A/B slots, install, and OTA updates

The A/B scheme

kernel/ota/ota_ab.c maintains two slots (A and B) plus a boot flag:

• GRUB reads the flag and boots the active slot; the menu offers force-slot entries as the manual override.
• An update installs onto the inactive slot while you keep running.
• The next boot flips to the updated slot; update rollback flips back if you reject it.

Installing to a disk

From a running ISO boot:

oc> dskstat                (pick a target disk, e.g. ata0)
oc> install ata0           (partition + copy the running kernel +
                            GRUB boot files + /etc volume)

install copies the multiboot2 module self - literally the kernel ELF that is running right now - onto the disk, so the installed system always matches its installer. abdisk <dev> does the same shaped as an A/B update disk (two slots + flags; destroys data).

abcfg shows the exact grub.cfg files those commands write.

Checking for updates

oc> checkupdate            (reads the update URL from /etc/opencube.conf
                            - http or https - and parses the manifest)
oc> config get update_url  (the configuration key it uses)

The manifest is the website's update.json: version, build, package URL, package SHA256, size.

Applying an update

oc> update download        (fetch the OTA package)
oc> update verify          (SHA256 + signature check)
oc> update install a       (install onto slot A/B - the inactive one)
oc> reboot                 (boot the updated slot)

The OTA package (opencube-<version>-update.tar.gz) carries the new kernel ELF plus a manifest. Verification is cryptographic: hash plus signature - a package that fails either is refused.

Rollback

If a slot misbehaves after a flip:

1. Pick the other slot from the GRUB menu (force entry), or
2. oc> update rollback from inside a running system.

ab_partition_test exercises slot discovery, flags and slot switching on a scratch disk without touching your real boot disk.

Update-related commands

checkupdate, checkupdate_test, update (the chain above), ab_partition_test, abcfg, abdisk, config (update URL keys). The full list with one page each: commands index - "OTA updates and A/B slots" domain.

Internals

For how the flags are stored, how install stages the inactive slot, and the boot-flag handshake with GRUB, see ../internals/ota-ab.md.