File Management, file system

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File Management, file system
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File-System Structure

• File structure
• Logical storage unit
• Collection of related information
• File system resides on secondary storage (disks)
• File system organized into layers
• File control block storage structure consisting
  of information about a file

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Model of I/O organization
Logical I/O
Basic File System
Basic I/O Supervisor file I/O initiation and
termination scheduling access to optimize
performance
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File System I/O and Software Architecture
Basic File System physical I/O, placement,
buffering of blocks
Logical I/O Enables programs to access records
provides general-purpose record I/O capability
maintains basic data about file
Basic I/O Supervisor file I/O initiation and
termination scheduling access to optimize
performance
create,delete open, close, read, write, append,
seek get-attributes, set-attributes
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File Structure

• Some kinds of files
• byte sequence
• record sequence
• tree

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File Operations

• File is an abstract data type
• Create
• Write
• Read
• Reposition within file
• Delete
• Truncate
• Open(Fi) search the directory structure on disk
  for entry Fi, and move the content of entry to
  memory
• Close (Fi) move the content of entry Fi in
  memory to directory structure on disk

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File Attributes

• Name only information kept in human-readable
  form
• Identifier unique tag (number) identifies file
  within file system
• Type needed for systems that support different
  types
• 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 for
  protection, security, and usage monitoring
• Information about files are kept in the directory
  structure, which is maintained on the disk

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Access Methods

• Sequential Access
• read next
• write next
• reset
• no read after last write
• (rewrite)
• Direct Access
• read n
• write n
• position to n
• read next
• write next
• rewrite n
• n relative block number

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More access methods Indexed File

• Index provides lookup capability
• Contains key field and a pointer to the main file
• May be exhaustive or partial index
• Scheme may use multiple indexes for different key
  fields
• new records are added to an overflow file
  structure

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More access methods The Hashed File

• Directly access a block at a known address
  (using, e.g. modulo operation on the main key)

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A Typical File-system Organization
A.T. MOS fig of same concept
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Directory Structure

• A collection of nodes containing information
  about all files

Directory
Files
F 1
F 2
F 3
F 4
F n
Both the directory structure and the files reside
on disk Backups of these two structures are kept
on tapes
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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 Java programs, all games,
  )

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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
• Limited grouping capability

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Tree-Structured Directories

• (sub)directory a file
• Grouping Capability
• Current directory (working directory)
• cd /spell/mail/prog
• type list
• Absolute or relative path name

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Acyclic-Graph Directories

• shared subdirectories and files
• Two different names (aliasing)
• w deletes list ? dangling ptr Solutions
• Backpointers, so we can delete all pointers
  -Variable size records a problem
• Entry-hold-count solution
• New directory entry type
• Link another name (pointer) to an existing file
• Resolve the link follow pointer to locate the
  file

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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 open
• File-open count counter of number of times a
  file is open to allow removal of data from
  open-file table when last processes closes it
• Disk location of the file cache of data access
  information
• Access rights per-process access mode information

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In-Memory File System Structures
fd
FCBs
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File System Mounting

• A file system must be mounted before it can be
  accessed
• file system is mounted at a mount point

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File Sharing

• allow files to be shared among users
• issues
• Access rights
• Management of simultaneous access

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Access Rights (managed by owner)

• Knowledge User can only determine that the file
  exists and who its owner is
• Execute user can load and execute a program but
  cannot copy it
• Read user can read the file
• Append user can add data to the file but cannot
  modify/delete contents
• Update user can modify, deleted, and add to the
  files data.
• Change protection user can change access rights
  granted to other users

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Domain Structure, Access Matrices

• Access-right ltobject-name, rights-setgt
• rights-set a subset of all valid operations on
  the object.

• For a process in Di to do op on object Oj, op
  must be in the access matrix.
• Can be expanded to dynamic protection.
• Operations to add, delete access rights.
• Special access rights
• owner of Oi
• copy op from Oi to Oj
• control Di can modify Dj access rights
• transfer switch from domain Di to Dj

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Simultaneous Access

• Locks
• on entire file when it is to be updated
• on the individual records during the update (cf
  2-phase locking in DBs)
• OS may provide different read/write locks
• Mutual exclusion and deadlock are issues for
  shared access

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Open File Locking

• Provided by some operating systems and file
  systems
• Mediates access to a file
• Mandatory or advisory
• Mandatory access is denied depending on locks
  held and requested
• Advisory processes can find status of locks and
  decide what to do

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File Sharing Consistency Semantics

• Consistency semantics specify how multiple users
  are to access a shared file simultaneously
• Similar to Ch 7 process synchronization
  algorithms
• Unix file system (UFS) implements
• Writes to an open file visible immediately to
  other users of the same open file
• Sharing file pointer to allow multiple users to
  read and write concurrently
• Andrew File System (AFS) implemented complex
  remote file sharing semantics
• AFS has session semantics
• Writes only visible to sessions starting after
  the file is closed

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Disks
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Secondary Storage Management

• Space must be allocated to files
• Must keep track of the space available for
  allocation
• Methods of File Allocation
• Contiguous allocation
• External fragmentation will occur
• Chained allocation
• Each block contains a pointer to next block
• No external fragmentation
• Best for sequential files
• Indexed allocation
• File allocation table contains a separate
  one-level index for each file

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Combined Scheme UNIX
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Implementing Directories (1)

• (a) A simple directory
• fixed size entries
• disk addresses and attributes in directory entry
• (b) Directory in which each entry just refers to
  an i-node

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Implementing Directories (2)

• Two ways of handling long file names in directory
• (a) In-line
• (b) In a heap

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Discussion

• Q how does the OS keep info on free/occupied
  disk blocks (memory frames)?
• What if file system is distributed/network-based?

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Free-Space Management

• Bit vector (n blocks)

0
1
2
n-1

0 ? blocki free 1 ? blocki occupied
biti
???
Block number calculation
(number of bits per word) (number of 0-value
words) offset of first 1 bit
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Free-Space Management (Cont.)

• Bit map requires extra space
• Example
• block size 212 bytes
• disk size 230 bytes (1 gigabyte)
• n 230/212 218 bits (or 32K bytes)
• Easy to get contiguous files
• Linked list (free list)
• Cannot get contiguous space easily
• No waste of space

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Schematic View of Virtual File System
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Schematic View of NFS Architecture

• Network interface client-server protocol
• Uses UDP (over IP over most commonly- ethernet)
• Mounting and caching