Chapter 11 File-System Interface
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Outline
• File Concept
• Access Methods
• Directory Structure
• File-System Mounting
• File Sharing
• Protection
11.1 File Concept
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Introduction
• OS abstracts from the physical storage devices to define a logical storage unit: File
• Types:
– Data: numeric, alphabetic, alphanumeric, binary – Program: source and object form
File Attributes
• Name – only information kept in human-readable form
• Type – needed for systems that support different types
• Location – pointer to file location on device
• Size – current file (and the maximum allowable) size
• Protection – controls who can read, write, execute
• 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 physical storage device.
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File Operations
• Create
– Allocate space
– Make an entry in the directory
• Write
– Search the directory
– Write is to take place at the location of the write pointer
• Read
– Search the directory
– Read is to take place at the location of the read pointer
• Reposition within file – file seek – Set the current file pointer to a
given value
• Delete
– Search the directory – Release all file space
• Truncate --
– All attributes to remain unchanged, but for the file to be reset to length zero
• Open(Fi)
– Search the directory structure on disk for Fi
– Move the content of the directory entry to memory
• Close(Fi)
– move the content of entry Fi in memory to directory structure on disk
Open-File Table
• OS keeps a small table containing information about all open files
– Each time a file is opened, the content of the directory entry of the file is moved into the table
• File Handle (file descriptor, file control block): an index into the table for a file
• Two levels of internal tables in multi-user OS
– Per-process table: All the files opened by a process
• Current file pointer
– System-wide open-file table: Process independent information
• Location of file on disk, access dates, file size
• File open count
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A Typical Open-File Table
Two-Level Open-File Table
File name Location on Disk File Open Count …….
File Pointer …..
File Pointer …..
System-Wide Open File Table Open File Table
for Proc. B
Open File Table for Proc. A
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File Types – Name,
Extension
If OS recognizes the type of a file, it can operate on the file in a reasonable wayFile Structure
• None - sequence of words, bytes
• Simple record structure – Lines
– Fixed length – Variable length
• Complex Structures – Formatted document – Relocatable load file
• All operating systems must support at least one structure – that of an executable file
• Who decides:
– Operating system – Program
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11.2 Access Methods
Access Methods
• Sequential Access: information is processed in order
– read next
– write next (append to the end of the file) – reset to the beginning of file
– skip forward or backward n records
• Direct Access: a file is made up of fixed length logical blocks or records
– read n – write n
– position to n – read next – write next – rewrite n
Sequential-access file
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Access Methods – Index and
Relative File
11.3 Directory Structure
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Directory Structure
• A collection of nodes containing information about all files.
• Both the directory structure and the files reside on disk.
• Backups of these two structures are kept on tapes.
A Typical File-System
Organization
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Information in a Device Directory
• Name
• Type: file, directory, symbolic link, special file…
• Address: device blocks to store a file
• Current length
• Maximum length
• Date last accessed (for archival)
• Date last updated (for dump)
• Owner ID (who pays)
• Protection information (discuss later)
Operations Performed on Directory
• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system
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Organize the Directory (Logically) to Obtain…
• 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 Pascal programs, all games, …)
Single-Level Directory
• A single directory for all users.
Naming problem
Grouping problem
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Two-Level Directory
• Separate directory for each user.
Path name
Can have the same file name for different user
Efficient searching
No grouping capability
Tree-Structured Directories
Subdirectory is just a special type of file…
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Tree-Structured Directories (Cont.)
• Efficient searching
• Grouping Capability
• Current directory (working directory)
– cd /spell/mail/prog – cat list
• Create a new file/subdirectory is done in current directory
• Absolute path name & relative path name
• How to handle the deletion of a directory
Acyclic-Graph Directories
• Can have shared subdirectories and files.
– A shared file can have different names (aliasing)
• Shared file implementation – Symbolic link: a pointer to
another file or directory
• ln -s /spell/count /dict/count – Hard link: duplicate all
information (especially device address) in shared directory
/spell/count
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Symbolic VS. Hard Link
Acyclic-Graph Directories (Cont.)
• If /dict/w/list is deleted ⇒ dangling pointer.
• Solutions: Preserve the file until all references to it are deleted
– Backpointers: a list of all references to a file (directory entries or symbolic links)
• Variable size records a problem – Entry-hold-count solution: a count
• A count of the number of references
• Used in UNIX for hard links
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General Graph Directory
General Graph Directory (Cont.)
• How do we guarantee no cycles?
– Allow only links to file not subdirectories – Garbage collection
– Every time a new link is added use a cycle detection algorithm to determine whether it is OK
• Traversal problem…
– Don’t traverse the same directory or file twice
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11.4 File-System Mounting
File-System Mounting
• A file system must be mounted before it can be accessed
• A file system is mounted at a directory called the mount point
• Mounting steps: mount /device/dsk /users
– Give the device name and the mount point
– Verify that the device contains a valid file system
• Check if the device directory has a valid format
– OS notes that a file system is mounted at the specified mount point
• What if the mount-point contains files?
– The original files are not available, until the file system is un-mounted
• Same file system can be mounted repeatedly, at different mount points
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File-System Mounting
Existing
Un-mounted Partition (/device/disk)
Mount Point
Unavailable when /device/disk is mounted
11.5 File Sharing
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File 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
• A remote file system must be mounted before using
– mount claven.lib.nctu.edu.tw:/OS /chow/book
– Once files are mounted, they are accessed by using the
concatenated logical path names without referencing either the remote hosts or local devices
Remote File-System Mounting Example
root
chow
paper book
root
OS
DFS DSM
Local Client Remote Server
Export
Mount
DFS DSM /chow/book/DSM
/OS/DSM
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Remote File-System Mounting
• Different clients may perceive a different FS view
– To achieve a global FS view – SA enforces mounting rules
• Export: a file server restricts/allows the mounting of all or parts of its file system to a predefined set of hosts
– The information is kept in the server’s export file
• File system mounting:
– Explicit mounting: clients make explicit mounting system calls whenever one is desired
– Boot mounting: a set of file servers is prescribed and all mountings are performed the client’s boot time
– Auto-mounting: mounting of the servers is implicitly done on demand when a file is first opened by a client
No Global Naming in NFS
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A Simple Automounter for
NFS
11.6 Protection
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Protection
• File owner/creator should be able to control
– What can be done – By whom
• Types of access
– Read – Write – Execute – Append – Delete – List
Access Lists and Groups
• Mode of access: read, write, execute
• Three classes of users: owner, group, universe
– User name and user identifier – Group name and group identifier
• 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
• In UNIX: three field of 3 bits each
– field: read, write, execute – Bits: r(4), w(2), x(1)
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