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.

Note that there are also some inodes allocated from non-reserved inode numbers for other filesystem features which are not referenced from standard directory hierarchy. These are generally reference from the superblock. They are:

Superblock field	Description
s_lpf_ino	Inode number of lost+found directory.
s_prj_quota_inum	Inode number of quota file tracking project quotas
s_orphan_file_inum	Inode number of file tracking orphan inodes.