Special inodes

ext4 reserves some inode for special features, as follows:

inode Number Purpose
0 Doesn't exist; there is no inode 0.
1 List of defective blocks.
2 Root directory.
3 User quota.
4 Group quota.
5 Boot loader.
6 Undelete directory.
7 Reserved group descriptors inode. (“resize inode”)
8 Journal inode.
9 The “exclude” inode, for snapshots(?)
10 Replica inode, used for some non-upstream feature?
11 Traditional first non-reserved inode. Usually this is the lost+found directory. See s_first_ino in the superblock.