Disks Disk Hardware 1

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DisksDisk 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

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

3
Disk Arm Scheduling Algorithms

• 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

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FCFS
Illustration shows total head movement of 640
cylinders.
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SSTF

• Selects the request with the minimum seek time
  from the current head position.
• SSTF scheduling is a form of SJF scheduling may
  cause starvation of some requests.
• Illustration shows total head movement of 236
  cylinders.

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SSTF (Cont.)
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SCAN/Elevator

• The disk arm starts at one end of the disk, and
  moves toward the other end, servicing requests
  until it gets to the other end of the disk, where
  the head movement is reversed and servicing
  continues.
• Sometimes called the elevator algorithm.
• Illustration shows total head movement of 208
  cylinders.

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SCAN (Cont.)
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C-SCAN

• Provides a more uniform wait time than SCAN.
• The head moves from one end of the disk to the
  other servicing requests as it goes. When it
  reaches the other end, however, it immediately
  returns to the beginning of the disk, without
  servicing any requests on the return trip.
• Treats the cylinders as a circular list that
  wraps around from the last cylinder to the first
  one.

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C-SCAN (Cont.)
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C-LOOK

• Version of C-SCAN
• Arm only goes as far as the last request in each
  direction, then reverses direction immediately,
  without first going all the way to the end of the
  disk.

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C-LOOK (Cont.)
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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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OS Review

• What is an OS?
• What is a Process?
• What is a Memory Manager?
• What is a File System?

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What is an OS?

• System Calls
• Resource Manager

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What is a process?

• Program in execution
• Process States
• Threads
• Synchronization
• Scheduling

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What is Memory Management?

• Multiprogramming and the need to manage memory
• MFT, MVT, security (bounds, limit)
• Virtual Memory
• Paging
• Segmentation
• Segments are paged
• Page replacement algorithms

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What is a File System?

• Files and Directories
• File operations, permissions, security
• File structure organization
• Allocation and De-allocation
• Disks, partitions, file systems
• Disk scheduling

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Questions?
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Disk Arm Scheduling Algorithms (3)

• The elevator algorithm for scheduling disk
  requests

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Disk Arm Scheduling Algorithms (2)
Pending requests
Initial position

• Shortest Seek First (SSF) disk scheduling
  algorithm