of day

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Clock Software (2)

Simulating multiple timers with a single clock

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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

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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