IBM Z DPM Partitions require a /boot partition when using LVM/RAID

[Lee Trager]

On IBM Z DPM Partitions the HMC firmware loads the kernel and initrd from the storage volume. This supports various filesystems but does not support RAID/LVM. To use RAID/LVM you need to keep the kernel and initrd on a separate /boot partition so the HMC can load the kernel and initrd. Then the Linux kernel can mount the RAID/LVM volume.

RAID and LVM fail in different ways. When RAID is used Curtin successfully finishes but the partition goes into a terminated state as it doesn't know how mount an mdraid block device. LVM actually causes Curtin to fail

Running command ['unshare', '--fork', '--pid', '--', 'chroot', '/tmp/tmphkwow1ir/target', 'update-initramfs', '-c', '-k', '5.4.0-66-generic'] with allowed return codes [0] (capture=False)
update-initramfs: Generating /boot/initrd.img-5.4.0-66-generic
Error: Unsupported setup: Directory '/boot' is located on a multi-target device-mapper device
Error: Script could not determine target parameters
Using config file '/etc/zipl.conf'
Run /lib/s390-tools/zipl_helper.device-mapper /boot
run-parts: /etc/initramfs/post-update.d//zz-zipl exited with return code 1
Running command ['udevadm', 'settle'] with allowed return codes [0] (capture=False)
TIMED subp(['udevadm', 'settle']): 0.028
Running command ['umount', '/tmp/tmphkwow1ir/target/sys'] with allowed return codes [0] (capture=False)
Running command ['umount', '/tmp/tmphkwow1ir/target/run'] with allowed return codes [0] (capture=False)
Running command ['umount', '/tmp/tmphkwow1ir/target/proc'] with allowed return codes [0] (capture=False)
Running command ['umount', '/tmp/tmphkwow1ir/target/dev'] with allowed return codes [0] (capture=False)
finish: cmd-install/stage-curthooks/builtin/cmd-curthooks/updating-initramfs-configuration: FAIL: updating initramfs configuration
finish: cmd-install/stage-curthooks/builtin/cmd-curthooks: FAIL: curtin command curthooks

I suspect this is a bit of an edge case as IBM Z volumes are multipath. This provides the redundancy of RAID and the flexibility of LVM.

[Frank Heimes (fheimes)]

I think it's good (not to say 'best') practice to have a separate /boot partition in LVM environments (even if s390x should support booting from /boot as part of a LVM with linear mapping).
A separate /boot outside the LVM is also the partition scheme that is used by subiquity (and d-i) in case of LVM installations.
And especially on server systems it's more robust to have a separate /boot in case of boot problems occure, since troubleshooting and fixing is easier.
See also LP 1680101.

[Jerzy Husakowski (jhusakowski)]

MAAS should offer a better set of supported storage layout configurations for IBM Z.