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

ITEC502 컴퓨터 시스템 및 실습 2 Contents  Disk Hardware  Disk Formatting  Disk Arm Scheduling Algorithms  Error Handling  Clocks

ITEC502 컴퓨터 시스템 및 실습 3 Moving-head Disk Machanism

ITEC502 컴퓨터 시스템 및 실습 4 Disk Hardware (1) Disk parameters for the original IBM PC floppy disk and a Western Digital WD hard disk

ITEC502 컴퓨터 시스템 및 실습 5 Disk Hardware (2)  Physical geometry of a disk with two zones  A possible virtual geometry for this disk

ITEC502 컴퓨터 시스템 및 실습 6 Disk Hardware (3)  Redundant Array of Inexpensive Disks (RAID)  Raid levels 0 through 2  Backup and parity drives are shaded

ITEC502 컴퓨터 시스템 및 실습 7 Disk Hardware (4)  Raid levels 3 through 5  Backup and parity drives are shaded

ITEC502 컴퓨터 시스템 및 실습 8 Disk Hardware (5) Recording structure of a CD or CD-ROM

ITEC502 컴퓨터 시스템 및 실습 9 Disk Hardware (6) Logical data layout on a CD-ROM

ITEC502 컴퓨터 시스템 및 실습 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

ITEC502 컴퓨터 시스템 및 실습 11 Disk Hardware (8) A double sided, dual layer DVD disk

ITEC502 컴퓨터 시스템 및 실습 12 Disk Formatting (1) A disk sector

ITEC502 컴퓨터 시스템 및 실습 13 Disk Formatting (2) An illustration of cylinder skew

ITEC502 컴퓨터 시스템 및 실습 14 Disk Formatting (3)  No interleaving  Single interleaving  Double interleaving

ITEC502 컴퓨터 시스템 및 실습 15 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

ITEC502 컴퓨터 시스템 및 실습 16 Disk Arm Scheduling Algorithms (2) Shortest Seek First (SSF) disk scheduling algorithm Initial position Pending requests

ITEC502 컴퓨터 시스템 및 실습 17 Disk Arm Scheduling Algorithms (3) The elevator algorithm for scheduling disk requests

ITEC502 컴퓨터 시스템 및 실습 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

ITEC502 컴퓨터 시스템 및 실습 19 Stable Storage Analysis of the influence of crashes on stable writes

ITEC502 컴퓨터 시스템 및 실습 20 Clocks: Clock Hardware A programmable clock

ITEC502 컴퓨터 시스템 및 실습 21 Clock Software (1) Three ways to maintain the time of day

ITEC502 컴퓨터 시스템 및 실습 22 Clock Software (2) Simulating multiple timers with a single clock

ITEC502 컴퓨터 시스템 및 실습 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

ITEC502 컴퓨터 시스템 및 실습 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