Update README.md

README.md

@@ -35,16 +35,39 @@ MBR/BIOS:
 
 ```
 /dev/disk:
-  /dev/disk1 - ext2  - /boot
-  /dev/disk2 - swap
-  /dev/disk3 - btrfs - /
+  /dev/disk1  ext4                   /boot
+  /dev/disk2  swap                   [SWAP]
+  /dev/disk3  btrfs@home             /home
+  /dev/disk3  btrfs@root-2021-04-01  /
+  /dev/disk3  btrfs@root-2021-04-20  /
+  /dev/disk3  btrfs@conf-2021-04-01  /config
+  /dev/disk3  btrfs@conf-2021-04-20  /config
 ```
 
-Subvolumes:
+- Every installed release has it's own subvolume for rootfs and config in case a configuration changes with an update
+- The user can choose which one to boot into
 
-  - 2021-04-01: Released version
-  - current: Currently active version in case next boot fails
-  - last: Last known good version
+### Update flow
+
+State before the update:
+
+- boot entry `last` is set to version 1 (last known working version)
+- boot entry `current` is also set to version 1
+- on every boot the default boot entry is set to `last` automatically in early boot
+
+Update process:
+
+- a new version is downloaded as a btrfs snapshot and put into place as version 2
+- `current` boot entry is modified to boot the new version
+- `current` is set as default boot entry
+
+- device reboots into new version 2 via `current` boot entry
+- early boot sets `last` as default boot entry
+- after successful boot:
+  - the user marks the system as functional which sets the `last` boot entry to the current version
+- if something goes wrong:
+  - the system reboots, now with the `last` entry being the default booting into the last known working system
+  - the user gets displayed an error because `last` and `current` is not the pointing to the same version and `last` is booted indicating an error
 
 ---