11.1

Chapter 6

Storage Management

File-System Interface

11.2

Chapter 6: File-System Interface

File Concept

Access Methods

Disk and Directory Structure

File-System Mounting

File Sharing

Protection

11.3

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

11.4

File Concept

Computers can store information on various storage

media, such as magnetic disks, magnetic tapes, and

optical disks.

So that the computer system will be convenient to use,

the operating system provides a uniform logical view of

information storage.

The operating system abstracts from the physical

properties of its storage devices to define a logical

storage unit, the file.

Files are mapped by the operating system onto physical

devices.

These storage devices are usually nonvolatile, so the

contents are persistent through power failures and

system reboots.

11.5

File Concept (Cont.)

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

From a user's perspective, a file is the smallest allotment of logical secondary storage; that

is, data cannot be written to secondary storage unless they are within a file.

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.

In general, a file is a sequence of bits, bytes, lines, or records, the meaning of which is

defined by the file's creator and user. The concept of a file is thus extremely general.

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, and so on.

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

o A text file is a sequence of characters organized into lines (and possibly pages).

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

11.6

File Attributes

A file's attributes vary from one operating system to another but

typically consist of these

o Name – only information kept in human-readable form

o Identifier – unique tag (number) identifies file within file system

o Type – needed for systems that support different types

o Location – pointer to file location on device

o Size – current file size

o Protection – controls who can do reading, writing, executing

o 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

Many variations, including extended file attributes such as file checksum

Information kept in the directory structure

11.7

File info Window on Mac OS X

11.8

File Operations

To define a file properly, we need to consider the operations that can be

performed on files. The operating system can provide system calls to create,

write, read, reposition, delete, and truncate files.

Creating a file. Two steps are necessary to create a file. First, space in the

file system must be found for the file. Second, an entry for the new file must be

made in the directory.

Writing a file. 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.

Repositioning within a file. The directory is searched for the appropriate

entry, and the current-file-position pointer is repositioned to a given value.

Repositioning within a file need not involve any actual I/O. This file operation is

also known as a file seek.

11.9

File Operations (Cont.)

Reading a file. To read from a file, we use a system call that specifies the

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

Again, the directory is searched for the associated entry, and the system

needs to keep a read pointer to the location in the file where the next read is to

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

Because a process is usually either reading from or writing to a file, the current

operation location can be kept as a per-process currentfile- position pointer.

Both the read and write operations use this same pointer, saving space and

reducing system complexity.

Deleting a file. To delete a file, we search the directory for the named file.

Having found the associated directory entry, we release all file space, so that it

can be reused bv other files, and erase the directory entry.

Truncating a file. The user may want to erase the contents of a file but keep

its attributes. Rather than forcing the user to delete the file and then recreate it,

this function allows all attributes to remain unchanged—except for file length—

but lets the tile be reset to length zero and its file space released.

11.10

File Types – Name, Extension

11.11

Access Methods

Files store information. When it is used, this information must be accessed and read into computer memory.

The information in the file can be accessed in several ways. Some systems provide only one access method for files. Other systems, such as those of IBM, support many access methods, and choosing the right one for a particular application is a major design problem. o Sequential Access o Direct Access

11.12

Sequential Access

The simplest access method is 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; for example, editors and

compilers usually access files in this fashion.

Reads and writes make up the bulk of the operations on a file.

A read operation—read next—reads the next portion of the file and

automatically advances a file pointer, which tracks the I/O location.

The write operation—write next—appends to the end of the file and

advances to the end of the newly written material (the new end of file).

Such a file can be reset to the beginning; and on some systems, a program

may be able to skip forward or backward n records for some integer n—

perhaps only for n = 1.

Sequential access, which is depicted in the following figure, is based on a

tape model of a file and works as well on sequential-access devices as it

does on random-access ones.

11.13

Sequential-access File

11.14

Direct Access (Relative Access)

A file is made up of fixed length logical records that allow programs to read and

write records rapidly in no particular order.

The direct-access method is based on a disk model of a file, since disks allow

random access to any file block.

For direct access, the file is viewed as a numbered sequence of blocks or records.

Thus, we may read block 14, then read block 53, and then write block 7.

There are no restrictions on the order of reading or writing for a direct-access file.

Direct-access files are of great use for immediate access to large amounts of

information.

Databases are often of this type. When a query concerning a particular subject

arrives, we compute which block contains the answer and then read that block

directly to provide the desired information.

The file operations must be modified to include the block number as a parameter.

Thus, we have read n, where n is the block number, rather than read next, and

write n rather than write next. An alternative approach is to retain read next and write next, as with sequential

access, and to add an operation position file to n, where n is the block number.

Then, to effect a read n, we would, position to n and then read next.

11.15

Direct Access (Cont.)

The block number provided by the user to the operating system is

normally a relative block number.

A relative block number is an index relative to the beginning of the

file.

Thus, the first relative block of the file is 0, the next is 1, and so on,

even though the actual absolute disk address of the block may be

14703 for the first block and 3192 for the second.

The use of relative block numbers allows the operating system to

decide where the file should be placed and helps to prevent the user

from accessing portions of the file system that may not be part of her

file.

Some systems start their relative block numbers at 0; others start at 1.

11.16

Access Methods

Sequential Access read next

write next

reset

no read after last write

(rewrite)

Direct Access – file is fixed length logical records read n

write n

position to n

read next

write next

rewrite n

n = relative block number

Relative block numbers allow OS to decide where file should be placed

11.17

Simulation of Sequential Access on Direct-access File

11.18

Chapter 6

File-System Interface

(Cont.)

Directories

11.19

Chapter 11: File-System Interface

Disk and Directory Structure

Objectives

o Explain some basic features of storage structure. o Explore the topic of directory structure.

11.20

Introduction

The file systems of computers, then, can be extensive. Some systems store

millions of files on terabytes of disk.

To manage all these data, we need to organize them. This organization

involves the use of directories.

11.21

Disk Structure

Disk can be subdivided into partitions

Disk or partition can be used raw (without a file

system, or formatted with a file system)

Partitions also known as minidisks, slices

Entity containing file system known as a volume

Each volume containing file system also tracks that

file system’s info in device directory or volume

table of contents

11.22

A Typical File-system Organization

11.23

Directory Structure

A collection of nodes containing information about all files

F 1 F 2 F 3

F 4

F n

Directory

Files

Both the directory structure and the files reside on disk

11.24

Operations Performed on Directory

Search for a file: We need to be able to search a directory structure to find

the entry for a particular file. Since files have symbolic names and similar

names may indicate a relationship between files, we may want to be able to

find all files whose names match a particular pattern.

Create a file: New files need to be created and added to the directory

Delete a file from a directory: When a file is no longer needed, we want to

be able to remove it from the directory.

List a directory: We need to be able to list the files in a directory and the contents of the directory entry for each file in the list.

Rename a file: Because the name of a file represents its contents to its users, we must be able to change the name when the contents or use of the file changes. Renaming a file may also allow its position within the directory structure to be changed.

Traverse the file system: We may wish to access every directory and every

file within a directory structure.

11.25

Directory Organization

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, …)

The directory is organized logically to obtain

11.26

Single-Level Directory

A single directory for all users

Naming problem

Grouping problem

11.27

Two-Level Directory

Separate directory for each user

Path name

Can have the same file name for different user

Efficient searching

No grouping capability

11.28

Tree-Structured Directories

11.29

Tree-Structured Directories (Cont.)

Efficient searching

Grouping Capability

Current directory (working directory)

cd /spell/mail/prog

type list

11.30

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

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

11.31

Acyclic-Graph Directories

Have shared subdirectories and files

11.32

Acyclic-Graph Directories (Cont.)

Two different names (aliasing)

If dict deletes list dangling pointer

Solutions:

Backpointers, so we can delete all pointers

Variable size records a problem

Backpointers using a daisy chain organization

Entry-hold-count solution

New directory entry type

Link – another name (pointer) to an existing file

Resolve the link – follow pointer to locate the file

11.33

General Graph Directory

11.34

Chapter 6

File-System Interface

Protection

Operating System Concepts – 9th Edition

11.35

Protection

File owner/creator should be able to control:

what can be done

by whom

Types of access

Read

Write

Execute

Append

Delete

List

11.36

Access Lists and Groups

Mode of access: read, write, execute

Three classes of users on Unix / Linux RWX

a) owner access 7 1 1 1 RWX

b) group access 6 1 1 0

RWX

c) public access 1 0 0 1

Ask manager to create a group (unique name), say G, and add some users to the group.

For a particular file (say game) or subdirectory, define an appropriate access.

Attach a group to a file chgrp G game

11.37

Windows 7 Access-Control List Management

11.38

A Sample UNIX Directory Listing