# preparation
ln -s $PWD/repo-reflink.repo websolv/deptool/Tumbleweed/repos/repo-reflink.repo
websolv/Deptool.py -C Tumbleweed ref
./getit small-kvm.t
rm -f testimg.raw && sudo ./makeimg testimg.raw data/small-kvm
./run_qemu testimg.raw
# General idea
x86_64 UEFI system. The partition table is GPT that adheres to
discoverable partitions for the EFI and "root" volume. The "root"
volume however contains a BTRFS with further sub-volumes. Ie the
root volume itself, /usr and /var.
The naming is in a way that a systemd generator can produce mount
units automatically.
# btrfs subvol list /
ID 256 gen 11 top level 5 path @auto
ID 257 gen 20 top level 256 path @auto/root-x86_64:opensuse-tumbleweed
ID 258 gen 14 top level 256 path @auto/usr-x86_64:opensuse-tumbleweed@10
ID 259 gen 14 top level 256 path @auto/sysimage-x86_64:opensuse-tumbleweed@10
ID 260 gen 21 top level 256 path @auto/var-opensuse-tumbleweed
The boot loader config tells the initrd which OS to mount:
# cat /efi/loader/entries/opensuse-tumbleweed-5.18.15-1-kvmsmall-10.conf
title openSUSE Tumbleweed@10
version 5.18.15-1-kvmsmall
sort-key opensuse-tumbleweed
options root=UUID=131dfa26-c7e6-4a7c-a8a0-2d43cf1fbd9f rootflags=rw,subvol=@auto/root-x86_64:opensuse-tumbleweed mount.usr=UUID=131dfa26-c7e6-4a7c-a8a0-2d43cf1fbd9f mount.usrflags=ro,subvol=@auto/usr-x86_64:opensuse-tumbleweed@10 console=tty0 console=ttyS0,115200 quiet
linux /opensuse-tumbleweed/5.18.15-1-kvmsmall/linux
initrd /opensuse-tumbleweed/5.18.15-1-kvmsmall/initrd
OS updates will create a new revision of the usr subvolume, eg
@auto/usr-x86_64:opensuse-tumbleweed@11, then call kernel-install
which would create a new boot loader config for that OS revision.