Because grub2 understands mdraid metadata, it's easy to boot a PC BIOS machine
off of mdraid. There are pretty good documents on the web to follow.

The setup is basically like this:

* ensure the /boot partition is marked active on each drive
* create an mdraid 1 or near-layout mdraid 10 on the /boot partitions
* create the /boot filesystem directly on the restulting md device
* install the grub stage1 into the MBR of each drive

Booting a UEFI system from mdraid is similar, but differs because UEFI needs to
load the stage1 bootloader from a fat filesystem. UEFI doesn't understand
mdraid metadata, so the system partition needs to look like a normal filesystem
when the firmware tries to boot from it.

mdraid metadata version 1.0 is located at the end of the device, where it is
hidden from UEFI. The system partition looks normal to UEFI. This lets an mdraid
1 or near-layout mdraid 10 be a UEFI system partion!

Here's how to do it:

* create identical GPT partition tables on each drive
** for simplicity's sake, create 3 partitions on each drive
** system
** boot
** LVM
* ensure the boot and system flags are set on the system partitions
* create an mdraid with version 1.0 metadata out of the system partitions. ex:
** mdadm -C /dev/md0 --level=10 -n=2 --metadata=1.0 /dev/sda1 /dev/sdb1
* format the system partition. ex:
** mkfs.fat /dev/md0
* create mdraids for /boot and LVM. ex:
** mdadm -C /dev/md1 --level=10 --n=2 /dev/sda2 /dev/sdb2
** mdadm -C /dev/md2 --level=10 --n=2 /dev/sda3 /dev/sdb3
** mkfs.ext4 /dev/md1
** pvcreate /dev/md2; vgcreate vgname /dev/md2
* Create the LVs you want and install Linux

This method works inside the Ubuntu 18.04 installer environment, with a few
minor difficulities becuase it doesn't totally do the grub install correctly.

To install Ubuntu 18.04 onto UEFI/mdraid:

* Add "advanced" to the kernel command line when booting the installer
** This will auto-load the mdraid tools
* Choose manual partitioning
* Use the menu system to create the partitions on your disks
** Set the EFI system partitions as EFI system partitions for now
** Set the other partitions as physical volume for RAID
* Use the menu system to set up mdraid
** At the first raid setup screen, press control-alt-F2 to get a terminal
** manually create the md device with metadata 1.0
** It will warn you that there's a partition there already. That's OK.
** presss control-alt-F1 to get back to the installer
** use the menu system to create the md devices for /boot and LVM
** exit the RAID setup menu
* At the main menu, configure the UEFI raid
** set the UEFI RAID to be used as EFI System Partition
** set the system partition of each drive to "do not use this partition"
*** they will then appear as "fat32"
* Use the menu system to set up LVM as you like it
* Complete the install
** It will fail at grub-install dummy. Proceed without installing a bootloader.
* First Boot
** Use the UEFI menu to boot from a file
** choose EFI/ubuntu/grubx64.efi or EFI/ubuntu/shimx64.efi
** grub will load and stop at a grub prompt
*** it can tab-complete file names for you
** type linux and initrd commands, then boot to boot. ex:
*** linux (md/1)/vmlinuz-tab-complete-this root=your-vg-name/your-root-lv-name
*** initrd (md/1)/initrd-tab-complete-this
*** boot
* Fix up bootloader
** Log in
** edit /etc/default/grub. add bootdegraded=true to GRUB_CMDLINE_LINUX_DEFAULT
** update-grub
* Reboot to ensure everything goes normally
* optionally pull out a drive to ensure the system fails and rebuild correctly

Happy Hacking!

-Tinker Fairy