File System Interface

File System Interface

1. File Concept

   • Contiguous logical address space
   • Types
     • Data
       • Numeric
       • Character
       • Binary
     • Program
   • Content defined by file's creator
   • File is an abstract data type

2. File Attributes

   • Name
     • Human readable string, the name of the file.
   • Identifier
     • Unique tag (number) that identifies the file within the file system.
   • Type
     • Type of the file. Necessary in systems that support different types of file.
   • Location
     • Pointer to the file location on the device.
   • Size
   • Protection
     • Control who is allowed to write, read, or execute the file.
   • Time, date, and user identification
     • Data for protection, security, and usage monitoring.
   • Information about files (metadata) is kept in the directory structure

3. File Operations

   • Create
   • Write
   • Read
   • Seek
     • Reposition read/write file offset
   • Delete
   • Truncate
   • Open(Fi)
     • Search the directory structure on disk for entry Fi , and move the content of entry to memory
   • Close(Fi)
     • Move the content of entry Fi in memory to directory structure on disk

4. Pieces of Data needed to manage opened files

   • Open-file Table
     • It is a table to track open files.
   • File Pointer
     • Pointer to last read/write location. Each for process that has the file opened.
   • File-open count
     • Counts the number of times a file was opened.
   • Disk location of the file
     • Cache of data access information
   • Access rights
     • per-process access mode information.

5. Open File Locking

   • • Mandatory
       • Access is denied depending on locks held and requested
     • Advisory
       • Processes can find status of locks and decide what to do
   • • Shared Lock
     • Exclusive lock

6. Possible file structures

   • The OS or the program will decide what is the structure of a given program.
   • None
     • Sequence of words or bytes.
   • Simple record structure
     • May have lines with fixed/variable length.
   • Complex Structures
     • Formatted document, or relocatable load file.

7. Access Methods

   • Sequential Access
   • Direct Access

8. Index File

   • Makes easier to search for a file based on an index value.. Each index table stores the index value and the ID of the file.

9. Directory

   • A directory is a collection of nodes that has information about files. It is stored in disk.
   • Operations
     • Search for file
     • Create file
     • Delete File
     • List a Directory
     • Rename file
     • Traverse the file system
   • Organization
     • For efficiency
       • Allows locating a file quickly
     • For naming
       • Convenient for users
     • For grouping
       • Logical grouping of files, based on their properties.
   • Levels
     • Single-level
       • A single directory for all users. It has naming and grouping problem.
     • Two-level directory
       • Each user has his own directory. Efficient search and can have the same file name for different users. No grouping capability.
     • Tree-structured directories
       • Efficient searching and grouping capability. It allows relative and absolute path name.
     • Acyclic-graph directories
       • Allows shared subdirectories and files with different names (aliasing).
       • Link
         • Another name (pointer) to an existing file. Resolving the link mean following pointer to locate the file.

10. Disk Structure

    • May be divided into partitions (minidisks, slices)
    • Partition or disk may be
      • RAID protected against failure or not
      • Raw of formatted with a file system
    • Volume
      • A volume is an entity that contains the file system
      • Tracks the info of the file system
    • File System
      • General purpose
      • Special purpose
      • Ex. for Solaris
        • ufs, zfs – general purpose file systems
        • tmpfs – memory-based volatile FS for fast, temporary I/O
        • objfs – interface into kernel memory to get kernel symbols for debugging
        • ctfs – contract file system for managing daemons
        • lofs – loopback file system allows one FS to be accessed in place of another
        • procfs – kernel interface to process structures

11. File System Mounting

    • A file system must be mounted before it can be used. It will be mounted at a mount point. The previous content of the mount point, if any, IS NOT subscribed by the new content. Only the reference is changed.

12. Protection

    • It is possible to control the types of access each file/directory can have and by whom.
    • Types of access
      • Read
      • Write
      • Execute
      • Append
      • List
      • Delete
    • Ex. Unix/Linux protection for files/directories

Images and files in the map