1 Chapter 10: File- System Interface. 2 File Concept Access Methods Directory Structure File-System...

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Chapter 10: File-Chapter 10: File-System InterfaceSystem Interface

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Chapter 10: File-System Chapter 10: File-System InterfaceInterface

File ConceptFile Concept

Access MethodsAccess Methods

Directory StructureDirectory Structure

File-System MountingFile-System Mounting

File SharingFile Sharing

ProtectionProtection

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ObjectivesObjectives

To explain the function of file systemsTo explain the function of file systems

To describe the interfaces to file systemsTo describe the interfaces to file systems

To discuss file-system design tradeoffs, To discuss file-system design tradeoffs, including access methods, file sharing, file including access methods, file sharing, file locking, and directory structureslocking, and directory structures

To explore file-system protectionTo explore file-system protection

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Contiguous logical address spaceContiguous logical address space

Types: Types: DataData

numericnumeric

charactercharacter

binarybinary ProgramProgram

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File StructureFile StructureNone - sequence of words, bytesNone - sequence of words, bytesSimple record structureSimple record structure Lines Lines Fixed lengthFixed length Variable lengthVariable length

Complex StructuresComplex Structures Formatted documentFormatted document Relocatable load fileRelocatable load file

Can simulate last two with first method by inserting Can simulate last two with first method by inserting appropriate control charactersappropriate control charactersWho decides:Who decides: Operating systemOperating system ProgramProgram

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File AttributesFile Attributes

NameName – only information kept in human- – only information kept in human-readable formreadable form

IdentifierIdentifier – unique tag (number) identifies – unique tag (number) identifies file within file systemfile within file system

TypeType – needed for systems that support – needed for systems that support different typesdifferent types

LocationLocation – pointer to file location on – pointer to file location on devicedevice

SizeSize – current file size – current file size

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File AttributesFile Attributes

ProtectionProtection – controls who can do reading, – controls who can do reading, writing, executingwriting, executing

Time, date, and user identificationTime, date, and user identification – – data for protection, security, and usage data for protection, security, and usage monitoringmonitoring

Information about files are kept in the Information about files are kept in the directory structure, which is maintained on directory structure, which is maintained on the diskthe disk

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File OperationsFile Operations

File is an File is an abstract data typeabstract data type

CreateCreate

WriteWrite

ReadRead

Reposition within fileReposition within file

DeleteDelete

TruncateTruncate

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File OperationsFile Operations

Open(FOpen(Fii)) – search the directory structure – search the directory structure

on disk for entry on disk for entry FFii, and move the content , and move the content

of entry to memoryof entry to memory

Close (FClose (Fii)) – move the content of entry – move the content of entry FFii in in

memory to directory structure on diskmemory to directory structure on disk

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Open FilesOpen FilesSeveral pieces of data are needed to manage Several pieces of data are needed to manage open files:open files: File pointer: pointer to last read/write location, File pointer: pointer to last read/write location,

per process that has the file openper process that has the file open File-open count: counter of number of times a File-open count: counter of number of times a

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

Disk location of the file: cache of data access Disk location of the file: cache of data access informationinformation

Access rights: per-process access mode Access rights: per-process access mode informationinformation

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Open File LockingOpen File Locking

Provided by some operating systems and Provided by some operating systems and file systemsfile systems

Mediates access to a fileMediates access to a file

Mandatory or advisory:Mandatory or advisory: MandatoryMandatory – access is denied – access is denied

depending on locks held and requesteddepending on locks held and requested AdvisoryAdvisory – processes can find status of – processes can find status of

locks and decide what to dolocks and decide what to do

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File Locking Example – Java APIFile Locking Example – Java API

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

public static final boolean EXCLUSIVE = false;public static final boolean EXCLUSIVE = false;public static final boolean SHARED = true;public static final boolean SHARED = true;public static void main(String arsg[]) throws public static void main(String arsg[]) throws IOException IOException { {

FileLock sharedLock = null;FileLock sharedLock = null;FileLock exclusiveLock = null;FileLock exclusiveLock = null;try try { {

continue ==>

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File Locking Example – (cont)File Locking Example – (cont)RandomAccessFile raf = new RandomAccessFile("file.txt", RandomAccessFile raf = new RandomAccessFile("file.txt",

"rw");"rw");

// get the channel for the file// get the channel for the file

FileChannel ch = raf.getChannel();FileChannel ch = raf.getChannel();

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

exclusiveLock = ch.lock(0, raf.length()/2, exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);EXCLUSIVE);

/** Now modify the data . . . *//** Now modify the data . . . */

// release the lock// release the lock

exclusiveLock.release();exclusiveLock.release();

continue ==>

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File Locking Example – (cont)File Locking Example – (cont)// this locks the second half of the file - shared// this locks the second half of the file - shared

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

/** Now read the data . . . *//** Now read the data . . . */// release the lock// release the locksharedLock.release();sharedLock.release();

}} catch (java.io.IOException ioe) {catch (java.io.IOException ioe) { System.err.println(ioe);System.err.println(ioe);

}finally {}finally { if (exclusiveLock != null)if (exclusiveLock != null)exclusiveLock.release();exclusiveLock.release();if (sharedLock != null)if (sharedLock != null)sharedLock.release();sharedLock.release();

} } }} } }

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File Types – Name, ExtensionFile Types – Name, Extension

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Access MethodsAccess MethodsSequential AccessSequential Access

read nextread nextwrite next write next resetresetno read after last write (rewrite)no read after last write (rewrite)

Direct AccessDirect Accessread read nnwrite write nnposition to position to nn

read nextread nextwrite next write next

rewrite rewrite nnnn = relative block number = relative block number

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Sequential-access FileSequential-access File

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Simulation of Sequential Access on Simulation of Sequential Access on Direct-access FileDirect-access File

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Example of Index and Relative Example of Index and Relative FilesFiles

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Directory StructureDirectory StructureA collection of nodes containing A collection of nodes containing information about all filesinformation 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

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Disk StructureDisk Structure

Disk can be subdivided into Disk can be subdivided into partitionspartitions

Disks or partitions can be Disks or partitions can be RAIDRAID protected protected against failureagainst failure

Disk or partition can be used Disk or partition can be used rawraw – without a – without a file system, or file system, or formattedformatted with a file systemwith a file system

Partitions also known as minidisks, slicesPartitions also known as minidisks, slices

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Disk StructureDisk Structure

Entity containing file system known as a Entity containing file system known as a volumevolume

Each volume containing file system also Each volume containing file system also tracks that file system’s info in a tracks that file system’s info in a device device directorydirectory or or volume table of contentsvolume table of contents

As well as As well as general-purpose file systemsgeneral-purpose file systems there are many there are many special-purpose filespecial-purpose file systemssystems, frequently all within the same , frequently all within the same operating system or computeroperating system or computer

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A Typical File-system A Typical File-system OrganizationOrganization

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Operations Performed on Operations Performed on DirectoryDirectory

Search for a fileSearch for a file

Create a fileCreate a file

Delete a fileDelete a file

List a directoryList a directory

Rename a fileRename a file

Traverse the file systemTraverse the file system

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Organize the Directory (Logically) Organize the Directory (Logically) to Obtainto Obtain

Efficiency – locating a file quicklyEfficiency – locating a file quickly

Naming – convenient to usersNaming – convenient to users Two users can have same name for different Two users can have same name for different

filesfiles The same file can have several different The same file can have several different

namesnames

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

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Single-Level DirectorySingle-Level DirectoryA single directory for all usersA single directory for all users

Naming problem

Grouping problem

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Two-Level DirectoryTwo-Level DirectorySeparate directory for each userSeparate directory for each user

Path name Can have the same file name for different user

Efficient searching No grouping capability

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Tree-Structured DirectoriesTree-Structured Directories

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Tree-Structured Directories (Cont)Tree-Structured Directories (Cont)

Efficient searchingEfficient searching

Grouping CapabilityGrouping Capability

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

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Tree-Structured Directories Tree-Structured Directories (Cont)(Cont)

AbsoluteAbsolute or or relativerelative path name path name

Creating a new file is done in the current Creating a new file is done in the current directorydirectory

Delete a fileDelete a file

rm <file-name>rm <file-name>

Creating a new subdirectory is done in the Creating a new subdirectory is done in the current directorycurrent directory

mkdir <dir-name>mkdir <dir-name>

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Tree-Structured Directories (Cont)Tree-Structured Directories (Cont)

Example: if in current directory Example: if in current directory /mail/mailmkdir countmkdir count

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

mail

prog copy prtexpcount

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Acyclic-Graph DirectoriesAcyclic-Graph DirectoriesHave shared subdirectories and filesHave shared subdirectories and files

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Acyclic-Graph Directories (Cont.)Acyclic-Graph Directories (Cont.)Two different names (aliasing)Two different names (aliasing)

If If dictdict deletes deletes listlist dangling pointer dangling pointer

Solutions:Solutions: Backpointers, so we can delete all pointersBackpointers, so we can delete all pointers

Variable size records a problemVariable size records a problem Backpointers using a daisy chain organizationBackpointers using a daisy chain organization Entry-hold-count solutionEntry-hold-count solution

New directory entry typeNew directory entry type LinkLink – another name (pointer) to an existing file – another name (pointer) to an existing file Resolve the linkResolve the link – follow pointer to locate the file – follow pointer to locate the file

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General Graph DirectoryGeneral Graph Directory

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General Graph Directory (Cont.)General Graph Directory (Cont.)

How do we guarantee no cycles?How do we guarantee no cycles? Allow only links to file not subdirectoriesAllow only links to file not subdirectories Garbage collectionGarbage collection Every time a new link is added use a Every time a new link is added use a

cycle detection algorithm to determine cycle detection algorithm to determine whether it is OKwhether it is OK

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File System MountingFile System Mounting

A file system must be A file system must be mountedmounted before it before it can be accessedcan be accessed

A unmounted file system (i.e. Fig. 11-11(b)) A unmounted file system (i.e. Fig. 11-11(b)) is mounted at a is mounted at a mount pointmount point

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(a) Existing. (b) Unmounted (a) Existing. (b) Unmounted PartitionPartition

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Mount PointMount Point

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Sharing of files on multi-user systems is Sharing of files on multi-user systems is desirabledesirable

Sharing may be done through a Sharing may be done through a protectionprotection scheme scheme

On distributed systems, files may be On distributed systems, files may be shared across a networkshared across a network

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

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File Sharing – Multiple UsersFile Sharing – Multiple Users

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

Group IDsGroup IDs allow users to be in groups, allow users to be in groups, permitting group access rights - useful for permitting group access rights - useful for project work with teamsproject work with teams

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File Sharing – Remote File SystemsFile Sharing – Remote File Systems

Uses networking to allow file system access Uses networking to allow file system access between systemsbetween systems Manually via programs like FTPManually via programs like FTP Automatically, seamlessly using Automatically, seamlessly using distributed file distributed file

systemssystems Semi automatically via theSemi automatically via the world wide webworld wide web

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File Sharing – Remote File File Sharing – Remote File SystemsSystems

Client-serverClient-server model allows clients to mount model allows clients to mount remote file systems from serversremote file systems from servers Server can serve multiple clientsServer can serve multiple clients Client and user-on-client identification is insecure Client and user-on-client identification is insecure

or complicatedor complicated NFSNFS is standard UNIX client-server file sharing is standard UNIX client-server file sharing

protocolprotocol CIFSCIFS is standard Windows protocol is standard Windows protocol Standard operating system file calls are Standard operating system file calls are

translated into remote callstranslated into remote calls

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File Sharing – Remote File File Sharing – Remote File SystemsSystems

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

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File Sharing – Failure ModesFile Sharing – Failure Modes

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

Recovery from failure can involve state Recovery from failure can involve state information about status of each remote information about status of each remote requestrequest

Stateless protocols such as NFS include all Stateless protocols such as NFS include all information in each request, allowing easy information in each request, allowing easy recovery but less securityrecovery but less security

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File Sharing – Consistency File Sharing – Consistency SemanticsSemantics

Consistency semanticsConsistency semantics specify how specify how multiple users are to access a shared file multiple users are to access a shared file simultaneouslysimultaneously Similar to Ch 6 process synchronization Similar to Ch 6 process synchronization

algorithmsalgorithmsTend to be less complex due to disk Tend to be less complex due to disk I/O and network latency (for remote file I/O and network latency (for remote file systemssystems

Andrew File System (AFS) implemented Andrew File System (AFS) implemented complex remote file sharing semanticscomplex remote file sharing semantics

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File Sharing – Consistency File Sharing – Consistency SemanticsSemantics

Unix file system (UFS) implements:Unix file system (UFS) implements:

Writes to an open file visible Writes to an open file visible immediately to other users of the same immediately to other users of the same open fileopen file

Sharing file pointer to allow multiple Sharing file pointer to allow multiple users to read and write concurrentlyusers to read and write concurrently

AFS has session semanticsAFS has session semantics

Writes only visible to sessions starting Writes only visible to sessions starting after the file is closedafter the file is closed

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

Types of accessTypes of access ReadRead WriteWrite ExecuteExecute AppendAppend DeleteDelete ListList

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Access Lists and GroupsAccess Lists and GroupsMode of access: read, write, executeMode of access: read, write, executeThree classes of usersThree classes of users

RWXRWXa) a) owner accessowner access 77 1 1 11 1 1

RWXRWXb) b) group accessgroup access 66 1 1 01 1 0

RWXRWXc) c) public accesspublic access 11 0 0 10 0 1

owner group public

chmod 761 game

Attach a group to a file chgrp G game

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Access Lists and GroupsAccess Lists and Groups

Mode of access: read, write, executeMode of access: read, write, execute

Three classes of usersThree classes of usersRWXRWX

a) a) owner accessowner access 77 1 1 11 1 1RWXRWX

b) b) group accessgroup access 66 1 1 01 1 0RWXRWX

c) c) public accesspublic access 11 0 0 10 0 1

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Access Lists and GroupsAccess Lists and Groups

Ask manager to create a group (unique name), Ask manager to create a group (unique name), say G, and add some users to the group.say G, and add some users to the group.For a particular file (say For a particular file (say gamegame) or subdirectory, ) or subdirectory, define an appropriate access.define an appropriate access.

owner group public

chmod 761 game

Attach a group to a file chgrp G game

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Windows XP Access-control List Windows XP Access-control List ManagementManagement

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A Sample UNIX Directory A Sample UNIX Directory ListingListing

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End of Chapter 10End of Chapter 10

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