Inodes

An inode stores file metadata + pointers to data blocks. NOT the file name (that's in the directory).

ext4 inode (256 bytes):

struct inode {
    uint16_t mode;          // file/dir/symlink + perms (rwx for u/g/o)
    uint16_t uid;
    uint32_t size_lo;
    uint32_t atime;          // access time
    uint32_t ctime;          // status change time
    uint32_t mtime;          // modification time
    uint32_t dtime;          // deletion time (0 if alive)
    uint16_t gid;
    uint16_t link_count;
    uint32_t blocks;         // number of blocks used (in 512B units)
    uint32_t flags;
    uint32_t block[15];      // direct + indirect pointers
    uint32_t version;
    uint32_t file_acl;
    uint32_t size_hi;
    ...
};

Inodes occupy a fixed-size table allocated at FS creation. Number of inodes is fixed unless you grow.

stat() returns mostly inode contents.

Direct vs indirect pointers (classic ext2/3):

• 12 direct: each points to a data block (12 × 4 KiB = 48 KiB).
• 1 indirect: points to a block of pointers → 1024 × 4 KiB = 4 MiB.
• 1 double-indirect: 1024 × 1024 × 4 KiB = 4 GiB.
• 1 triple-indirect: 4 TiB.

Total max: ~4 TiB per file (ext2/3). ext4 added extents.

Extent: (start_block, length, file_offset). One extent describes a contiguous run.

• ext4 inode has 4 extents inline; more in an extent tree.
• Better for large contiguous files.

Hardlinks:

• link_count ≥ 1.
• Multiple directory entries pointing to same inode.
• Delete decrements; reaches 0 → free inode + blocks.

Symlinks:

• A separate inode with mode = SYMLINK.
• Data: target path string.
• Following requires re-resolving path.

ls -i shows inode numbers. Inodes are scarce: a FS with 10M inodes can hold only 10M files regardless of disk space.