File Systems

File Systems

Overview

• File systems are essential for organizing and retrieving data on random access devices.
• They act as an index, providing a structured way to access data.
• Formatting a disk involves structuring it for a specific file system, allocating blocks for bookkeeping data structures.
• Key file information is duplicated across the disk to prevent data loss.
  • This redundancy introduces a trade-off between security and potential inconsistencies.

Common File Systems

• FAT (File Allocation Table):
  • Used by older Microsoft Windows versions and still common in portable USB storage devices.
• EXT (Extended File System):
  • A family of file systems used in Linux, including EXT, EXT2, EXT3, and EXT4.
• HFS+:
  • Used by Mac OS.

Hierarchical File Systems

• Most file systems use a hierarchical structure, representable as a tree.
• Root Directory:
  • The top-level directory in the file system, serving as the root of the directory tree.
• Directories and Files:
  • Directories contain files and other directories.
  • In Windows, directories are referred to as folders (the concept remains the same).
  • Files can be text files or binary files.

Special Data Structures

• Free Block List:
  • Maintains a list of available blocks for writing data.
  • Deleting a file marks its blocks as free without removing the content immediately.
  • This allows for potential recovery of deleted files using specialized tools.
  • Implemented as bitmap.
• Metadata Blocks:
  • A contiguous cluster of blocks storing file metadata (information about files).
  • In POSIX systems, this is where inodes are stored.
  • The number of metadata blocks limits the maximum number of files that can be referenced on the disk.
  • The number of files is independent of the physical disk size and depends on the allocated space for metadata.

Directories as Files

• Directories are special files that contain file name-location pairs for their member files.
• Directory files are accessed like normal files and have their own metadata blocks (inodes in POSIX).
• Directory files contain pointers to the files and directories they contain.
• This structure enables the hierarchical file system.
• Root Directory:
  • The top-level directory is the root of the directory tree.
• Root Terminology:
  • Root can refer to the root user, root directory, or root process.

Finding Files

• To locate a file, the OS needs to find the file's metadata block (inode), which contains pointers to the data blocks on the disk.
• The OS starts searching from the root directory.
• The OS traverses the directory tree, reading each directory file to find the location of the next directory file.
  • The last directory file contains the filename and the location of the file metadata block.
• This process involves traversing the entire tree, opening one directory file after another.

Soft Links (Symbolic Links)

• Soft links (symbolic links) are supported by most file systems.
• Hard links are a related concept in POSIX systems, but are not supported by Microsoft Windows.
• Soft links allow files/directories to be accessible from multiple locations in the hierarchy.
• They provide multiple paths to the same file through different branches of the file system.
• A soft link is a new directory entry that points to another file or directory.
  • In Windows, it is often called a shortcut.
• In Linux (EXT file systems), the metadata block of a soft link contains the path to the linked file or directory.

Side Effects of Soft Links

• Deleting a soft link does not affect the original file or directory.
• Deleting the original file leaves the soft link dangling (pointing to a nonexistent file).
• Dangling references should be avoided.
• Cyclic paths can be created with soft links which can lead to infinite loops.
• Linux systems limit traversal to 20 symbolic links to avoid infinite loops.
  • This number is configurable.

Mounting File Systems

• Mounting is commonly used with external drives (e.g., USB thumb drives) and network drives.
• The OS must recognize the mass storage device and its file system.
• The OS assigns a unique identifier, allowing users/applications to reference the root directory of the mounted file system.
• Hardware is attached, whereas the file system is mounted.

Mounting in Different OS

• Microsoft Windows:
  • Assigns a unique letter to each new file system (e.g., C for the main drive, D for data, H/J for network drives).
• POSIX Systems:
  • The mounted file system becomes part of the main file system hierarchy.
  • The mount point is the location where the external drive is mounted.
  • The content of the mount point is replaced by the content of the mounted disk.