Usr-merge to become required in HH

Way back in Disco, we have made a switch for newly installed runtime systems to have usr-merged by default. All our buildd chroots remained split-usr.

I think it is time to migrate all systems to usr-merge too. I intented to achieve this by seeding “usrmerge” package into “Required” seed, such that “ubuntu-minimal” wants it.

On upgrades, user systems will be configured to merged usr, if possible.

This will allow us to switch buildd to merged-usr by default in 22.10 with all systems requiring merged usr. Meaning, in 22.10 usrmerge package will become required.

Our initrds have been mounting both / and /usr since before Disco. The only known case of unable to automatically convert systems to merged-usr, is where “/usr” is an network nfs mount, which is setup from “/” and initrd doesn’t know how to do that. In such cases, users would need to errange to expose this information in the initrd, and make initrd mount both “/” and “/usr”. Such systems will continue to work through to 22.04 LTS unmodified, and will need manual migration to 22.10.

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