Move /boot filesystem to partition 3

[bgilbert]

Describe the enhancement

The current x86_64 partition layout looks like this:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048          788479   384.0 MiB   8300  boot
   2          788480         1048575   127.0 MiB   EF00  EFI-SYSTEM
   3         1048576         1050623   1024.0 KiB  EF02  BIOS-BOOT
   4         1050624        16777182   7.5 GiB     8300  root

with the BIOS-BOOT partition omitted from the 4Kn image. Change it to look like this:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048            4095   1024.0 KiB  EF02  BIOS-BOOT
   2            4096          264191   127.0 MiB   EF00  EFI-SYSTEM
   3          264192         1050623   384.0 MiB   8300  boot
   4         1050624        16777182   7.5 GiB     8300  root

with the BIOS-BOOT partition included in the 4Kn image. Likewise move /boot to partition 3 on non-x86 arches.

System details

All platforms

Additional information

We're planning to implement #581 via Dracut glue in the initramfs, activated by an Ignition config. Users will be able to write some FCC sugar to specify a mirrored disk layout. This will desugar to an Ignition config which creates the appropriate partitions on each of the mirrored disks, and the Dracut glue will then handle copying the necessary data. The detailed proposal is in coreos/enhancements#3 and a WIP PR is in coreos/fedora-coreos-config#718.

To avoid rerunning grub-install from the initramfs, the recreated BIOS-BOOT partition needs to be at the same offset as the old one. Currently that offset is 512 MiB, since BIOS-BOOT comes after /boot and the ESP. That constant is awkward to hardcode into FCCT. Thus, we're planning to swap BIOS-BOOT and /boot so BIOS-BOOT is at the beginning of the disk.

Everything that wants to mount /boot should be doing it by filesystem label (or partition label), not by index, but we wanted to raise this issue for visibility.

[jamescassell]

Everything that wants to mount /boot should be doing it by filesystem label (or partition label), not by index, but we wanted to raise this issue for visibility.

I admire your optimism!

Are there plans to transition existing systems? (would bootupd handle something like this?)

[bgilbert]

Are there plans to transition existing systems?

Nope. The change should only affect Ignition, which isn't relevant after the first boot.

[dustymabe]

The fix for this went into stable stream release 32.20201104.3.0.