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.