File Systems: Concept, Structure, and Operations, Slides of Computer Science

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File-System Interface

Chapter 10

Objectives

• To explain the function of file systems

• To describe the interfaces to file systems

• To discuss file-system design tradeoffs,

including access methods, file sharing, file

locking, and directory structures

• To explore file-system protection

File Structure

• None - sequence of words, bytes (UNIX)

• Simple record structure

• Lines
• Fixed length
• Variable length

• Complex Structures

• Formatted document
• Relocatable load file

• Can simulate last two with first method by inserting

appropriate control characters

• Who decides:

• Operating system
• Program

File Attributes

• Files typically have the following attributes
  • Name – only information kept in human-readable form.
  • Identifier – Unique tag, usually a number, identifies the file within the file system.
  • Type – needed for systems that support different types.
  • Location – pointer to file location on device.
  • Size – current file size.
  • Protection – controls who can do reading, writing, executing.
  • Time , date , and user identification – data for protection, security, and usage monitoring.
• Information about files are kept in the directory structure,

which is maintained on the disk

File Operations (Con’t)

• Most file operations require searching the directory

for the entry associated with the named file.

• To avoid constant searching, many systems require that an

open system call be used

• The operating system keeps a small table containing

information about all open files (open-file table)

• The file is specified via an index into the table, so no searching is required.

Open Files

• Several pieces of data are needed to manage

open files:

• File pointer: pointer to last read/write location, per process that has the file open
• File-open count: counter of number of times a file

is open – to allow removal of data from open-file table when last processes closes it

• Disk location of the file: cache of data access information
• Access rights: per-process access mode information

Access Methods

• Sequential Access: Simplest access method. Information in the file is processed in order, one record after the other

read next write next reset

• Direct Access: A file is made up of fixed-length logical records that allow rapid read/write access in no particular order read n write n position to n read next write next rewrite n n = relative block number

Simulation of Sequential Access on a Direct-

access File

• Not all operating systems support both sequential and direct

access file methods.

• Easy to simulate sequential access method on a direct-access file
• Extremely inefficient to simulate a direct-access file method on a sequential access file.

A Typical File-system Organization

Operations Performed on Directory

• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

Single-Level Directory

• A single directory for all users

Naming problem

Grouping problem

Two-Level Directory

• Separate directory for each user

n Path name n Can have the same file name for different user n Efficient searching n No grouping capability

Tree-Structured Directories (Cont)

• Absolute or relative path name
• Creating a new file is done in current directory
• Delete a file

rm <file-name>

• Creating a new subdirectory is done in current directory

mkdir <dir-name>

Example: if in current directory /mail

mkdir count

mail

prog copy prt exp count

Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”

Acyclic-Graph Directories

• Have shared subdirectories and files