# --------------------
# RAID 1 of /boot/efi partition on Debian
# --------------------

# see: https://unix.stackexchange.com/questions/644108/raid-1-of-boot-efi-partition-on-debian

The steps are mainly:

1.) Install Debian without setting up the RAID for the ESP partition.

         During the partitioning, I've already created two identical partitions
         marked as ESP partitions. They were on /dev/sda1 and /dev/sdb1


2.) Copie the contents of /boot/efi somewhere else (/boot/eficopy).

      # cp -a /boot/efi /boot/eficopy


3.) Unmount EFI boot partition

      # umount /boot/efi


4.) Create an array for use as EFI partion.

      NOTE:
      use metadata version 1.0

      Version 1.0 still has the requirement (for this usecase) of placing the
      superblock at the end of the device, but also includes "the modern features of mdadm",
      by using common layout format as 1.1 & 1.2.

      # mdadm --create --verbose /dev/md3 --level=1 --raid-devices=2 --metadata=1.0 /dev/sda1 /dev/sdb1


5.) Create vfat filesystem on created raid array

         # mkfs.vfat /dev/md3


6.) found the UUID of the partition in /dev/disk/by-uuid:

         # blkid /dev/md3
         # ls -al /dev/disk/by-uuid/


7.) Changed the /boot/efi entry in /etc/fstab with the new UUID

      for example:

      UUID=C354-2922  /boot/efi       vfat    umask=0077      0       1


8.) Reload systemd

         # systemctl daemon-reload


9.) Mount new EFI partition as configured in #/etc/fstab'

         # mount /boot/efi


10.) Copy the data from the backup into /boot/efi again

         # cp -a /boot/eficopy/* /boot/efi/


11) mdadm: Scan config file or /proc/mdstat for missing information:

         # mdadm --examine --scan

     add row for new array to file /etc/mdadm/mdadm.conf


12.) Update initramfs:

         # update-initramfs -u -k all