ext2

Concept

• blocks are grouped into block groups
• uses indexed allocation, files/directories use i-nodes
  • contain file metadata and data block address
• fast access for small files + flexibility for huge files

Superblock

• one for whole file system, duplicated in each block group for redundancy
• describes the whole file system
  • total number of i-nodes
  • i-nodes per group
  • total number of blocks
  • blocks per group

Group descriptors

• number of free blocks/i-nodes
• location of the bitmaps

Block bitmap

• keep track of free blocks in the group
• 1 - occupied, 0 - free

i-node bitmap

• keep track of i-nodes in the group
• 1 - occupied, 0 - free

i-node table

• array of i-nodes for this group
• stores the file metadata
• i-nodes:
  • fixed size
  • 12 pointers - direct pointers, block with actual file data
  • 13th - single indirect, block with direct pointers (2-level)
  • 14th - double indirect, block with single indirect pointers (3-level)
  • 15th - triple indirect, block with double indirect pointers (4-level)

Directory

• directory’s data block -> linked list of directory entries
• contains:
  • i-node number - look up the file in the i-node table
  • pointer to next entry/size of this entry
  • length of file/sub-directory name
  • type - file or directory
  • file/sub-directory name - 255 characters

Linking

Hard link

• two directory entries point to the same i-node
• i-node keeps track of reference count
• data is only deleted when reference count is 0

Symbolic link

• entry has its own i-node that points to the other directory entry
• only pathname is stored, need to search for the target i-node
• can delete original file -> dangling pointer