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Systems Architecture, Fifth Edition

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Title: Systems Architecture, Fifth Edition Subject: Chapter 12: File and Secondary Storage Management Keywords: Presenter - Anne Ketchen Last modified by – PowerPoint PPT presentation

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Title: Systems Architecture, Fifth Edition


1
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2
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

3
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

4
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5
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)

6
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
7
Logical and Physical Storage 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.
10
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)

11
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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14
Directory Content and Structure
  • Contain information about files and other
    directories, typically name, file type, location,
    size, ownership, access controls, and time stamps

15
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

16
Active (working) directory
17
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

20
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

21
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

22
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.
25
All of a file allocations units are chained
together in sequential order by a series of
pointers.
26
Blocking
  • Logical record grouping within physical records
  • Described by a numeric ratio of logical records
    to physical records (blocking factor)

27
Blocking factor 43
Blocking factor 23
28
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

31
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

32
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

33
Access Controls
  • Granted by file owners and system administrators
    for reading, writing, and executing files
  • Provide security at the expense of additional FMS
    overhead

34
File Migration, Backup, and Recovery
  • Provided by most FMSs to protect files against
    damage or loss

35
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

36
Original
Copy that has been updated to reflect new data
37
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

38
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

39
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

40
Fault Tolerance
  • Methods of securing file content against hardware
    failure
  • File backup
  • Recovery
  • Transaction logging
  • Mirroring
  • RAID (Redundant Array of Inexpensive Disks)

41
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

42
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

43
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44
Data striping Each segment is written in
parallel to a separate disk.
45
If the parity disk fails, the other disks still
retain their original data bits.
46
RAID 10 Mirrors individual disks (RAID 1), then
stripes data (RAID 0) across multiple mirrored
pairs.
47
Storage Consolidation
  • Overcomes inefficiencies of direct-attached
    storage (DAS) in multiple-server environments
  • Common approaches
  • Storage area network (SAN)
  • Network-attached storage (NAS)

48
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
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