Exercises

UNIX file system

Blocks and Partitions

UNIX Disk Architecture

Block 0 – boot block
Block 1 – superblock
Block 2 – i-node
…
Block n – i-node
Block n+1 – file data zone
…
Block n+m – file data zone

• UNIX file = sequence of bytes, each byte being individually addressable
  • a byte can be accessed both sequentially as well as directly
• blocks = basic units of data storage in the file system
  • data exchange between the memory and the disk is done in blocks
  • older systems have a block size of 512 bytes
  • for more efficient space management, newer systems use larger blocks (sizes up to 4KB)
• UNIX file system = a structure stored on disk, and structured as shown in the image above in four block types
  • UNIX file system is stored either on a device such as a hard-drive or CD or on a hard-drive partition
  • hard-drive partitioning = operation relatively independent of the operating system stored there
  • partitions and physical devices = disks
• Block 0 = the operating system boot loader
  • a program dependent on the machine architecture on which it is installed
• Block 1 (superblock) = information that defines the file systems layout on the disk
  • info like:
    • number of i-nodes
    • number of disk zones
    • pointers to the i-node allocation map
    • pointers to the free space map
    • disk zone dimensions
• Blocks 2 to n, where n is a defined when the disk is formatted are i-nodes
  • i-node = UNIX name for the file descriptor, data structures that store information about files and directories, such as permissions, ownership, size, and pointers to data blocks
    • stored on disk as a list named i-list
    • order number of an i-node in the i-list is represented on two bytes and is called i-number and is the link between the file and the user programs
  • largest part of the disk is reserved for file data
  • space allocation for files is done using an elegant indexing mechanism
  • starting point for this allocation is stored in the i-node
• directories = containers for files and other directories

  • directory entry contains only the file name and its corresponding i-node reference

    Mode	File access permissions
    Link count	number of directories that contain references to this i-number, which is basically the number of links to this file
    User ID	Owner user ID (UID)
    Group ID
    Owner group ID (GID)
    Size	number of bytes in the file (file length)
    Access time	time the file was last accessed
    Modification time	time the file was last modified
    i-node time	time the i-node was last modified
    Block list	address list of the first few file blocks
    Indirect list	references to the rest of the blocks belonging to the file

• i-node occupies usually between 64 and 128 bytes and contains the info
• hierarchical, starting from the root directory ("/")
• everything is treated as a file, including hardware devices
• a file entry in a directory is File name (practically of unlimited i-number length)
• file types = regular files, directories, symbolic links, device files, sockets, and FIFOs
• every UNIX file system has a few individual constants, such as: block size, i-node size, disk address size, how many initial block addresses are stored directly in the i-node and how many references are stored in the indirect list
  • regardless of these values, the storage and retrieval principles are the same