Linux Kernel: Fix Root Filesystem Mount Failure in Hybrid Cloud VM Migration
Quick Fix Summary
TL;DRBoot from rescue media, chroot, and regenerate initramfs with correct drivers.
The kernel cannot find or mount the root filesystem, typically due to missing storage drivers in the initramfs after a cross-cloud VM migration.
Diagnosis & Causes
Recovery Steps
Step 1: Verify Boot Parameters and Environment
From the kernel panic screen or a rescue boot, check the kernel command line and identify the expected root device.
cat /proc/cmdline
lsblk -f
blkidStep 2: Boot into Rescue/Recovery Environment
Use cloud provider's rescue image, installation ISO, or a live CD to gain shell access to the failed system's disks.
# Mount the root filesystem from rescue environment. Example:
mkdir /mnt/rescue
mount /dev/sda1 /mnt/rescue
mount --bind /dev /mnt/rescue/dev
mount --bind /proc /mnt/rescue/proc
mount --bind /sys /mnt/rescue/sysStep 3: Chroot and Identify Missing Drivers
Change root into the failed system and check the kernel modules required for the current storage hardware.
chroot /mnt/rescue /bin/bash
lspci -k | grep -i storage -A3
lsmod | grep -E 'virtio|nvme|xen|scsi'Step 4: Rebuild Initramfs with All Necessary Drivers
Force regeneration of the initramfs for the current kernel, ensuring all storage drivers are included. Adjust for your distribution.
# For RHEL/CentOS/Fedora/AlmaLinux/Rocky:
dracut --force --add-drivers "virtio_scsi virtio_blk virtio_pci nvme xen_blkfront"
# For Ubuntu/Debian:
update-initramfs -u -k allStep 5: Verify and Update Bootloader Configuration
Check that the GRUB configuration points to the correct root device by UUID or persistent name. Update if necessary.
cat /etc/default/grub
grub2-mkconfig -o /boot/grub2/grub.cfg
# For legacy BIOS systems, also run:
grub2-install /dev/sdaStep 6: Test Boot and Implement Prevention
Exit chroot, reboot, and verify normal boot. To prevent recurrence, ensure initramfs tools are configured for hybrid environments.
exit
umount -R /mnt/rescue
reboot
# Post-boot, verify initramfs content:
lsinitrd /boot/initramfs-$(uname -r).img | grep -E 'virtio|nvme|xen'Architect's Pro Tip
"This often happens when migrating a VM from a hypervisor using SCSI emulation (like VMware) to one using virtio (like KVM/AWS) without rebuilding initramfs. Always test-boot in the target environment before cutting over."
Frequently Asked Questions
How can I prevent this during future migrations?
Before migration, pre-install common cloud drivers (e.g., virtio drivers on VMware) and rebuild initramfs. Use a standardized, hardware-agnostic image build process.
What if I don't know which storage driver is needed?
Boot a working, generic instance in the target cloud, run `lspci -k` and `lsmod`, and note the storage controller driver (e.g., 'virtio_pci'). Add this driver to the dracut command.