A junior administrator cloned a workstation's ext4 root partition to an older production server that still runs a 4.4-series kernel. On first boot the system drops to emergency mode and the console shows:
EXT4-fs (sda1): couldn't mount because of unsupported optional features (0x400)
The goal is to restore normal boot without re-creating the filesystem or losing data. Which action is most likely to achieve this?
Re-create the filesystem with mkfs.ext4 -F /dev/sda1 and restore the data from backup.
Boot from rescue media, run tune2fs -O ^metadata_csum /dev/sda1 and then e2fsck -f /dev/sda1.
Run fsck.ext4 -y /dev/sda1 from the emergency shell and reboot.
Add the mount option nobarrier for /dev/sda1 in /etc/fstab and reboot.
The hexadecimal flag 0x400 corresponds to the metadata_csum feature that was enabled when the filesystem was created on the newer workstation. Kernels and e2fsprogs versions older than 4.5/1.43 cannot mount an ext4 volume with this feature enabled. Booting from rescue media and using tune2fs -O ^metadata_csum /dev/sda1 clears the feature bit in the superblock; an immediate e2fsck rewrites the metadata so that the filesystem is consistent. Because the feature is removed rather than the filesystem being re-formatted, the data remain intact and the next boot proceeds normally.
Re-formatting with mkfs.ext4 would work but destroys all data, so it violates the requirement to avoid data loss.
Adding nobarrier in /etc/fstab only changes write-barrier behavior and does not affect feature compatibility bits.
Running fsck alone cannot disable an unsupported feature bit, so the mount would still fail.
Therefore, disabling metadata_csum with tune2fs followed by e2fsck is the correct corrective action.
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