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Chapter 10: FileChapter 10: File--System InterfaceSystem Interface
10.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Chapter 10: FileChapter 10: File--System InterfaceSystem Interface
File ConceptAccess MethodsDirectory StructureFile-System MountingFile SharingProtection
10.3 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
ObjectivesObjectives
To explain the function of file systemsTo describe the interfaces to file systemsTo discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structuresTo explore file-system protection
10.4 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File ConceptFile Concept
Contiguous logical address space
Types: Data
numericcharacterbinary
Program
10.5 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File StructureFile Structure
None - sequence of words, bytesSimple record structure
Lines Fixed lengthVariable length
Complex StructuresFormatted documentRelocatable load file
Can simulate last two with first method by inserting appropriatecontrol charactersWho decides:
Operating systemProgram
10.6 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File AttributesFile Attributes
Name – only information kept in human-readable formIdentifier – unique tag (number) identifies file within file systemType – needed for systems that support different typesLocation – pointer to file location on deviceSize – current file sizeProtection – controls who can do reading, writing, executingTime, date, and user identification – data for protection, security, and usage monitoringInformation about files are kept in the directory structure, which is maintained on the disk
10.7 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File OperationsFile Operations
File is an abstract data typeCreateWriteReadReposition within fileDeleteTruncateOpen(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memoryClose (Fi) – move the content of entry Fi in memory to directory structure on disk
10.8 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Open FilesOpen 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-file table when last processes closes itDisk location of the file: cache of data access informationAccess rights: per-process access mode information
10.9 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Open File LockingOpen File Locking
Provided by some operating systems and file systemsMediates access to a fileMandatory or advisory:
Mandatory – access is denied depending on locks held and requestedAdvisory – processes can find status of locks and decide what to do
10.10 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Locking Example File Locking Example –– Java APIJava 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 fileFileChannel ch = raf.getChannel();// this locks the first half of the file - exclusiveexclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);/** Now modify the data . . . */// release the lockexclusiveLock.release();
10.11 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Locking Example File Locking Example –– Java API (cont)Java API (cont)
// this locks the second half of the file - sharedsharedLock = ch.lock(raf.length()/2+1, raf.length(),
SHARED);/** Now read the data . . . */// release the lockexclusiveLock.release();
} catch (java.io.IOException ioe) {System.err.println(ioe);
}finally {if (exclusiveLock != null)exclusiveLock.release();if (sharedLock != null)sharedLock.release();
}}
}
10.12 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Types File Types –– Name, ExtensionName, Extension
10.13 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Access MethodsAccess Methods
Sequential Accessread nextwrite next resetno read after last write
(rewrite)Direct Access
read nwrite nposition to n
read nextwrite next
rewrite nn = relative block number
10.14 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
SequentialSequential--access Fileaccess File
10.15 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Simulation of Sequential Access on a DirectSimulation of Sequential Access on a Direct--access Fileaccess File
10.16 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Example of Index and Relative FilesExample of Index and Relative Files
10.17 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Directory StructureDirectory 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
10.18 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
A Typical FileA Typical File--system Organizationsystem Organization
10.19 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Operations Performed on DirectoryOperations Performed on Directory
Search for a fileCreate a fileDelete a fileList a directoryRename a fileTraverse the file system
10.20 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Organize the Directory (Logically) to ObtainOrganize the Directory (Logically) to Obtain
Efficiency – locating a file quicklyNaming – convenient to users
Two users can have same name for different filesThe same file can have several different names
Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)
10.21 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
SingleSingle--Level DirectoryLevel Directory
A single directory for all users
Naming problem
Grouping problem
10.22 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
TwoTwo--Level DirectoryLevel Directory
Separate directory for each user
Path nameCan have the same file name for different userEfficient searchingNo grouping capability
10.23 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
TreeTree--Structured DirectoriesStructured Directories
10.24 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
TreeTree--Structured Directories (Cont)Structured Directories (Cont)
Efficient searching
Grouping Capability
Current directory (working directory)cd /spell/mail/progtype list
10.25 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
TreeTree--Structured Directories (Cont)Structured Directories (Cont)
Absolute or relative path nameCreating a new file is done in current directoryDelete 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
prog copy prt exp count
Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”
10.26 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
AcyclicAcyclic--Graph DirectoriesGraph Directories
Have shared subdirectories and files
10.27 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
AcyclicAcyclic--Graph Directories (Cont.)Graph Directories (Cont.)
Two different names (aliasing)
If dict deletes list ⇒ dangling pointerSolutions:
Backpointers, so we can delete all pointersVariable size records a problemBackpointers using a daisy chain organizationEntry-hold-count solution
New directory entry typeLink – another name (pointer) to an existing fileResolve the link – follow pointer to locate the file
10.28 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
General Graph DirectoryGeneral Graph Directory
10.29 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
General Graph Directory (Cont.)General Graph Directory (Cont.)
How do we guarantee no cycles?Allow only links to file not subdirectoriesGarbage collectionEvery time a new link is added use a cycle detectionalgorithm to determine whether it is OK
10.30 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File System MountingFile System Mounting
A file system must be mounted before it can be accessedA unmounted file system (i.e. Fig. 11-11(b)) is mounted at a mount point
10.31 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
(a) Existing. (b) (a) Existing. (b) UnmountedUnmounted PartitionPartition
10.32 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Mount PointMount Point
10.33 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File SharingFile 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
10.34 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Sharing File Sharing –– Multiple UsersMultiple 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
10.35 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Sharing File Sharing –– Remote File SystemsRemote File Systems
Uses networking to allow file system access between systemsManually via programs like FTPAutomatically, seamlessly using distributed file systemsSemi automatically via the world wide web
Client-server model allows clients to mount remote file systems from servers
Server can serve multiple clientsClient and user-on-client identification is insecure or complicatedNFS is standard UNIX client-server file sharing protocolCIFS is standard Windows protocolStandard operating system file calls are translated into remote calls
Distributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing
10.36 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Sharing File Sharing –– Failure ModesFailure Modes
Remote file systems add new failure modes, due to network failure, server failureRecovery from failure can involve state information about status of each remote requestStateless protocols such as NFS include all information in each request, allowing easy recovery but less security
10.37 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
File Sharing File Sharing –– Consistency SemanticsConsistency Semantics
Consistency semantics specify how multiple users are to access a shared file simultaneously
Similar to Ch 7 process synchronization algorithmsTend to be less complex due to disk I/O and network latency (for remote file systems
Andrew File System (AFS) implemented complex remote file sharing semanticsUnix file system (UFS) implements:
Writes to an open file visible immediately to other users of the same open fileSharing file pointer to allow multiple users to read and write concurrently
AFS has session semanticsWrites only visible to sessions starting after the file is closed
10.38 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
ProtectionProtection
File owner/creator should be able to control:what can be doneby whom
Types of accessReadWriteExecuteAppendDeleteList
10.39 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Access Lists and GroupsAccess Lists and Groups
Mode of access: read, write, executeThree classes of users
RWXa) owner access 7 ⇒ 1 1 1
RWXb) group access 6 ⇒ 1 1 0
RWXc) 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.
owner group public
chmod 761 game
Attach a group to a filechgrp G game
10.40 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Windows XP AccessWindows XP Access--control List Managementcontrol List Management
10.41 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
A Sample UNIX Directory ListingA Sample UNIX Directory Listing
End of Chapter 10End of Chapter 10
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