One answer to how your Ubuntu 26.04 server boots when its root filesystem is on a software RAID array is that it just does and you don't need to think about it. Unfortunately this wasn't the case in the pre-beta version . That's been fixed since ( contrary to what I thought until now ), but that raises the question of what changed between 24.04 LTS and 26.04 LTS to cause and then fix this.

At some point between 24.04 and 26.04, Canonical switched from booting servers using initramfs-tools (and an initial ramdisk built with them) to booting them with Dracut . Initial ramdisks built with initramfs-tools will automatically assemble all software RAID arrays they see, or possibly anything they see in their embedded /etc/mdadm/mdadm.conf file (it's not clear to me right now). Initramfses using Dracut will normally only assemble software RAID arrays that are explicitly specified with rd.md.uuid= arguments , regardless of what's in their /etc/mdadm.conf. The 26.04 pre-beta server installer didn't add any such rd.md.uuid arguments to the kernel command line, causing the obvious problems.

(At the time our workaround was to create an /etc/default/grub.d file that explicitly added the right rd.md.uuid argument to the kernel arguments. This had the UUID of the root filesystem's software RAID array hard-coded, which is the style of these files.)

Now that 26.04 has been released, if you install a 26.04 server system with a mirrored root filesystem and inspect the resulting server's /proc/cmdline, you'll still find a striking lack of rd.md.uuid parameters (or rd.auto, cf dracut.cmdline(7) ). However, if you peek into the initial ramdisk itself, what you'll find is an /etc/cmdline.d/20-mdraid.conf that contains an rd.md.uuid setting. For example:

rd.md.uuid=838c6235:90e967bb:8e64be9f:3c38b0f8

(/etc/cmdline.d in the initramfs also contains a '20-root-dev.conf', at least on a test system. This is partially redundant with the kernel command line itself.)

As covered in the Dracut manual page , when the initramfs boots, Dracut (in the initramfs) will look at /etc/cmdline.d/ and effectively take them as extra Dracut-directed parameters. So apparently the installer has been updated to write this file, fixing the problem without making the fix visible in /proc/cmdline.

(The initramfs also has an /etc/mdadm.conf, which probably opens you up to the traditional issues with it , although some of them may not be an issue any more .)

This 20-mdraid.conf file only exists in the initramfs, not on the booted system, but the overall Ubuntu kernel update system arranges for each new kernel and initramfs to get this 20-mdraid.conf, so your newly installed kernel boots just as your old one did. (I don't know which bit of the overall setup is responsible for this; it might be Dracut itself.)