Suppose, not hypothetically, that you have some Ubuntu servers that have local data filesystems (with stuff you want to keep) on separate disks from your system disks. These servers might be running Ubuntu 22.04 LTS or 24.04 LTS, and you'd like to update them to 26.04 LTS. Since you now have a kexec based network reinstall system , reinstalling them in place is attractive, but you need to figure out how to not destroy your data filesystems during the reinstall.

The 26.04 server installer offers some attractive options for this. It will recognize your existing software RAID arrays so you don't have to touch them, and then you can tell the installer to mount the extra RAID array somewhere as an ext4 filesystem and not format it. Well, in theory you can. In practice, the installer seems to always reformat anything you've told it to mount somewhere , even if you tried to tell it not to do so. I don't know if this is a bug or if it's me holding things wrong, but either way I don't think it's safe to let the installer touch anything you want to stay intact.

(The installer can definitely leave disks totally alone, because otherwise it would destroy its own USB memory stick when it's running from one. But you normally don't touch the USB memory stick at all in the installer.)

If you really care about the data, for example if it's years of historical metrics data and you're reinstalling the server in place, I think you (we) want to physically remove the data disks before hand. But if you're going to remove the data disks and you have the extra hardware (which we do), you probably want to install on new hardware and then move the data disks over. Certainly I'd have to test an in-place reinstall first to make sure the disks and the software RAID array would stay untouched, and I'm not sure I'd fully trust it even then, because who knows what the installer might decide to do under some situation I don't realize.

Even if everything works out, you'll probably want to correct the software RAID array's name by adding it to /etc/mdadm/mdadm.conf so it doesn't get called 'md127' or whatever. And you'll want to preserve things like the old system's /etc/fstab so you know what UUID to use to mount the filesystem on the data disks (or filesystems, if there's more than one).

(One reason I'm considering this crazy idea in the first place is my virtualization host for testing stuff is running Ubuntu 22.04 and needs an upgrade. I know how to move its setup around , but it would be so much easier if I could just network reinstall it, mount /virt again to get the VM images, and put a few configuration and data files back into place. I could do the entire reinstall from my office and much faster than any other way.)