Chapter 11

File Systems and Directories

Chapter Goals• Files, directories, and file systems

• Text and Binary files

• Disk-scheduling algorithms

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File Systems

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File Systems

• How do we make this:

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• Look like this?

File Systems

• File – A named collection of related data– A sequence of bytes

• File system – The logical view that an operating system provides– Physically, information on the disk is scattered

• Directory or Folder – A named group of files

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Directory Trees

• Directory tree – A logical view of a file system; – uses a tree structure to represent organization of a file

system

• Root The directory at the highest level

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The Disk – A Physical View

• Data is stored in blocks• One file may be spread across several blocks

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Directory Trees - A Logical View

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File Types

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2 File Types1) Text2) Binary

•Depends on:

–Are the bytes ASCII Characters or not?

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Text and Binary Files

• Text file – Each byte represents an ASCII character

• Binary file– Each byte or group of bytes represents something

other than a character– The specific meaning of the format of bytes must

be understood by the program using the file

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Disk Scheduling

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Disk Scheduling

• The File System is a shared resource • Many programs ask the operating system for

access to the disk

• Disk scheduling The technique that the operating system uses to determine which disks access requests to satisfy first

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Disk Scheduling

• The File System must be accessed in an efficient manner

• As a computer deals with multiple processes over a period of time, a list of requests to access the disk builds up

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Disk Scheduling

• How should we move the head to service requests?

Figure 5.5 The organization of a magnetic disk

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Disk Scheduling Algorithms

• First-Come, First-Served (FCFS)

• Shortest-seek-time-first (SSTF)

• Scan (aka Elevator Algorithm)

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FCFS

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• First-Come, First-Served

• Requests are serviced in the order they arrive in the list, without regard to the current position of the heads

• Problem: Inefficient – The disk heads are moving too much

SSTF

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• Shortest-seek-time-first (SSTF)

• The closest item in the list is services first

• Problem: Starvation – New requests may be closer than older requests, so older requests never get serviced.

Scan

• Scan (aka Elevator Algorithm)• Like an elevator• Travels from top to bottom, servicing requests along

the way• Problems?? Problems may exist for special cases.

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