File System Interface

File Concept
1. Contiguous logical address space
2. Types
  1. Data
    1. Numeric
    2. Character
    3. Binary
  2. Program
3. Content defined by file's creator
4. File is an abstract data type
File Attributes
1. Name
  1. Human readable string, the name of the file.
2. Identifier
  1. Unique tag (number) that identifies the file within the file system.
3. Type
  1. Type of the file. Necessary in systems that support different types of file.
4. Location
  1. Pointer to the file location on the device.
5. Size
6. Protection
  1. Control who is allowed to write, read, or execute the file.
7. Time, date, and user identification
  1. Data for protection, security, and usage monitoring.
8. Information about files (metadata) is kept in the directory structure
File Operations
1. Create
2. Write
3. Read
4. Seek
  1. Reposition read/write file offset
5. Delete
6. Truncate
7. Open(Fi)
  1. Search the directory structure on disk for entry Fi , and move the content of entry to memory
8. Close(Fi)
  1. Move the content of entry Fi in memory to directory structure on disk
Pieces of Data needed to manage opened files
1. Open-file Table
  1. It is a table to track open files.
2. File Pointer
  1. Pointer to last read/write location. Each for process that has the file opened.
3. File-open count
  1. Counts the number of times a file was opened.
4. Disk location of the file
  1. Cache of data access information
5. Access rights
  1. per-process access mode information.
Open File Locking
1. 1. Mandatory
    1. Access is denied depending on locks held and requested
  2. Advisory
    1. Processes can find status of locks and decide what to do
2. 1. Shared Lock
  2. Exclusive lock
Possible file structures
1. The OS or the program will decide what is the structure of a given program.
2. None
  1. Sequence of words or bytes.
3. Simple record structure
  1. May have lines with fixed/variable length.
4. Complex Structures
  1. Formatted document, or relocatable load file.
Access Methods
1. Sequential Access
2. Direct Access
Index File
1. 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.
Directory
1. A directory is a collection of nodes that has information about files. It is stored in disk.
3. Operations
  1. Search for file
  2. Create file
  3. Delete File
  4. List a Directory
  5. Rename file
  6. Traverse the file system
4. Organization
  1. For efficiency
    1. Allows locating a file quickly
  2. For naming
    1. Convenient for users
  3. For grouping
    1. Logical grouping of files, based on their properties.
5. Levels
  1. Single-level
    1. A single directory for all users. It has naming and grouping problem.
  2. Two-level directory
    1. Each user has his own directory. Efficient search and can have the same file name for different users. No grouping capability.
  3. Tree-structured directories
    2. Efficient searching and grouping capability. It allows relative and absolute path name.
  4. Acyclic-graph directories
    2. Allows shared subdirectories and files with different names (aliasing).
    3. Link
      1. Another name (pointer) to an existing file. Resolving the link mean following pointer to locate the file.
Disk Structure
1. May be divided into partitions (minidisks, slices)
2. Partition or disk may be
  1. RAID protected against failure or not
  2. Raw of formatted with a file system
3. Volume
  1. A volume is an entity that contains the file system
  2. Tracks the info of the file system
4. File System
  1. General purpose
  2. Special purpose
  3. Ex. for Solaris
    1. ufs, zfs – general purpose file systems
    2. tmpfs – memory-based volatile FS for fast, temporary I/O
    3. objfs – interface into kernel memory to get kernel symbols for debugging
    4. ctfs – contract file system for managing daemons
    5. lofs – loopback file system allows one FS to be accessed in place of another
    6. procfs – kernel interface to process structures
File System Mounting
1. 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.
Protection
1. It is possible to control the types of access each file/directory can have and by whom.
2. Types of access
  1. Read
  2. Write
  3. Execute
  4. Append
  5. List
  6. Delete
3. Ex. Unix/Linux protection for files/directories

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File System Interface

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	Created by Artur Assis almost 3 years ago