Rebooting from a faulty kernel

Rescue mode allows you to boot your Instance into a temporary operating system, running from the Instance’s RAM. Rescue mode is often used to diagnose and resolve issues such as faulty kernel configurations, networking issues, firewall configurations…etc.

1. Switch the Instance’s boot-type to rescue and reboot your Instance into rescue mode using the CLI-Tools:

   scw instance server update {Instance_ID} boot-type=rescue
   scw instance server reboot {Instance_ID}Copy

   Note:

   Replace {Instance_ID} with the unique ID of your Instance, e.g. 0500ebd2-d70d-49af-a969-3ac09b6f7fff.

2. Once the Instance is rebooted, log into your Instance using SSH and setup the environment to be able to chroot into it:

   cat /proc/partitions
   major minor #blocks name
   8 0 9765625 sda
   8 1 9634536 sda1
   8 14 3072 sda14
   8 15 126976 sda15Copy

   Then mount the partitions:

   mount /dev/sda1 /mnt
   mount /dev/sda15 /mnt/boot/efi
   mount -o bind /sys /mnt/sys
   mount -o bind /proc /mnt/proc
   mount -o bind /dev /mnt/devCopy

3. Once mounted, use the chroot command to get into your Instances’ root filesystem. You can then change the GRUB_DEFAULT value to boot using the previous kernel:

   chroot /mnt
   nano /etc/default/grubCopy

   Tip:

   In the example above we use nano as text editor. Feel free to use your favorite text editor to edit the file.

   Change the value of GRUB_DEFAULT to `“1 > 2”:

   # head /etc/default/grub
   # If you change this file, run 'update-grub' afterwards to update
   # /boot/grub/grub.cfg.
   # For full documentation of the options in this file, see:
   #   info -f grub -n 'Simple configuration'
   GRUB_DEFAULT="1>2"
   GRUB_TIMEOUT=5
   GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
   GRUB_CMDLINE_LINUX_DEFAULT=""
   GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200 earlyprintk=ttyS0,115200 consoleblank=0"Copy

   Save the file and exit the text editor.
4. Update the grub boatloader` to apply the kernel’s update

   # update-grub
   Generating grub configuration file ...
   Found linux image: /boot/vmlinuz-6.0.0-0.deb11.2-cloud-amd64
   Found initrd image: /boot/initrd.img-6.0.0-0.deb11.2-cloud-amd64
   Found linux image: /boot/vmlinuz-5.19.0-0.deb11.2-cloud-amd64
   Found initrd image: /boot/initrd.img-5.19.0-0.deb11.2-cloud-amd64
   Found linux image: /boot/vmlinuz-5.10.0-19-cloud-amd64
   Found initrd image: /boot/initrd.img-5.10.0-19-cloud-amd64
   doneCopy

5. Switch back the Instance’s boot_type to local and reboot the Instance:

   scw instance server update {Instance_ID} boot-type=local
   scw instance server reboot {Instance_ID}Copy

   Your Instance reboots on the previously installed kernel.

Examples of failed boots

• In the following example, the Instance may only boot with the root filesystem in read-only mode:

  [  OK  ] Finished Remove Stale Onli…ext4 Metadata Check Snapshots.
  [    4.219158] cloud-init[542]: Traceback (most recent call last):
  [    4.220328] cloud-init[542]:   File "/usr/bin/cloud-init", line 33, in <module>
  [    4.221518] cloud-init[542]:     sys.exit(load_entry_point('cloud-init==20.4.1', 'console_scripts', 'cloud-init')())
  [    4.223064] cloud-init[542]:   File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 891, in main
  [    4.224435] cloud-init[542]:     retval = util.log_time(
  [    4.225251] cloud-init[542]:   File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 2348, in log_time
  [    4.226838] cloud-init[542]:     ret = func(*args, **kwargs)
  [    4.227743] cloud-init[542]:   File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 600, in status_wrapper
  [    4.229478] cloud-init[542]:     util.ensure_dirs((data_d, link_d,))
  [    4.230501] cloud-init[542]:   File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 1534, in ensure_dirs
  [    4.232014] cloud-init[542]:     ensure_dir(d, mode)
  [    4.232540] cloud-init[542]:   File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 1591, in ensure_dir
  [    4.233980] cloud-init[542]:     chmod(path, mode)
  [    4.234756] cloud-init[542]:   File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 1830, in chmod
  [    4.235672] cloud-init[542]:     os.chmod(path, real_mode)
  [    4.236227] cloud-init[542]: OSError: [Errno 30] Read-only file system: '/var/lib/cloud/data'
  [FAILED] Failed to start Apply the …ngs specified in cloud-config.Copy