chapter 10: file-system interface 10.1 silberschatz, galvin and gagne ©2011 operating system...

Chapter 10: File-System Interface

10.1 Silberschatz, Galvin and Gagne ©2011Operating System Concepts – 8th Edition 2014

Chapter 10: File-System Interface

File Concept Access Methods Directory Structure File-System Mounting File Sharing Protection

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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

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10.4Silberschatz, Galvin and Gagne ©2011Operating System Concepts – 8th Edition 2014

The file system consists of two distinct parts:

1. a collection of files, each storing related data, and

2. a directory structure, which organizes and provides information about all the files in the system.

File Concept

A file is a named collection of related information that is recorded on secondary storage.

Commonly, files represent programs (both source and object forms) and data.

Data files may be numeric, alphabetic, alphanumeric, or binary. Files may be free form, such as text files, or may be formatted rigidly.

The information in a file is defined by its creator.

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File Concept (cont.) Many different types of information may be stored in a file

source programs, object programs, executable programs, numeric data, text, payroll records, graphic images, sound recordings, …..

A file has a certain defined structure, which depends on its type.

o e.g: A text file is a sequence of characters organized into lines

o A source file is a sequence of subroutines and functions, each of which is

further organized as declarations followed by executable statements.

o An object file is a sequence of bytes organized into blocks understandable

by the system’s linker.

o An executable file is a series of code sections that the

loader can bring into memory and execute.

File Structure

None - sequence of words, bytes 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

Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within 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

File is an abstract data type

Create : Two steps are necessary to create a file.

First, space in the file system must be found for the file how? Second, an entry for the new file must be made in the directory.

Write: To write a file, we make a system call specifying both the name of the file and the information to be written to the file.

Given the name of the file, the system searches the directory to find the file’s location.

The system must keep a write pointer to the location in the file where the next write is to take place. The write pointer must be updated whenever a write occurs.



File Operations (cont.)

Read: To read from a file, we use a system call that

1. specifies file name and where (in memory) the next block of the file should be put.

2. the directory is searched for the associated entry,

3. the system keeps a read pointer to the location in the file where the next read is to take place.

4. Once the read has taken place, the read pointer is updated.

reading from or writing to a file, are similar so, the current operation location can be kept as a per-process currentfile-position pointer.

saving space and reducing system complexity.



File Operations (cont.)

Delete: To delete a file

Provide name and search the directory for it Having found it, we release all file space, so that it can be reused by

other files, and erase the directory entry.

Truncate

Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory

Close (Fi) – move the content of entry Fi in memory to directory structure on disk

Silberschatz, Galvin and Gagne ©2011Operating System Concepts – 8th Edition 2014

File Types – Name, Extension


Access Methods

1. Sequential Access : Information in the file is processed in order, one record after the other. This mode of access is by far the most common

read next, write next reset , no read after last write

(rewrite)

2. Direct Access: (or relative access). A file is made up of fixed length of logical records that allow programs to read and write records rapidly in no particular order. Restrictions

on the order of reading or writing for a direct-access file.read n, write n , position to n

read next, write next , rewrite n

n = relative block number


Sequential-access File


Simulation of Sequential Access on a Direct-access File


Example of Index and Relative Files


Directory Structure

A collection of nodes containing information about all files

F 1 F 2F 3

F 4

F n

Directory

Files

Both the directory structure and the files reside on diskBackups of these two structures are kept on tapes


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


Organize the Directory (Logically) to Obtain

Efficiency – locating a file quickly Naming – convenient to users

Two users can have same name for different files The same file can have several different names

Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)


Single-Level Directory

A single directory for all users

Naming problem

Grouping problem


Two-Level Directory

Separate directory for each user

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


Tree-Structured Directories


Tree-Structured Directories (Cont)

Efficient searching

Grouping Capability

Current directory (working directory) cd /spell/mail/prog type list


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”


File Sharing

Sharing of files on multi-user systems is desirable

Sharing may be done through a protection scheme

On distributed systems, files may be shared across a network

Network File System (NFS) is a common distributed file-sharing method


File Sharing – Multiple Users

User IDs identify users, allowing permissions and protections to be per-user

Group IDs allow users to be in groups, permitting group access rights


Protection

File owner/creator should be able to control: what can be done by whom

Types of access Read Write Execute Append Delete List


End of Chapter 10


chapter 10: file-system interface 10.1 silberschatz, galvin and gagne ©2011 operating system...

Documents

file concepta file

source file

object file

executable file

file locking

filesystem protection

filesystem interface10

file source programs