File Management

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File Management

• Chapter 12

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File System

• Permits users to create data collections (files)
  which has the following properties
• Long-term existence stored on disk/LT-storage
• Sharable between processes access permissions
• Structure an internal structure convenient for
  particular applications
• Functions that can be performed on files
• Create / Delete Open / Close Read / Write
• File Structure
• Field the basic element of data
• Record a collection of related fields that can
  be treated as a unit (e.g. an employee record
  with name, SSN, address, etc.)
• Database a collection of related data and
  operations defined upon them (usually database
  management system is independent of the OS)
• Note that on UNIX systems, the basic file
  structure is just a stream of bytes (e.g. a C
  program file does not have fields, records, and
  so on)

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File Management System

• Set of system software that provides services to
  users and applications in the use of files
• Objectives
• Meet the data management needs of the user (e.g.
  file storage and operations R/W, create/delete,
  access rights)
• Optimize performance (e.g. maximize
  throughput/response time)
• Provide I/O support for a variety of storage
  device types
• Minimize or eliminate the potential for lost or
  destroyed data
• Provide a standardized set of I/O interface
  routines
• Provide I/O support for concurrent access to
  files
• Functions
• Identify and locate a selected file
• Use a directory to describe the location of all
  files plus their attributes
• I/O is done on a block basis conversions
  between blocks and records
• Allocate files to free blocks
• Manage organize the free storage

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File Organization and Access

• File Organization logical structuring of the
  records as determined by the way in which they
  are accessed
• Physical Organization depends on the blocking
  and file allocation strategy
• Criteria for choosing a file organization
• Short access time
• Ease of update
• Economy of storage
• Simple maintenance
• Reliability
• Different Organizations
• The pile
• The sequential file
• The indexed sequential
• The indexed
• The direct, or hashed, file

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• Different file Organizations
• The pile
• The sequential file
• The indexed sequential
• The indexed

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File Directories

• Contains information about files
• Attributes
• Location
• Ownership
• Directory itself is a file owned by the OS
• Provides mapping between file names and the files
  themselves

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• Hierarchical, or
• Tree-Structured Directory
• Master directory with user directories underneath
  it
• Files can be located by following a path from the
  root
• Current directory working directory
• Files are referenced relative to the working
  directory

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UNIX File Management

• Types of files
• Regular / ordinary
• Directory
• Special
• Named pipes
• Links
• Symbolic links
• Inodes
• Index node
• Control structure that contains key information
  for a particular file

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File Allocation Methods for Secondary Storage
Management

• Contiguous allocation
• single entry in the File Allocation Table (FAT) -
  Starting block and the length
• External fragmentation may occur
• ? compaction needed

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File Allocation (cont.)

• Chained allocation
• Each block contains a pointer to the next block
  in the chain
• single entry in the FAT - Starting block and the
  length
• No external fragmentation
• Best for sequential files quick access to the
  record
• No accommodation of the principle of locality

After consolidation ? ?
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File Allocation (cont.)

• Indexed allocation
• File allocation table contains a one-level index
  for each file
• The index has one entry for each portion
  allocated to the file
• The file allocation table contains block number
  for the index

Indexed Allocation with variable-length portions
Indexed Allocation with block portions
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Linux Virtual File System

• Designed to support a wide variety of file
  management systems
• VFS provides a uniform file system interface to
  user processes
• Represents any conceivable file systems general
  feature and behavior
• Assumes files are objects that share basic
  properties regardless of the target system

Files on secondary Storage maintained By file
system X