1

A Typical File-system OrganizationA Typical File-system Organization

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DirectoriesDirectories

Record information about groups of filesRecord information about groups of files

Management of filesManagement of files

Single-Level directory: most simple; all files in the same directorySingle-Level directory: most simple; all files in the same directory

Two-Level directory: separate directory for each userTwo-Level directory: separate directory for each user

Tree-Structured (hierarchical) directories: most commonTree-Structured (hierarchical) directories: most common

Operations on directories:Operations on directories:

Search for a file Search for a file

Create a file (directory entry) Create a file (directory entry)

Delete a file (directory entry) Delete a file (directory entry)

List a directory List a directory

Rename a file Rename a file

Traverse the file system (recursive) Traverse the file system (recursive)

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Single-Level DirectorySingle-Level Directory

A single directory for all usersA single directory for all users

Naming problem

Grouping problem

4

Two-Level DirectoryTwo-Level Directory

Separate directory for each userSeparate directory for each user

Path name

Can have the same file name for different user

Efficient searching

No grouping capability

5

Tree-Structured DirectoriesTree-Structured Directories

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Directory StructureDirectory Structure

A collection of nodes containing information about all filesA 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

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Operations Performed on DirectoryOperations Performed on Directory

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) to Organize the Directory (Logically) to ObtainObtain

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 properties, (e.g., all Java programs, all games, …)games, …)

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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 (Cont)Tree-Structured Directories (Cont)AbsoluteAbsolute or or relativerelative path name path name

Creating a new file is done in current directoryCreating a new file is done in current directory

Delete a fileDelete a file

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

Creating a new subdirectory is done in current directoryCreating a new subdirectory is done in current directory

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

Example: if in current directory Example: if in current directory /mail/mail

mkdir countmkdir countmail

prog copy prt exp count

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

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Acyclic-Graph DirectoriesAcyclic-Graph Directories

Have 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 pointersVariable 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 cycle Every time a new link is added use a cycle detectiondetectionalgorithm to determine whether it is OKalgorithm to determine whether it is OK

15

UNIX DirectoriesUNIX DirectoriesFully hierarchical, tree-structuredFully hierarchical, tree-structured

Directories are represented as files Directories are represented as files

Problem: Truncation Problem: Truncation

Processes have a current working directory Processes have a current working directory

pwd commandpwd command

Each user has a home directory Each user has a home directory

cd; echo $HOME – commands to obtain info about the home dir.cd; echo $HOME – commands to obtain info about the home dir.

The file system has a single root directory The file system has a single root directory

cd / - command changes working directory to root directorycd / - command changes working directory to root directory

Special names identify neighbors in the directory treeSpecial names identify neighbors in the directory tree

./ - the current directory./ - the current directory

../ - the directory one level above the current directory../ - the directory one level above the current directory

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Linking Names and File ContentLinking Names and File Content

UNIX separates file names and UNIX separates file names and file contentfile content

file content may have file content may have multiple (different) namesmultiple (different) names

ln command associates new ln command associates new name with existing file name with existing file

File content identified by:File content identified by:

(Device, File system on (Device, File system on device, i-node)device, i-node)

i-node contains references to i-node contains references to all blocks making up a fileall blocks making up a file

a free-node list is maintained a free-node list is maintained for each file systemfor each file system

Information contained in a UNIX i-node

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File File ProtectionProtection

Access rights can be independently defined for:Access rights can be independently defined for:

(u) user (u) user – Owner (creator) of a file – Owner (creator) of a file

(g) group (g) group – Group– Group

(o) other (o) other – all other users of the UNIX system – all other users of the UNIX system

Example:Example:

luna test ( 48 )-% ls -lisaluna test ( 48 )-% ls -lisa

total 2total 2

421908 1 drwxr-xr-x 421908 1 drwxr-xr-x 2 2 apolze 1024 apolze 1024 Jan 7 15:06 .Jan 7 15:06 .

116884 1 drwxr-xr-x 116884 1 drwxr-xr-x 13 13 apolze 2048 apolze 2048 Jan 7 15:06 ..Jan 7 15:06 ..

116992 0 -rw------- 116992 0 -rw------- 1 1 apolze apolze 0 0 Jan 7 15:05 Mail.txtJan 7 15:05 Mail.txt

116991 0 -rw-rw-rw- 116991 0 -rw-rw-rw- 1 1 apolze 0 apolze 0 Jan 7 15:05 test.cJan 7 15:05 test.c

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File ProtectionFile Protection (contd.)(contd.)

Access rights for a file:Access rights for a file:

(r) (r) Read access right; List right for directorisyRead access right; List right for directorisy

(w) (w) Write access right; includes delete/append rights Write access right; includes delete/append rights

(x) (x) Execute access right; Traverse right for directoriesExecute access right; Traverse right for directories

Binary representation:Binary representation:

(x): (x): Bit 0 (+1) Bit 0 (+1)

(w): (w): Bit 1 (+2) Bit 1 (+2)

(r): (r): Bit 2 (+4) Bit 2 (+4)

Rights can be combinedRights can be combined

Read+Write access right: 6 Read+Write access right: 6

Read+Execute access right: 3 Read+Execute access right: 3

Read-only: 2 Read-only: 2

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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 Groups

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

Three classes of usersThree classes of users

RWXRWX

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

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

RWXRWX

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

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

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

22

A Sample UNIX Directory ListingA Sample UNIX Directory Listing

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File SharingFile Sharing

Sharing of files on multi-user systems is desirableSharing of files on multi-user systems is desirable

Sharing may be done through a Sharing may be done through a protectionprotection schemescheme

On distributed systems, files may be shared On distributed systems, files may be shared across a networkacross 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 identify users, allowing permissions and protections to be per-userpermissions and protections to be per-user

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

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File Sharing – Remote File File Sharing – Remote File SystemsSystems

Uses networking to allow file system access between systemsUses networking to allow file system access between systems

Manually via programs like FTPManually via programs like FTP

Automatically, seamlessly using Automatically, seamlessly using distributed file systemsdistributed file systems

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

Client-serverClient-server model allows clients to mount remote file systems model allows clients to mount remote file systems from serversfrom servers

Server can serve multiple clientsServer can serve multiple clients

Client and user-on-client identification is insecure or complicatedClient and user-on-client identification is insecure or complicated

NFSNFS is standard UNIX client-server file sharing protocol is standard UNIX client-server file sharing protocol

CIFSCIFS is standard Windows protocol is standard Windows protocol

Standard operating system file calls are translated into remote callsStandard operating system file calls are translated into remote calls

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

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File Sharing – Failure ModesFile Sharing – Failure Modes

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

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

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File Sharing – Consistency File Sharing – Consistency SemanticsSemantics

Consistency semanticsConsistency semantics specify how multiple users are to specify how multiple users are to access a shared file simultaneouslyaccess a shared file simultaneously

Similar to process synchronization algorithmsSimilar to process synchronization algorithms

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

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

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

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

Sharing file pointer to allow multiple users to read and write Sharing file pointer to allow multiple users to read and write concurrentlyconcurrently

AFS has session semanticsAFS has session semantics

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

28

Distribution – Network File SystemsDistribution – Network File Systems

Various approaches towards distributed file systems:Various approaches towards distributed file systems:

SUN Network File System (Standard) SUN Network File System (Standard)

UNIX United UNIX United

Andrew FileAndrew File S System ystem

Sprite Sprite

Locus Locus

SUN NFSSUN NFS

Client/Server-System (based on remote procedure call (RPC))Client/Server-System (based on remote procedure call (RPC))

File system operations are forwarded from client to server File system operations are forwarded from client to server

Server executes actual file system operations, returns results Server executes actual file system operations, returns results

Client has access to remote resourcesClient has access to remote resources

Stateless operation (Reliability !)Stateless operation (Reliability !)

29

Operation of a Network File SystemOperation of a Network File System

A set of operations is implemented as RPC-callable functions:A set of operations is implemented as RPC-callable functions:

Searching for a file in a directory Searching for a file in a directory

Reading a set of directory entries Reading a set of directory entries

Manipulating links and directories Manipulating links and directories

Accessing file attributes Accessing file attributes

Reading and writing files Reading and writing files

Logical connection between client and server has to be establishedLogical connection between client and server has to be established

mount protocolmount protocol

NFS works in heterogeneous environmentsNFS works in heterogeneous environments

Machine-independent protocol for data representation (XDR) Machine-independent protocol for data representation (XDR)

Stateless protocolStateless protocol

Network file system may tolerate client crashes (reboots)Network file system may tolerate client crashes (reboots)

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Mounting a Remote File SystemMounting a Remote File System

Computer "sun" exports the "/local"-file system to computer "moon" Computer "sun" exports the "/local"-file system to computer "moon"

Exported to moonExported to all computers on the net

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Windows – UNIX File System Windows – UNIX File System InteroperabilityInteroperability

Windows uses per default the Server Message Block (SMB) Windows uses per default the Server Message Block (SMB) protocol to implement distributed file systemsprotocol to implement distributed file systems

SMB/CIFS (remote) resource sharing:SMB/CIFS (remote) resource sharing:

Samba – de.samba.orgSamba – de.samba.org

POSIX libraries/tool chains for Windows API:POSIX libraries/tool chains for Windows API:

Cygwin – sources.redhat.com/cygwin/Cygwin – sources.redhat.com/cygwin/

U/WIN – www. research.att.com/sw/tools/uwin/U/WIN – www. research.att.com/sw/tools/uwin/

NuTCracker / MKS Toolkit – www.datafocus.comNuTCracker / MKS Toolkit – www.datafocus.com

Interix, SFU – www.microsoft.com/windowsserversystem/sfu/Interix, SFU – www.microsoft.com/windowsserversystem/sfu/

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What's Samba all What's Samba all about?about?

Free SMB and CIFS client/server for UNIX & other Free SMB and CIFS client/server for UNIX & other OSOS

Functionality:Functionality:

a SMB server, to provide Windows NT and LAN a SMB server, to provide Windows NT and LAN Manager-style file and print services to SMB clients Manager-style file and print services to SMB clients such as Windows, Warp Server, smbfs and others.such as Windows, Warp Server, smbfs and others.

a NetBIOS (rfc1001/1002) nameserver, which amongst a NetBIOS (rfc1001/1002) nameserver, which amongst other things gives browsing support. Samba can be the other things gives browsing support. Samba can be the master browser on your LAN if you wish. master browser on your LAN if you wish.

a ftp-like SMB client so you can access PC resources a ftp-like SMB client so you can access PC resources (disks and printers) from UNIX, Netware and other (disks and printers) from UNIX, Netware and other operating systemsoperating systems