The ext2 File System

Objectives

1. Look at an example file system.

Notes

• I am using Kerrisk chapter 14.
• ext2 - the second extended file system
  • ext2 was viable from 1993 until 2001
  • The design was based on Berkeley Fast File System (ufs),
  • It was replaced by ext3 and quickly ext4
  • wikipedia claims that it is still the choice for usb, sd cards, ...
  • But a fat file system may also be in contention there.
• Disks support physical blocks.
  • The smallest unit of storage.
  • 512 bytes is the suggested size.
• These are treated as logical blocks at the file system
  • 1k, 2k and 4k in ext2
• From Kerrisk.
  • The boot block is for the os use
    • Kerrisk states that all file systems have one, even if it is unused.
  • The superblock
    • Holds file system information
      • size of the i-node table (in a second)
      • Size of a logical block
      • Size of the file system in logical blocks.
      • And much more
    • There are backup superblocks distributed across the disk.
  • The i-node table
    • The closest thing we have to a list of files.
• i-nodes
  • index nodes.
  • Each file on the file system has an inode
  • This contains
    • The file type
      • On linux all files are regular files except
      • directories
      • symbolic links
      • character and block special files (hardware interface)
      • named pipes
      • unix domain sockets.
    • The owner (an integer from the /etc/passwd)
    • The group (an integer from /etc/group)
    • Access permissions (rwx for ugo + s)
    • Time stamps
      • last access time ls -lu
      • last modification time ls -l
      • last i-node modification time ls -c
    • hard link count
    • size in bytes
    • the number of blocks, measured in 512 byte blocks.
    • Pointers to the data blocks.
  • This does not contain the file name!
  • The data block pointers
    • Blocks 0-11 are direct pointers: they point to a data block
    • Block 12 points to a block of pointers.
      • Assume each pointer takes 4 bytes (32 bit number)
        • A 1K block points to 256 other blocks.
        • A 2K block points to 512 other blocks
        • And a 4K block points to 1024 other blocks.
    • Block 13 is a double indirect pointer
      • At 1K, 256 pointers point to 256 blocks for a total of 65K blocks.
      • 2K 242K blocks
      • 4K 1048K blocks.
    • And block 14 is a triple indirect block 1024 pointers to 1024 pointers to 1024 pointers to 4k blocks or 2⁴²
    • From Kerrisk.