Restoring the OS bootloader

In the event of BareMetal server issues with the GRUB bootloader for Linux distributions running in legacy mode, you can use a Rescue CD boot image to restore GRUB. Proceed in the KVM console.

To restore the bootloader:

1. Start the server from the Rescue CD image.

   In the main SystemRescue menu, select Boot SystemRescue using default options, press ENTER and wait for SystemRescue to load.

2. View information about the server storage devices (disks and partitions created on them):

   fdisk -l
   

   Result:

   Disk /dev/sda: 838.36 GiB, 900185481216 bytes, 1758174768 sectors
   Disk model: SAMSUNG MZ7GE900
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/0 size (minimum/optimal): 512 bytes / 512 bytes
   Disklabel type: gpt
   Disk identifier: D79F5407-DEEC-4A1D-983F-3AC5********
   
   Device      Start      End         Sectors      Size  Type
   /dev/sda1   2048       614399      612352       299M  BIOS boot
   /dev/sda2   614400     21585919    20971520     10G   Linux filesystem
   /dev/sda3   21585920   34168831    12582912     6G    Linux filesystem
   /dev/sda4   34168832   1732861951  1698693120   810G  Linux filesystem
   
   
   Disk /dev/sdb: 838.36 GiB, 900185481216 bytes, 1758174768 sectors
   Disk model: SAMSUNG MZ7GE900
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/0 size (minimum/optimal): 512 bytes / 512 bytes
   Disklabel type: gpt
   Disk identifier: CDZACB4C-1618-4BAF-A6BB-D2B9********
   
   Device      Start      End         Sectors      Size  Type
   /dev/sdb1   2048       614399      612352       299M  BIOS boot
   /dev/sdb2   614400     21585919    20971520     10G   Linux filesystem
   /dev/sdb3   21585920   34168831    12582912     6G    Linux filesystem
   /dev/sdb4   34168832   1732861951  1698693120   810G  Linux filesystem
   
   Disk /dev/md127: 6 GiB, 6438256640 bytes, 12574720 sectors
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/0 size (minimum/optimal): 512 bytes / 512 bytes
   
   Disk /dev/md126: 9.99 GiB, 10729029632 bytes, 20955136 sectors = 512 bytes
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/0 size (minimum/optimal): 512 bytes / 512 bytes
   
   Disk /dev/md125: 809.88 GiB, 869596659712 bytes, 1698430976 sectors
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/0 size (minimum/optimal): 512 bytes / 512 bytes
   
   Disk /dev/loop: 824.97 MiB, 865046528 bytes, 1689544 sectors
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/O size (minimum/optimal): 512 bytes / 512 bytes
   

   In the above example, the fdisk utility outputs information about the physical disks (/dev/sda and /dev/sdb), their partitions, and the RAID created from them containing three partitions (/dev/md127, /dev/md126, and /dev/md125).

   The example server is partitioned as follows:

   • /dev/md126: /boot partition.
   • /dev/md127: SWAP partition.
   • /dev/md125: / partition with the root file system.

   You need to mount the /boot and / partitions in SystemRescue.

3. Mount the partitions with the root file system and boot files for the BareMetal server OS in SystemRescue:

   mount /dev/md125 /mnt/
   mount /dev/md126 /mnt/boot
   

   Note

   If there is no RAID in the OS, you need to mount the physical disk partitions (/dev/sda1, /dev/sda2, etc.).

   If the OS was installed on the disk using LVM, you need to mount the /dev/mapper partition.

   If using the UEFI bootloader, mount the relevant EFI partition as well, e.g., mount /dev/sda3 /mnt/boot/efi, where /dev/sda3 is the EFI partition.

4. Mount the /sys, /proc, and /dev directories:

   mount --bind /sys /mnt/sys
   mount --bind /proc /mnt/proc
   mount --bind /dev /mnt/dev
   

5. Create a new SystemRescue OS shell with the environment in the previously mounted BareMetal server OS file system:

   chroot /mnt /bin/bash
   

6. If GRUB is not installed on the disks, install it. GRUB must be installed on all the disks you are going to boot the server OS from. If GRUB is already installed, proceed to the next step.

   For the example server, you need to install GRUB on the /dev/sda and /dev/sdb disks.

   grub-install /dev/sda
   grub-install /dev/sdb
   

   Result:

   Installing for i386-pc platform.
   Installation finished. No error reported.
   

7. Generate a GRUB configuration file:

   update-grub
   

   Result:

   Sourcing file '/etc/default/grub'
   Sourcing file '/etc/default/grub.d/init-select.cfg'
   Generating grub configuration file ...
   Found linux image: /boot/uml inuz-5.15.0-130-generic
   Found initrd image: /boot/initrd.img-5.15.0-130-generic
   Found linux image: /boot/vmlinuz-5.15.0-113-generic
   Found initrd image: /boot/initrd.img-5.15.0-113-generic
   Found linux image: /boot/vmlinuz-5.15.0-25-generic
   Found initrd image: /boot/initrd.img-5.15.0-25-generic
   Warning: os-prober will not be executed to detect other bootable partitions.
   Systems on them will not be added to the GRUB boot configuration.
   Check GRUB_DISABLE_OS_PROBER documentation entry.
   done
   

8. Exit the chroot environment you created:

   exit
   

9. Disable the virtual CD drive in the KVM console:

   1. In the KVM console window, in the top menu, select Media → Virtual Media Wizard... or click the CD icon.
   2. In the window that opens, click Disconnect under CD/DVD Media1.
   3. Click Close.

10. Restart the server:

    reboot
    

See alsoSee also

• Using the Rescue CD recovery and diagnostics disk
• Resetting the root user password on a server
• Adding a new user SSH key
• Replacing a disk in a RAID array