How is the Ext4 file system organized?

Ext4 is a journaling file system that uses an inode- and block-based structure.

Key concepts:

Superblock: Stores metadata about the file system (size, number of blocks, inodes, etc.). Located at the beginning of the file system and has backup copies.
Block groups: The file system is divided into block groups to improve data locality and reduce fragmentation. Each group has its own descriptor.
Block group descriptor: Contains information about a specific group: address of the inode table, block bitmap, inode bitmap.
Block bitmap: A map of block usage within the group.
Inode bitmap: A map of inode usage within the group.
Inode table: An array of inode structures.
Inode: A structure describing a file or directory. Stores metadata: file type, permissions, owner, group, size, timestamps, data block pointers.
Data blocks: Contain the actual content of files. Block size is configured at filesystem creation (usually 4KB).
Directory: A file containing a list of entries. Each entry links a filename or directory name to its inode number.

Pointers to data blocks in inode:

Direct pointers: Point directly to data blocks. Usually several (e.g., 12), allowing quick access to small files.
Indirect pointers: Point to blocks containing pointers to data blocks. Single, double, triple indirect. Used for large files.
Extents: Ext4 introduced extents to improve performance and handle large files. An extent is a range of contiguous blocks, reducing the number of pointers in the inode.

Journaling: Ext4 has a journal that records changes before applying them to the main file system. This allows quick recovery after a crash.

Features of Ext4: