Report Hans-Lanie File-System Interface

8/3/2019 Report Hans-Lanie File-System Interface

1/45

FILE-SYSTEM INTERFACE

ADVANCED OPERATING SYSTEM

FILE MANAGEMENT


2/45

FILE-SYSTEM INTERFACE

File ConceptAccess Methods

Directory Structure

File Sharing

Protection


3/45

OBJECTIVES

To explain the function of file systemsTo describe the interfaces to file

systems

To discuss file-system designtradeoffs, including accessmethods, file sharing, file locking,and directory structures

To explore file-system protection


4/45

A. FILE CONCEPT

Computer can store information on several

different storage media, such as magneticdisks, magnetic tapes, and optical disks.

The operating system abstracts form thephysical properties of its storage devicesto define a logical storage, the file.

Filesare mapped, by the O.S., onto physicaldevices.

A Fileis a named collection of relatedinformation that is recorded on secondarystorage.


5/45

FILE CONCEPT (CONT)

From a users perspective, a file is the

smallest allotment of logical secondarystorage; that is, data cannot be written tosecondary memory unless they are within afile.

A file represents:Datanumericcharacter

binaryProgram (both source and object forms)


6/45

FILE CONCEPT (CONT)

A file has a certain defined structureaccording to its type.

Text file (characters organized into lines)Source file (sequence of subroutines &functions)Object file (sequence of bytes)Executable file (series of code sections)


7/45

A.1 FILE ATTRIBUTES

Name only information kept in human-readable form

Identifier unique tag (number) identifies file withinfile system

Type needed for systems that support differenttypes

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 forprotection, security, and usage monitoring

Information about files are kept in the directorystructure, which is maintained on the disk


8/45

A.2 FILE OPERATIONSFile is an abstract data type

Create

Write

Read

Reposition within file

DeleteTruncate

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

Close (Fi) move the content of entry Fi in memory to

directory structure on disk

per-process current file- position pointer

open-file table,


9/45

A.2 FILE OPERATIONS

The implementation of the open() and close()

operations is more complicated in an environmentwhere several processes may open the file at thesame time

per-process table

information regarding the use of the file by the processsystem-wide open-file table

table contains process-independent information, such asthe location of the file on disk, access dates, and file size


10/45

A.3 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 openFile-open count: counter of number of times a file

is open to allow removal of data from open-filetable when last processes closes itDisk location of the file: cache of data accessinformation

Access rights: per-process access modeinformation


11/45

A.4 OPEN FILE LOCKING

Provided by some operating systems and

file systemsMediates access to a file

A shared lock is akin to a reader lock in

that several processes can acquire thelock concurrently.

An exclusive lock behaves like a writerlock; only one process at a time can

acquire such a lock.


12/45

A.4 OPEN FILE LOCKING

Mandatory or advisory:

Mandatory access is denied dependingon locks held and requested

Advisory processes can find status oflocks and decide what to do


13/45

FILE LOCKING EXAMPLE JAVA API

import java.io.*;

import java.nio.channels.*;public class LockingExample {

public static final boolean EXCLUSIVE = false;

public static final boolean SHARED = true;

public static void main(String arsg[]) throws IOException {

FileLock sharedLock = null;

FileLock exclusiveLock = null;try {

RandomAccessFile raf = new RandomAccessFile("file.txt", "rw");

// get the channel for the file

FileChannel ch = raf.getChannel();

// this locks the first half of the file - exclusive

exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);/** Now modify the data . . . */

// release the lock

exclusiveLock.release();


14/45

FILE LOCKING EXAMPLE JAVA API (CONT)

// this locks the second half of the file - shared

sharedLock = ch.lock(raf.length()/2+1, raf.length(),SHARED);

/** Now read the data . . . */

// release the lock


}catch (java.io.IOException ioe) {

System.err.println(ioe);

}finally {

if (exclusiveLock != null)


if (sharedLock != null)

sharedLock.release();}

}

}


15/45

A.5 FILE TYPES NAME, EXTENSION


16/45

A.6 FILE STRUCTUREWho decides:

Operating systemProgram


17/45

B. ACCESS METHODS

Sequential Access (simplest method)

read nextwrite nextreset


18/45

ACCESS MOTHOD (CONT)

Direct Access (relative access)read n

write n

position to n

read nextwrite next

rewrite n

n= relative block number

A file is make up of fixed-length logical recordsthat allowprograms to read and write records rapidly in noparticular order.

Direct access method is based on a disk model of file,since disks allow random access to any file block.


19/45

SEQUENTIAL-ACCESS FILE

S O O S Q CC SS O C CC SS


20/45

SIMULATION OF SEQUENTIAL ACCESS ON A DIRECT-ACCESS

FILE


21/45

OTHER ACCESS METHOD :

EXAMPLE OF INDEX AND RELATIVE FILES


22/45

C. 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


23/45

A TYPICAL FILE-SYSTEM ORGANIZATION


24/45

OPERATIONS PERFORMED ON DIRECTORY

Search for a file

Create a file

Delete a file

List a directory

Rename a fileTraverse the file system It is useful to be able to access every directory, and every file

within a directory structure. For reliability, it is a good idea to savethe contents and structure of the entire file system at regular

intervals.


25/45

ORGANIZE THE DIRECTORY (LOGICALLY) TO OBTAIN

Efficiency locating a file quicklyNaming convenient to users

Two users can have same name for differentfiles

The same file can have several differentnames

Grouping logical grouping of files by

properties, (e.g., all Java programs, allgames, )


26/45

C.1 SINGLE-LEVEL DIRECTORY

A single directory for all users

Naming problem


27/45

C.2 TWO-LEVEL DIRECTORY

Separate directory for each user

Path name

Can have the same file name for different user

Efficient searching


28/45

C.3 TREE-STRUCTURED DIRECTORIES


29/45

TREE-STRUCTURED DIRECTORIES (CONT)

Efficient searching

Grouping Capability

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

type list


30/45

TREE-STRUCTURED DIRECTORIES (CONT)

Absolute or relative path name

Creating a new file is done in current directory

Delete a file

rm

Creating a new subdirectory is done in current

directorymkdir

Example: if in current directory /mail

mkdir count

mail

prog copy prt exp count

Deleting maildeleting the entire subtree rooted by mail


31/45

C.4 ACYCLIC-GRAPH DIRECTORIES

Have shared subdirectories and files

Acyclic graph is graph with no cycle


32/45

C.5 GENERAL GRAPH DIRECTORY


33/45

GENERAL GRAPH DIRECTORY (CONT.)

How do we guarantee no cycles (self-

referencing)?Allow only links to file not subdirectories.

Garbage collection (involves traversing the filesystem)Used to determine when the last reference has been deleted and

the disk space can be reallocated.

It is necessary only because of possible cycles in the graph

Every time a new link is added use a cycle

detection algorithm to determine whether it is OK


34/45

C.6 FILE SHARING

Sharing of files on multi-user systems is

desirable

Sharing may be done through aprotection scheme

On distributed systems, files may beshared across a network

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


35/45

FILE SHARING MULTIPLE USERS

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

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


36/45

FILE SHARING REMOTE FILE SYSTEMS

Uses networking to allow file system accessbetween systems

Manually via programs like FTP

Automatically, seamlessly using distributed filesystems

Semi automatically via theworld wide web


37/45

FILE SHARING REMOTE FILE SYSTEMS

(CONT)

Client-server model allows clients to mount remote file

systems from servers Server can serve multiple clients

Client and user-on-client identification is insecure orcomplicated

NFS is standard UNIX client-server file sharing protocol

Standard operating system file calls are translated into remotecalls

Distributed Information Systems (distributed naming services)such as LDAP, DNS, NIS, Active Directory implementunified access to information needed for remote

computing


38/45

FILE SHARING FAILURE MODES

Remote file systems add new failure modes,due to network failure, server failure

Recovery from failure can involve stateinformation about status of each remote

request

Stateless protocols such as NFS include allinformation in each request, allowing easy

recovery but less security


39/45

D. PROTECTION

When information is kept in a computer

system, a major concern is itsprotection from both physicaldamage (reliability) and improper

access (protection).File owner/creator should be able to

control:

what can be doneby whom


40/45

PROTECTION (CONT)

Types of accessRead. Read from the file.

Write. Write or rewrite the file.

Execute. Load the file into the memory and execute.

Append. Write new information at the end of the file.

Delete. Erase the file and free its space for possiblereuse.

List. Display the name and attributes of the file.


41/45

D.1 ACCESS LISTS AND GROUPS

Mode of access: read, write, execute

Three classes of usersRWX

a) owner access 7 1 1 1RWX

b) group access 6 1 1 0RWX

c) public access 1 0 0 1Ask 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.

owner group public

chmod 761 game

Attach a group to a filechgrp G game


42/45

D.2 OTHER PROTECTION APPROACHAnother protection approach is the association of

Passwordin a file.

If the passwords are chosen randomly and changedoften, this scheme may be effective in limiting accessto a file to only those users who know the password.But, if we associate a separate password with each file,the number of passwords that a user needs to

remember may become large, making the schemeimpractical.

If one password is used for all the files, then, once it isdiscovered, all files are accessible.

Some systems (for example, TOP-20) allow a user to

associate a password with a subdirectory, rather thanwith an individual file, to deal with this problem.

-MANAGEMENT


43/45

MANAGEMENT


44/45

A SAMPLE UNIX DIRECTORY LISTING

Status/protn #L CN GN FS DC FN


45/45

THANKS FOR LISTENING

Have a nice day to all

Documents

Report Hans-Lanie File-System Interface