Systems Architecture, Fifth Edition

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Chapter Goals

• 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

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Chapter Goals (continued)

• 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
• Compare and contrast storage area networks and
  network-attached storage

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File Management Systems

• Collection of system software that manages all
  aspects of user and program access to secondary
  storage
• Usually part of the operating system
• Translates operations into commands to physical
  storage devices
• Implemented in four layers (command layer, file
  control, storage I/O control, and secondary
  storage devices)

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Bridges between logical and physical views of
secondary storage
Allocates secondary storage locations to
individual files and directories Includes
software modules for device drivers for each
storage device or device controller, interrupt
handlers, buffers and cache managers
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Logical and PhysicalStorage Views

• Logical view
• Collection of files organized within directories
  and storage volumes
• Physical view
• Collection of physical storage locations
  organized as a linear address space

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The file is subdivided into multiple records and
each record is composed of multiple fields.
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File Content and Type

• FMS supports limited number of file types
• Executable programs
• Operating system commands
• Textual or unformatted binary data
• Modern FMSs can define new file types and install
  utility programs to manipulate them (file
  association)

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File Types

• Normally declared when a file is created and
• Stored within a directory, or
• Declared through a filename convention
• Determine
• Physical organization of data items and data
  structures within secondary storage
• Operations that may be performed upon the file
• Filename restrictions

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Directory Content and Structure

• Contain information about files and other
  directories, typically name, file type, location,
  size, ownership, access controls, and time stamps

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Hierarchical Directory Structure

• Directories can contain other directories,
  creating a tree structure, but cannot be
  contained within more than one parent
• Ways that names of access paths can be specified
• Complete path (fully qualified reference)
• Relative path
• Each storage device has a root directory

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Active (working) directory
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Graph Directory Structure

• More flexible than hierarchical directory
  structure
• Files and subdirectories can be contained within
  multiple directories
• Directory links can form a cycle

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Storage Allocation

• Secondary storage devices
• Large number of storage locations low frequency
  of allocation changes
• Divided into allocation units

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Allocation Units

• Smallest number of secondary storage bytes that
  can be allocated to a file cannot be smaller
  than unit of data transfer between storage device
  and controller (block)
• Assigned/reclaimed by FMS as files and
  directories are created or expanded/shrink or are
  deleted
• Size difficult to change once set

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Allocation Unit Size

• Tradeoffs
• Efficient use of secondary storage space for
  files
• Size of storage allocation data structures
• Efficiency of storage allocation procedures
• Smaller units More efficient use of storage
  space
• Larger units Allow smaller storage allocation
  data structures

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Storage Allocation Tables

• Data structures that record which allocation
  units are free and which belong to files
• Format and content vary across FMSs
• Can contain linked lists in simpler FMSs or
  indices or other complex data structures in more
  complex FMSs

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Free allocation units are assigned to a hidden
system file called SysFree.
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All of a file allocations units are chained
together in sequential order by a series of
pointers.
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Blocking

• Logical record grouping within physical records
• Described by a numeric ratio of logical records
  to physical records (blocking factor)

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Blocking factor 43
Blocking factor 23
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Buffering

• Temporary storage of data as it moves between
  programs and secondary storage devices
• Physical records are stored in the buffer as they
  are read from secondary storage
• FMS extracts logical records from buffers and
  copies them to data area of the application
  program
• Each buffer is the size of one allocation unit
• Improves I/O performance if enough are used

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File Manipulation

• Exact set of service layer functions varies among
  FMSs, but typically includes create, copy, move,
  delete, read, and write
• Application programs interact directly with FMS
  through OS service layer
• Users interact indirectly with FMS through
  command layer

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File Open and Close Operations

• File open
• Causes FMS to find the file, verify access
  privileges, allocate buffers, and update internal
  table of open files
• File close
• Causes FMS to flush buffer content to the storage
  device, release buffers, update file time stamps,
  and update table of open files

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Delete and Undelete Operations

• Delete
• Does not immediately remove files some content
  remains on secondary storage unit all allocation
  units have been reassigned and overwritten
• File content can be visible to intruders
• Undelete
• Can be used to reconstruct directory and storage
  allocation table contents

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Access Controls

• Granted by file owners and system administrators
  for reading, writing, and executing files
• Provide security at the expense of additional FMS
  overhead

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File Migration, Backup, and Recovery

• Provided by most FMSs to protect files against
  damage or loss

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File Migration(Version Control)

• Automatic storage and backup of old file versions
  
• Balances storage cost of each file version with
  anticipated user demand for that version

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Original
Copy that has been updated to reflect new data
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File Backup

• Protects against data loss (file content,
  directory content, and storage allocation tables)
• Store backup copies on a different storage device
  in a different physical location
• Manual or automatic
• Full or incremental

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Transaction Logging

• Automatically records all changes to file content
  and attributes in a separate storage area also
  writes them to the files I/O buffer
• Provides high degree of protection against data
  loss due to program or hardware failure
• Imposes a performance penalty used only when
  costs of data loss are high

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File Recovery

• Automated and manual components
• Can search backup logs for copies of lost or
  damaged files
• Can perform consistency checking and repair
  procedures for crashed system or physically
  damaged storage device

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Fault Tolerance

• Methods of securing file content against hardware
  failure
• File backup
• Recovery
• Transaction logging
• Mirroring
• RAID (Redundant Array of Inexpensive Disks)

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Mirroring

• All disk write operations are made concurrently
  to two different storage devices
• Provides high degree of protection against data
  loss with no performance penalty if implemented
  in hardware
• Disadvantages
• Cost of redundant disk drives
• Higher cost of disk controllers that implement
  mirroring

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RAID

• Disk storage technique that improves performance
  and fault tolerance
• All levels except RAID 1 use data striping
• Breaks a unit of data into smaller segments and
  stores them on multiple disks
• Multiple levels can be layered to combine their
  best features (e.g. RAID 10)
• Can be implemented in hardware or software

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Data striping Each segment is written in
parallel to a separate disk.
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If the parity disk fails, the other disks still
retain their original data bits.
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RAID 10 Mirrors individual disks (RAID 1), then
stripes data (RAID 0) across multiple mirrored
pairs.
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Storage Consolidation

• Overcomes inefficiencies of direct-attached
  storage (DAS) in multiple-server environments
• Common approaches
• Storage area network (SAN)
• Network-attached storage (NAS)

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Storage Consolidation
Storage Area Network (SAN) Network-Attached Storage (NAS)
High-speed interconnection among general-purpose servers and one or more storage servers Block-oriented access Common in multi-server environments with mainframes or supercomputers and substantial overlap among server storage needs Expensive to purchase and administer, but avoid costs of duplicate storage and storage administration Dedicated to managing one or more file systems Accessed by other servers and clients over a local or wide area network File-oriented access Common when geographically dispersed servers need access to a common file system Cheaper to acquire than SAN, but at the price of lower performance
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Summary

• File management systems
• Directory content and structure
• Storage allocation
• File manipulation
• Access controls
• File migration, backup, and recovery
• Storage consolidation