ITEC 502 Chapter 9 2 Disk Scheduling MiJung

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

Contents § § § Disk Hardware Disk Formatting Disk Arm Scheduling Algorithms Error Handling Clocks ITEC 502 컴퓨터 시스템 및 실습 2

Moving-head Disk Machanism ITEC 502 컴퓨터 시스템 및 실습 3

Disk Hardware (1) Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disk ITEC 502 컴퓨터 시스템 및 실습 4

Disk Hardware (2) § Physical geometry of a disk with two zones § A possible virtual geometry for this disk ITEC 502 컴퓨터 시스템 및 실습 5

Disk Hardware (3) § Redundant Array of Inexpensive Disks (RAID) § Raid levels 0 through 2 § Backup and parity drives are shaded ITEC 502 컴퓨터 시스템 및 실습 6

Disk Hardware (4) § Raid levels 3 through 5 § Backup and parity drives are shaded ITEC 502 컴퓨터 시스템 및 실습 7

Disk Hardware (5) Recording structure of a CD or CD-ROM ITEC 502 컴퓨터 시스템 및 실습 8

Disk Hardware (6) Logical data layout on a CD-ROM ITEC 502 컴퓨터 시스템 및 실습 9

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 ITEC 502 컴퓨터 시스템 및 실습 10

Disk Hardware (8) A double sided, dual layer DVD disk ITEC 502 컴퓨터 시스템 및 실습 11

Disk Formatting (1) A disk sector ITEC 502 컴퓨터 시스템 및 실습 12

Disk Formatting (2) An illustration of cylinder skew ITEC 502 컴퓨터 시스템 및 실습 13

Disk Formatting (3) § No interleaving § Single interleaving § Double interleaving ITEC 502 컴퓨터 시스템 및 실습 14

Disk Arm Scheduling Algorithms (1) § Time required to read or write a disk block determined by 3 factors 1. Seek time 2. Rotational delay 3. Actual transfer time § § Seek time dominates Error checking is done by controllers ITEC 502 컴퓨터 시스템 및 실습 15

Disk Arm Scheduling Algorithms (2) Initial position Pending requests Shortest Seek First (SSF) disk scheduling algorithm ITEC 502 컴퓨터 시스템 및 실습 16

Disk Arm Scheduling Algorithms (3) The elevator algorithm for scheduling disk requests ITEC 502 컴퓨터 시스템 및 실습 17

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 ITEC 502 컴퓨터 시스템 및 실습 18

Stable Storage Analysis of the influence of crashes on stable writes ITEC 502 컴퓨터 시스템 및 실습 19

Clocks: Clock Hardware A programmable clock ITEC 502 컴퓨터 시스템 및 실습 20

Clock Software (1) Three ways to maintain the time of day ITEC 502 컴퓨터 시스템 및 실습 21

Clock Software (2) Simulating multiple timers with a single clock ITEC 502 컴퓨터 시스템 및 실습 22

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 ITEC 502 컴퓨터 시스템 및 실습 23

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 ITEC 502 컴퓨터 시스템 및 실습 24