Restored Linux virtual machine does not boot up

Challenge

Restored Linux VM does not boot up.

RHEL/CentOS system drops into dracut emergency shell and during boot process displays the following error messages:
  dracut-initqueue timeout - starting timeout scripts
  /dev/vgroot/root does not exist

By default, Debian does not use dracut to generate initramfs image, so it may display a different error message similar to the following:
  Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block()

Cause

Kernel initramfs image does not have necessary block device kernel modules (drivers) in it.

The problem usually occurs when restoring virtual machines backed up with VAL to completely new hardware, for example when a virtual machine was backed up in VMware and then restored to a Hyper-V environment.

Solution

Regenerate the initramfs image.

RHEL/CentOS instructions:
1. During the boot process, select rescue kernel option in grub instead of usual kernel, for example:
 CentOS Linux (0-rescue-1edb64) 7 (Core)
2. Then, if the system starts successfully, run dracut to update existing initramfs image for your usual kernel, for example:
 dracut -f /boot/initramfs-3.10.0-862.2.3.el7.x86_64.img 3.10.0-862.2.3.el7.x86_64
If the system could not be started with rescue kernel or if you do not have rescue kernel option in grub, please follow instructions for Debian.

1. Mount VAL Recovery Media LiveCD or any other LiveCD to problematic VM.
2. Boot into LiveCD and switch to command line/open bash shell.
3. Check if all VM block devices are present in /dev/sd*. If you do not see some of VM block devices you might need to use LiveCD with newer kernel.
4. Mount VM block devices, skip LVM steps if VM does not have LVM:
 mkdir /media/disk
 vgdisplay
 vgchange -a y centos
 vgscan --mknodes
 mount /dev/centos/root /media/disk
 mount /dev/sda1        /media/disk/boot 
 mount -t proc  proc    /media/disk/proc
 mount -t sysfs sys     /media/disk/sys
 mount -o bind  /dev    /media/disk/dev
5. Then chroot to new environment:
 chroot /media/disk /bin/bash
6. Update initramfs using update-initramfs (use dracut for RHEL/CentOS here)
 update-initramfs -u -t -k 3.16.0-6-amd64

More Information

https://access.redhat.com/solutions/2515741
https://access.redhat.com/solutions/24029