File Management Systems

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File Management Systems

• Chapter 12

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Objectives

• Describe the components and functions of a file
  management system
• Compare the logical and physical organization of
  files and directories
• Explain how secondary storage locations are
  allocated to files and describe the data
  structures used to record those allocations
• Describe file manipulation operations, including
  open, close, read, delete, and undelete
  operations
• List access controls that can be applied to files
  and directories
• Describe security, backup, recovery, and fault
  tolerance methods and procedures

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Chapter Topics
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File Management Systems

• A File Management System (FMS) is
• implemented in layers
• Command layer or application program
• File control
• Storage I/O control
• Secondary storage devices

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File Management Systems
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File Management Systems

• Logical and Physical Storage Views
• Logical Storage Views viewed by users are a
  collection of files organized within directories
  and storage volumes
• Physical Storage Views a collection of physical
  storage locations organized as a linear address
  space

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File Management Systems
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File Management Systems
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File Management Systems

• File Content and Type
• A file can store many different data types
  including text, numbers, complex data structures,
  and executable instructions
• Modern file management systems provide a
  framework to support additional file types

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File Management Systems
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File Management Systems
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Directory Content and Structure

• Typical directory contents include
• Name Size Time stamp
• File Type Ownership
• Location Access controls

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Directory Content and Structure

• Hierarchical Directory Structure
• Directories can contain other directories
• Directories can not have more than one parent
• Sometimes called a tree structure

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Directory Content and Structure
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Directory Content and Structure

• Graph Directory Structure
• Files and subdirectories can be contained within
  multiple directories
• Directory links can form a cycle

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Directory Content and Structure
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Storage Allocation

• Allocation Units
• the smallest number of secondary storage bytes
  that can be allocated to a file
• can not be smaller than the unit of data transfer
  between the storage device and controller, which
  is normally called a block

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Storage Allocation

• Allocation unit size is usually a
• Tradeoff among
• Efficient use of secondary storage space for
  files.
• Size of storage allocation data structures.
• Efficiency of storage allocation procedures.

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Storage Allocation

• Storage Allocation Tables
• A storage allocation table is a data structure
  that records which allocation units are free and
  which belong to files.

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Storage Allocation
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Storage Allocation
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Storage Allocation
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Storage Allocation

• Blocking and Buffering
• A logical record is a collection of data items,
  or fields, that is accessed by an application
  program as a single unit
• A physical record is the unit of storage
  transferred between the device controller and
  memory in a single operation

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Storage Allocation

• Blocking and Buffering
• Logical record grouping within physical records
  is called blocking
• If a physical record contains just one logical
  record, then the file is said to be unblocked

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Storage Allocation
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Storage Allocation

• Blocking and Buffering
• A FMS uses buffers in primary storage to store
  data temporarily as it moves between programs and
  secondary storage devices
• A buffer is a scratchpad for extracting logical
  records from physical records

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Storage Allocation
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File Manipulation

• File Open and Close Operations
• The FMS must perform several tasks, collectively
  called a file open operation, before an
  application program can read or write a files
  contents.

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File Manipulation

• File Open Operation
• Locates the file within the directory structure
  and reads its directory entry.
• Searches an internal table of open files to see
  if the file already is open.
• Ensures that process has sufficient privileges to
  access the files.
• Allocates one or more buffers.
• Updates an internal table of open files.

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File Manipulation

• File Close Operation
• Flushing the programs file I/O buffers to
  secondary storage.
• Deallocating buffer memory.
• Updating the files directory entry time stamps.
• Updating the open file table.

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File Manipulation

• Delete and Undelete Operations
• In most file management systems, files are not
  removed immediately from secondary storage when
  they are deleted.
• The files storage allocation units are marked as
  free and its directory entry is marked as unused.
• A user might be able to use the undelete
  operation to recover the file.

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Access Controls

• A File Management System helps prevent loss,
  corruption and unauthorized access to files.
• The operating system is used to identify and
  authenticate users and their processes.
• The file access is authenticated through ids and
  passwords.

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Access Controls

• UNIX defines three access control types
• Read
• Write
• Execute

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Technology Focus
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File Migration, Backup, and Recovery

• File Migration management technique for
  secondary storage in which older versions of a
  file are moved automatically to less costly
  storage media or devices such as magnetic tape.

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File Migration, Backup, and Recovery
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File Migration, Backup, and Recovery

• File Backup
• Full Backup the FMS copies all files and
  directories for an entire storage volume
• Incremental Backup only the files that have
  been modified are archived
• Differential Backup only the changed portions
  of the files are archived

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File Migration, Backup, and Recovery

• File Recovery
• The file management system maintains backup logs
  to aid in locating backup copies of lost or
  damages files.
• The recovery utility reconstructs as much of the
  directory and storage allocation data structures
  as possible and makes a consistency check.

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File Migration, Backup, and Recovery

• Fault Tolerance
• Describes methods of securing file content
  against hardware failure.
• File backup, recovery, and transaction logging
  are forms or protection against disk failure.

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File Migration, Backup, and Recovery

• Mirroring
• A fault tolerance technique in which all disk
  write operations are made simultaneously or
  concurrently to two different storage devices.
• Disk mirroring provides a high degree of
  protection against data loss with no performance
  penalty if implemented in hardware.

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Technology Focus
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Technology Focus
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Technology Focus
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Summary

• The file management system (FMS) is usually a
  part of the operating system, manages all aspects
  of user and program access to secondary storage.
• With directories, users can organize the
  thousands of files stored in a typical computer
  system.
• Secondary storage units are divided into
  allocation units, which are typically a few
  kilobytes in size.

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Summary

• The FMS allocates buffers to support program file
  I/O.
• The FMS enforces access controls when accessing
  files on behalf of a user or program.
• FMSs provide utilities to make backup copies of
  files and directories and to recover them if
  needed.