Please enable JavaScript.

Coggle requires JavaScript to display documents.

TOPIC 3 : FILE MANAGEMENT - Coggle Diagram

- - - - II) Commonly, files represent programs (both source and object forms) and data.
        III) Data files may be: numeric, alphabetic, alphanumeric, or binary..
        IV) Files may be: free form, (e.g: text files), or may be formatted rigidly.
      - V) Many different types of information may be stored in a file; source programs, object programs, executable programs, numeric data, text, payroll records, graphic images, sound recordings, and so on.
        VI) A file has a certain defined structure, which depends on its type.
        -> A text file is a sequence of characters organized into lines (and possibly pages).
        -> A source file is a sequence of subroutines and functions, each of which is further organized as declarations followed by executable statements.
        -> An object file is a sequence of bytes organized into blocks understandable by the system's linker.
        -> An executable file is a series of code sections that the loader can bring into memory and execute.
      - I)A file is a :
        -> sequence of bits, bytes, lines, or records, the meaning of which is defined by the file's creator and user.
        -> named collection of related information that is recorded on secondary storage.
    - - I) File is named, for the convenience of its human users, and is referred to by its name.
        II) A name is usually a string of characters, such as example.c.
      - III) Some systems differentiate between uppercase and lowercase characters in names, whereas other systems do not.
        IV) A file's attributes usually consist of these:
        -> Name – only information kept in human-readable form
        -> Identifier – unique tag (number) identifies file within file system
        -> Type – needed for systems that support different types
        -> Location – pointer to file location on device
        -> Size – current file size
        -> Protection – controls who can do reading, writing, executing
        -> Time, date, and user identification – data for protection, security, and usage monitoring
    - - The operating system can provide system calls to:
        I) create,
        II) write,
        III) read,
        IV) reposition,
        V) delete, and
        VI) truncate files.
        VII) Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory
        VIII) Close (Fi) – move the content of entry Fi in memory to directory structure on disk
      - Meaning to say, these are what we can do to a file.
    - - I) The question we always ask after downloading something from the internet,
        eg : “Can my computer read / run this file?”
        II) It is the operating system that enable files to be read or run by a computer.
        III) If an operating system recognizes the type of a file, it can then operate on the file in reasonable ways.
        IV) A common technique for implementing file types is to include the type as part of the file name. The name is split into two parts—a name and an extension, usually separated by a period character. The user and the operating system can tell from the name alone what the type of a file is.
    - - III) Certain files must conform to a required structure that is understood by the operating system.
        IV) For example, the operating system requires that an executable file have a specific structure so that it can determine where in memory to load the file and what the location of the first instruction is.
      - I) File types also can be used to indicate the internal structure of the file.
        II) Source and object files have structures that match the expectations of the programs that read them.
      - V) Some operating systems impose (and support) a minimal number of file structures. This approach has been adopted in UNIX, MS-DOS, and others.
        VI) The Macintosh operating system supports a minimal number of file structures. It expects files to contain two parts: a resource fork and a data fork.
        ->The resource fork contains information of interest to the user.
        ->The data fork contains program code or data—the traditional file contents.
- - - - other name : relative
      - file is made up of fixed length logical records that allow programs to read and write records rapidly in no particular order.
        
        based on a disk model of a file, since disks allow random access to any file block.
        
        for this technique, file is viewed as a numbered sequence of blocks or records
        
        no restrictions on the order of reading or writing
        
        in other words :
        
        Random access file organization provides access the records directly.
        
        Each record has its own address on the file with by the help of which it can be directly accessed for reading or writing.
        
        The records need not be in any sequence within the file and they need not be in adjacent locations on the storage medium.
      - e.g : we may read block 14, then read block 53, and then write block 7
      - great use for immediate access to large amounts of information such as databases
    - - contains pointers to the various blocks
      - Index is searched sequentially and its pointer is used to access the file directly to find the desired record
      - in large files, the index file itself may become too large to be kept in memory.
        
        Solution : create an index for the index file,
        and the primary index file would contain pointers to secondary index files, which would point to the actual data items.
    - - the records are accessed in some sequence, i.e., the information in the file is processed in order, one record after the other
      - most common techniques
        
        e.g compilers & editors usually access files using this technique
      - most operation on a file are read & write
        
        A read operation : read next - reads the next portion of the file and automatically advances a file pointer, which tracks the I/O location.
        
        The write operation : write next - appends to the end of the file and advances to the end of the newly written material (the new end of file)
    - - not all operating systems support both sequential and direct access for files.
        
        some systems allow only sequential file access otherwise allow only direct access.
        
        some systems require that a file be defined as sequential or direct when it is created and such a file can be accessed only in a manner consistent with its declaration.
      - Direct
        read n
        write n
        position to n
        read next
        write next
        rewrite n
        
        n = relative block number
      - Sequential
        read next
        write next
        reset
        no read after last write
        (rewrite)
- - - - This is because:
        
        Give users flexibility of accessing any record in any order without having to begin search from the beginning of file.
        
        Uses direct access files which can be implemented only on direct access storage devices
        
        Records are identified by their relative addresses (their addresses relative to beginning of file)
        
        Logical addresses computed when recoeds are stored & again when records are retrieved.
        
        Use hashing algorithms
      - Advantages
        
        Fast access to records
        
        Can be updated more quickly than sequential files because records quickly rewritten to original addresses after modifications
      - Disadvantage
        
        Several records with unique keys may generate same logical address (collision)
- - - - The file systems can be extensive.
        
        Some systems store millions of files on terabytes .
        
        To manage this data, we need organize them.
        
        This organization involves the use of directories.
      - a collection of nodes containing information about all files.
        
        Both the directory structure and the files reside on disk.
        
        Backups of these two structures are kept on tapes
        
        A disk can be used in its entirety for a file system. it is desirable to place multiple file systems on a disk or to use parts of a disk for a file system and other parts for other things
        
        These parts are known variously as partitions, slices, or minidisks (IBM).
        
        A file system can be created on each of these parts of the disk
        
        The parts can also be combined to form larger structures known as volumes,
        
        a chunk of storage that holds a file system as a volume
        
        Each volume can be thought of as a virtual disk
        
        Volumes can also store multiple operating systems
        
        This information is kept in entries in a device directory or volume table of contents.
        
        Each volume that contains a file system must also contain information.
        
        The device directory (known as directory) records information
        
        Operations Performed on Directory:
        
        Search for a file
        
        Create a file
        
        Delete a file
        
        List a directory
        
        Rename a file
        
        Traverse the file system (to access every directory
        and every file within a directory structure)
    - - Single-Level Directory
        
        All files are contained in the same director.
        
        Has significant limitations.
        
        when the number of files increases or when the system has more than one user.
        
        may find it difficult to remember the names of all the files as the number of files increases for single user do a single-level directory.
        
        not uncommon for a user to have hundreds of files and an equal number of additional files on another system.
      - Two-Level Directory
        
        create a separate directory for each user.
        
        Each user has his own user file directory (LTD).
        
        The UFDs have similar structures, but each lists only the files of a single user.
        
        When a user job starts or a user logs in, the system's master file directory (MFD) is searched.
        
        The MFD is indexed by user name or account number