UNIX File System Structure UFS

A UFS file system is made of:

• Boot block, the first block of every file system (block 0)
• Superblock
  Block 1 that contains:
  Total size of the file system (in blocks)
  Number of blocks reserved for inodes
  Name of the file system
  Device identification
  Date of the last superblock update
  Head of the free-block list
  List of free inodes
  Inode blocks and, for assigned inodes:
  File type: regular, device, named pipes, socket, symbolic link
  File owner: UID and GID
  Protection information: rwe for ugo
  Link count: name and inode of master file
  Size of the file in bytes
  Last file access date
  Last file modification date
  Last inode modification date
  Pointers to data blocks: actual location of blocks on physical disk
  Data blocks with user data or system files
  

New blocks allocated to a file are obtained from the free-block list to which blocks are returned when a file is deleted.

The superblock is followed by blocks containing inodes and associated inumber pairs. An inode describes an individual file with one inode for each file in the file system. For each file system is allocated a maximum number of inodes and therefore a maximum number of files. The maximum values depend on the the file system size.

Inode 1 on each file system is unnamed and unused. Inode 2 must correspond to the file system root directory that supports all other files in the file system. Inodes after inode 2 are free and can be any file. Inodes and blocks are not allocated in any particular order.

A directory entry, file or link, consists of the name and the inumber representing the file. The link count indicates the number of directory entries that refer to the same file. A file is deleted if the link count is zero. When the file is deleted the associated inode is returned to the free-inode list and its associated blocks are returned to the free-block list.