File Allocation Methods and Access Techniques
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Questions and Answers

What is the primary advantage of contiguous allocation for files?

  • It supports the most flexible allocation of disk space for files.
  • It allows for efficient random access to any part of the file.
  • It simplifies file management and reduces fragmentation. (correct)
  • It enables the use of indexing techniques for faster retrieval.
  • How does the "write next()" operation work in sequential access to files?

  • It replaces existing data at the end of the file.
  • It appends new data to the end of the file and advances the pointer. (correct)
  • It overwrites existing data at the beginning of the file.
  • It inserts new data at a specific location within the file.
  • Which type of file access is typically employed in databases for efficient data retrieval?

  • Linked allocation.
  • Sequential access
  • Indexed sequential access
  • Direct access (correct)
  • How does indexed allocation differ from direct access?

    <p>Indexed allocation uses an index to locate data blocks, while direct access accesses blocks directly by their number.</p> Signup and view all the answers

    What is a potential drawback of using a single index for a very large file?

    <p>It can slow down data access due to the need to search through a large index.</p> Signup and view all the answers

    What is the primary function of a primary index in a multi-level indexing system?

    <p>To act as a secondary index, pointing to other indexes in the system.</p> Signup and view all the answers

    Which of the following is NOT an advantage of linked allocation for files?

    <p>It supports random access to any part of the file.</p> Signup and view all the answers

    Which allocation method is best suited for files that are frequently updated or resized?

    <p>Linked allocation</p> Signup and view all the answers

    What is the main purpose of a protection system in a computer system?

    <p>To provide a mechanism for enforcing policies regarding resource use.</p> Signup and view all the answers

    Which principle dictates that systems, users, and programs should only have the minimum privileges required for their tasks?

    <p>Principle of least privilege</p> Signup and view all the answers

    What is the main reason why protection policies may need to change over time?

    <p>User needs and applications evolve.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of a system that implements the principle of least privilege?

    <p>Centralized control over all user privileges</p> Signup and view all the answers

    What role does a protection mechanism play in relation to protection policies?

    <p>Protection mechanisms implement the rules specified by protection policies.</p> Signup and view all the answers

    What is the most significant consequence of an unprotected resource in a computer system?

    <p>Potential for unauthorized or improper use</p> Signup and view all the answers

    Why is it essential to have a flexible protection system?

    <p>To adapt to evolving user needs and application requirements.</p> Signup and view all the answers

    What is a key benefit of managing users with the principle of least privilege?

    <p>Reduced risk of data breaches and security vulnerabilities.</p> Signup and view all the answers

    What is a major limitation of linked allocation when accessing blocks in a file?

    <p>It requires starting from the beginning to find any block.</p> Signup and view all the answers

    What is one of the disadvantages of linked allocation in terms of space usage?

    <p>Pointers consume a significant portion of disk space.</p> Signup and view all the answers

    Which solution helps mitigate reliability issues in linked allocation?

    <p>Implementing doubly linked lists.</p> Signup and view all the answers

    What does the file-allocation table (FAT) do in linked allocation?

    <p>It maps each block to its subsequent block.</p> Signup and view all the answers

    How are the pointers in the index block initialized for indexed allocation?

    <p>They are set to null until a block is written.</p> Signup and view all the answers

    What is indicated by a table value of 0 in the FAT?

    <p>The block is unused.</p> Signup and view all the answers

    In indexed allocation, what does the directory contain?

    <p>The address of the index block.</p> Signup and view all the answers

    What happens when the ith block is first written in indexed allocation?

    <p>A block is retrieved from the free-space manager.</p> Signup and view all the answers

    What is tracked by the per-process table in an operating system?

    <p>Current file pointer for each file opened by the process</p> Signup and view all the answers

    What type of information is stored in the system-wide table?

    <p>The location of the file on disk</p> Signup and view all the answers

    What does the file-open count indicate?

    <p>The number of processes that have opened a specific file</p> Signup and view all the answers

    What is the purpose of the current-file-position pointer?

    <p>To point to the last read-write location in a file for the process</p> Signup and view all the answers

    Which access method processes information in order, one record at a time?

    <p>Sequential Access</p> Signup and view all the answers

    What information about a file is typically kept in memory for efficient operations?

    <p>Information needed to locate the file on disk</p> Signup and view all the answers

    What happens to an entry in the system-wide table when all processes close the file?

    <p>It is immediately removed after every close</p> Signup and view all the answers

    Which of the following information is NOT typically included in a per-process table?

    <p>File size of opened files</p> Signup and view all the answers

    What is the main advantage of indexed allocation compared to linked allocation?

    <p>Indexed allocation avoids external fragmentation</p> Signup and view all the answers

    In a multilevel index, how many levels of index blocks are involved in accessing a data block?

    <p>Three</p> Signup and view all the answers

    In a combined indexed allocation scheme, what is the purpose of the first 12 pointers in the file's inode?

    <p>They point to direct blocks containing file data</p> Signup and view all the answers

    What is the difference between a single indirect block and a double indirect block?

    <p>A single indirect block points to data blocks, while a double indirect block points to other indirect blocks</p> Signup and view all the answers

    Which of the following is a disadvantage of indexed allocation?

    <p>It has high pointer overhead compared to linked allocation</p> Signup and view all the answers

    What is a key advantage of organizing directories logically?

    <p>It enhances efficiency in locating files.</p> Signup and view all the answers

    Which of the following correctly describes a two-level directory structure?

    <p>It allows a single file name for multiple users.</p> Signup and view all the answers

    What is a limitation of a single-level directory system?

    <p>Naming conflicts among files.</p> Signup and view all the answers

    Which of the following is NOT a goal of protection in a file system?

    <p>Ensuring fast access to frequently used files.</p> Signup and view all the answers

    What type of file system is specifically designed for managing daemons in Solaris?

    <p>ctfs</p> Signup and view all the answers

    Which operation is NOT typically performed on a directory?

    <p>Compress a file</p> Signup and view all the answers

    What is a key characteristic of tree-structured directories?

    <p>They enable logical grouping of files.</p> Signup and view all the answers

    Which of the following file systems is a general-purpose file system?

    <p>ufs</p> Signup and view all the answers

    Study Notes

    File System

    • A file system is a logical storage unit
    • Files are mapped to non-volatile physical devices
    • A file is a named collection of related information stored on secondary storage
    • For users, a file is the smallest allotment of logical secondary storage
    • Files represent programs (source and object code) and data
    • Data can be numeric, alphabetic, alphanumeric, or binary
    • Files can be free-form (e.g., text files) or rigidly formatted
    • Generally, a file is a sequence of bits, bytes, lines, or records whose meaning is defined by the creator and user

    File Attributes

    • File attributes vary between operating systems, but typically include:
      • Name (human-readable symbolic name)
      • Identifier (unique, non-human-readable name for the file within the file system)
      • Type (needed for systems supporting different file types)
      • Location (pointer to device and file location)
      • Size (current size in bytes, words, or blocks; maximum allowed size)
      • Protection (access control information determining who can read, write, execute, etc.)
      • Time, date, and user identification for creation, modification, and last use (useful for protection, security, and usage monitoring)

    File Operations

    • Operating systems provide system calls for creating, writing, reading, repositioning, deleting, and truncating files
    • Creating a file: File is initially empty
    • Writing to a file: System call specifies file name and information to be written; system searches the directory to locate the file; A write pointer marks location in the file for the next write, updating whenever necessary
    • Reading a file: System call specifies file name and memory location for the next block; system searches the directory for the associated entry; reads from the file location indicated by the read pointer; updates the read pointer
    • Repositioning within a file (file seek): Positioning the current-file-position pointer to a given value; used for random access
    • Deleting a file: Searching directory for the file; releasing file space; erasing the directory entry
    • Truncating a file: Erases contents but keeps attributes, setting file length to zero, releasing file space

    File Operations (Continued)

    • Systems typically use open() system calls before file use
    • An open-file table keeps information on all open files
    • Operating systems usually use internal tables:
      • Per-process table—tracks files open to a process; stores information on how the process is using the file; includes the current file pointer
      • System-wide table—process-independent table, containing file location on disk, access dates, and file size; an open count for each file
    • Information associated with an open file:
      • File pointer—The last read/write location in the file
      • File-open count—Tracks the number of opens and closes; zero on the last close, allowing removal of the entry

    File Operations (Continued)

    • Disk location of the file: Information needed for file location on disk is kept in memory for faster access during file operations
    • Access rights: Each process opens a file in an access mode; information is stored in the per-process table for the system to allow or deny further I/O requests

    File Types

    • Various computer files exist - Examples:
      • executable
      • object
      • source code
      • batch
      • markup
      • word processor
      • library
      • print/view
      • archive
      • multimedia

    File Access Methods

    • Files store information accessible in several ways

    • Sequential Access:

      • Simplest method; processes information in order, one record after the other
      • Commonly used by editors and compilers
      • Read operation (read next()) reads the next portion and automatically advances the file pointer
      • Write operation (write next()) appends to the end and advances to the end of the newly written material
    • Direct Access:

      • File is viewed as numbered blocks or records
      • Allows programs to read and write records rapidly in any order (no specific order)
      • Examples: Databases are often direct access—a query computes the block number containing the answer, then reads that block directly to provide the desired information
        • Read operation is read(n), where n is the block number
        • Write operation for direct access is write(n)
    • Other Access Methods:

      • Built on top of direct access
      • An index structure is usually used, with the index containing pointers to the various blocks
      • To locate a record, first search the index, then use the pointer to access the file and locate the desired record; secondary index files may be used if the index file is large

    Allocation Methods

    • Techniques for allocating file data on storage media
    • Contiguous Allocation:
      • Allocates contiguous blocks on the disk
      • Defined by disk address and length (in block units) of the first block
      • Directory entry for each file includes the address of the starting block and the length of the allocated area

    Allocation Methods (Continued)

    • Contiguous Allocation (Problem & Solution):
      • Problem: External fragmentation
      • Solution: Copy entire file system to another disk, then copy files back to the original, allocating free contiguous space for compacting fragmentation

    Allocation Methods (Continued)

    • Linked Allocation:
      • Solves contiguous allocation problems by arranging file as a chain of disk blocks—blocks may be scattered
      • Directory stores pointers to the first and last blocks of the file

    Allocation Methods (Continued)

    • Linked Allocation(Disadvantages):

      • Inefficient for random access files—to locate the ith block must start at the first block and follow the pointers
      • Space required for file pointers—example: if a pointer occupies 4 bytes out of a 512-byte block, the wasted space is large
    • Linked Allocation (other factors):

      • Not reliable if the operating system software or disk hardware has a bug in the software, and may result in the wrong pointer, leading to linking to free space or another file—solutions:
        • doubly linked lists—file name and relative block number in each block
        • File-allocation table (FAT): a table containing entries for each disk block, indexed by block number. Directory points to the first block of the file; the table entry indicates the next block. Chain continues till the end-of-file value; unused blocks are assigned a value of 0

    Allocation Methods (Continued)

    • Indexed Allocation:
      • Each file has its own index block—an array of disk block addresses—the ith entry of the index block points to the ith block of the file
      • Directory maintains the index address
      • Direct access—use index-block entry to locate the ith block
      • All pointers are set to null during file initialization
      • To write the ith block, a block is acquired from the free-space manager

    Allocation Methods (Continued)

    • Indexed Allocation (Disadvantage):
      • Index block pointer overhead is greater than linked allocation pointer overhead
      • Normally, an index block is one disk block; thus, it can be read and written directly; linking several index blocks can support larger files

    Allocation Methods (Continued)

    • Indexed Allocation (Multilevel Index):
      • First-level index block points to several second-level index blocks, which in turn point to the file blocks—used for accessing a specific block
    • Combined Scheme (Indexed Allocation):
      • Inodes (file's inode) contain the first 15 pointers of the index block—the first 12 pointers in this block may be direct block pointers to the blocks containing the file's data; this helps reduce the space required for small files by eliminating need for a separate index block
      • The next three pointers point to indirect blocks, thus accessing data blocks indirectly

    Directory Structure

    • A collection of nodes containing information about all files
    • Both the directory structure and files reside on disk; Examples:
      • Single-level directory—simple structure but has naming and grouping issues
      • Two-level directory—separate directories for each user, efficiently addressing the searching problem but lacking grouping capabilities
      • Tree-structured directories—the most used method, using a tree structure

    File System Types

    • General-purpose file systems
    • Special-purpose file systems: Examples include memory-based volatile file systems (tmpfs) for temporary I/O; the object file system (objfs) for kernel symbols; and contract file system (ctfs) for daemons

    Operations Performed on Directory

    • Searching for a file
    • Creating a file
    • Deleting a file
    • Listing a directory
    • Renaming a file
    • Traversing the file system

    Directory Organization

    • Directory organization is logically created for fast file retrieval
    • Efficiency: Quickly locate files
    • Naming: Convenient for users (Different users can have files with similar names. Files themselves can also have multiple names.)
    • Grouping: Logic grouping of files based on similar properties

    Protection

    • The role of protection in a computer system is to provide a mechanism for the enforcement of policies governing resource use—these may be a fixed system design or set by the system, and/or individual users
    • Goals of protection: -prevent mischievous/intentional violation of access restrictions -Ensure system components use system resources only as per policies -Protect against unauthorized or incompetent user misuse

    Protection (Continued)

    • Principles of protection: Least privilege—programs, users, and systems are given only the necessary privileges to perform their tasks

    Protection (Continued)

    • Domain of protection:
      • A computer system is a collection of processes and objects (hardware and software objects)
      • Each object has a unique name allowing it to be distinguished, and each object can be accessed only through defined operations
      • Access to a resource requires the process to have authorization. Access to resources is by domains with assigned permissions for that domain; switching between domains is permissible—domain switching occurs based on various criteria: User login, User logout, Process sending messages to a process
      • A domain can be a user, a process, or a procedure—if a domain is equivalent to a user, then its access rights depend on the identity of the user; or if the domain is equivalent to a process, then its access rights depend on the identity of the process
      • Domain access rights are defined by their rights—using access rights, a domain can access an object if the access right includes that permission
      • Each domain can be realized in ways (user, process or procedure)
      • Access matrix—a matrix containing each domain and object with specified rights; each row/column is an access right
      • Copying access rights—allow access right copying from one domain into another (indicated by an asterisk (*)) only within the column (object)

    Protection (Continued)

    • Access Matrix (continued):
      • Addition/removal of rights—the owner right controls these; if access(i, j) includes the owner right, then the process executing in domain Di can add/remove rights in Column j
      • Control rights: applicable to domain objects; if access(i, j) includes the control right, then a process executing in domain Di can remove access rights in row j

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    Description

    Explore the various methods of file allocation and the principles of file access. This quiz covers contiguous allocation, indexed allocation, linked allocation, and the principles of protection systems in computing. Test your knowledge on how these concepts apply to databases and security.

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