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4.7. Using rescue mode for instances

Compute has a method to reboot a virtual machine in rescue mode. Rescue mode provides a mechanism for access when the virtual machine image renders the instance inaccessible. A rescue virtual machine allows a user to fix their virtual machine by accessing the instance with a new root password. This feature is useful if the file system of an instance is corrupted. By default, rescue mode starts an instance from the initial image attaching the current boot disk as a secondary one.

4.7.1. Preparing an image for a rescue mode instance

Due to the fact that both the boot disk and the disk for rescue mode have same UUID, sometimes the virtual machine can be booted from the boot disk instead of the disk for rescue mode.

To avoid this issue, you should create a new image as rescue image based on the procedure in 「Creating an image」:


The rescue image is stored in glance and configured in the nova.conf as a default, or you can select when you do the rescue. Rescuing an image that uses ext4 file system

When the base image uses ext4 file system, you can create a rescue image from it by using the following procedure:

  1. Change the UUID to a random value by using the tune2fs command:

    # tune2fs -U random /dev/<device_node>

    Replace <device_node> with the root device node, for example, sda or vda.

  2. Verify the details of the file system, including the new UUID:

    # tune2fs -l
  3. Update the /etc/fstab to use the new UUID. You might need to repeat this for any additional partitions that you have that are mounted in the fstab by UUID.
  4. Update the /boot/grub2/grub.conf file and update the UUID parameter with the new UUID of the root disk.
  5. Shut down and use this image as your rescue image. This causes the rescue image to have a new random UUID that does not conflict with the instance that you are rescuing.

The XFS file system cannot change the UUID of the root device on the running virtual machine. Reboot the virtual machine until the virtual machine is launched from the disk for rescue mode.