So... What are the hidden and reserved inode numbers on ext2/3/4 filesystems?
| Name | Number | Function |
|---|---|---|
| EXT2_BAD_INO | 1 | Allocated over bad blocks by mke2fs/e2fsck -c |
| EXT2_ROOT_INO | 2 | The root directory of the file system |
| EXT4_USR_QUOTA_INO | 3 | The now integrated and hidden user quota file. Previously used for ACL index? |
| EXT4_GRP_QUOTA_INO | 4 | The now integrated and hidden group quota file. Previously used for ACL data? |
| EXT2_BOOT_LOADER_INO | 5 | Apprently unused. Was probably intended to hide stage2 loaders, just like the fs starts padded with zeroes to allow stage1 loaders. |
| EXT2_UNDEL_DIR_INO | 6 | Apparently unused. Was supposedly meant for undeletion functionality that was never implemented. |
| EXT2_RESIZE_INO | 7 | Used to speed up resizing, by reserving room to let the block group descriptor table grow (mke2fs resize=) option. |
| EXT2_JOURNAL_INO | 8 | The ext3 journal (mke2fs -j) |
| EXT2_EXCLUDE_INO | 9 | Unused in vanilla, but used by the Next3 fs for snapshots. |
| EXT4_REPLICA_INO | 10 | Unused in vanilla, but used for ext4 metadata replication out of tree in a Google patch. |