Two System Slots

Two system slots

What this page will help you do

Understand the one rule that decides what happens to your old system, what a NebulaOS update overwrites, and how to get back to Creality. Most questions about "will this replace my firmware" have the same answer, and it is this page.

The one rule

The Ender-3 V3 KE has two system slots on its internal storage. One is running; the other is not. And:

Installing a system image always writes the slot that is not running.

That is true of NebulaOS's installer, and it is true of Creality's own updater - it is how the printer was designed, not something NebulaOS added. Everything below follows from it.

The printer always starts one slot or the other. A marker on the storage says which, and both systems can change that marker.

What happens, in order

Before you start. One slot holds the system you bought the printer with - Creality's, or OpenKE if you installed that. The other is empty or holds an old copy.

          slot 1                      slot 2
   ┌────────────────────┐      ┌────────────────────┐
   │ Creality  ◀running │      │ (empty / old)      │
   └────────────────────┘      └────────────────────┘

1. You install NebulaOS. You run Creality's own updater on the NebulaOS .img, from the stock touchscreen or over SSH. It writes the slot that is not running - slot 2 - and the printer starts it.

   ┌────────────────────┐      ┌────────────────────┐
   │ Creality           │      │ NebulaOS  ◀running │
   └────────────────────┘      └────────────────────┘

Your old system is untouched. Settings → Reset → Swap boot slot starts it again, as you left it.

2. You update NebulaOS with a new system image. Same rule: it writes the slot that is not running - which is now the one holding your old system.

   ┌────────────────────┐      ┌────────────────────┐
   │ NebulaOS (new) ◀run│      │ NebulaOS (previous)│
   └────────────────────┘      └────────────────────┘

This is the step that removes Creality's system. After it, Swap boot slot moves between two NebulaOS versions. That is also what gives you the automatic way back from a bad update - the previous NebulaOS is right there - but it is a real trade and it is worth knowing before you take the first update rather than after.

You can see it coming. Settings → Updates → System image lists both slots with what each one holds and which is running, before you confirm.

If an install stops, nothing changed on the system you are running. Just install again. An install can stop by itself - for example when a print starts while it is writing. The other slot was already being written by then, so it is cleared: until an install completes, you cannot switch back to the system that was in the other slot.

If you want to keep Creality's system

Nothing stops you taking NebulaOS updates and losing stock; most people will. If you would rather keep it:

Run the update while Creality's system is the one running. The rule does the rest: the slot that is not running is then the NebulaOS one, so that is what gets overwritten and stock stays put. In practice this means swapping to stock, installing the NebulaOS .img with Creality's own updater there, and swapping back.

That needs a way to run the updater on the stock side - its touchscreen, or SSH if your stock side is OpenKE. Creality's stock firmware does not normally give you SSH, and NebulaOS does not change its password.

Most things update without touching a slot at all

A system image is only needed for OS-level parts: the kernel, drivers, Wi-Fi, the touchscreen stack, SSH.

Everything else updates on its own, through Moonraker's update manager, with no image and no slot written:

Component	How it updates
Klipper	Moonraker's update manager
Moonraker	Moonraker's update manager
Mainsail	Moonraker's update manager (type: web, channel stable, from mainsail-crew/mainsail)
NebulaOS Klipper extensions	Moonraker's update manager
NebulaScreen	its own releases

So a NebulaOS image that ships Mainsail 2.18.2 is not stuck on 2.18.2 - Mainsail updates itself from its own repository. See Updating NebulaOS.

Could a Creality update overwrite NebulaOS?

Yes, by the same rule, if you ran one from the stock side: Creality's updater would write the slot that is not running, which would be the NebulaOS one.

Getting back to Creality

What you want	How
Your old system is still in the other slot	Settings → Reset → Swap boot slot
It is gone, and you want Creality back	Install Creality's official .img from Settings → Updates → System image. NebulaOS's installer reads the kernel inside an image and recognises Creality's; it goes into the slot you are not running
Neither works	Recovery - a .ingenic file over micro-USB rewrites the printer completely, and erases everything

And the other direction - you are on stock and want NebulaOS back, without reinstalling it:

Over SSH on the stock side, Creality's own built-in switcher flips which slot starts next:

sh -c '. /etc/ota_bin/ota_local_method.sh; local_set_next_boot_device'

It toggles rather than taking a target: it flips to whichever system you are not on. Then reboot. Nothing is reinstalled, so nothing is recalibrated - your configuration and calibration live on a shared data area that neither slot overwrites.

From the NebulaOS side the matching command is sh -c '. /etc/ota_marker.sh; write_ota_marker "ota:kernel"' for slot 1, or ota:kernel2 for slot 2 - but from NebulaOS you would normally just use Swap boot slot on the screen.

What survives all of this

Your printer.cfg, macros, calibration, print files, Wi-Fi networks and screen settings live on a shared data area that is not part of either slot. Swapping slots does not touch them, and installing an image does not touch them. That is why coming back to NebulaOS does not mean calibrating again.

A NebulaOS update does replace its own copies of Klipper, Moonraker, Mainsail and the extensions when the image expects newer ones, and takes a backup first, under /usr/data/nebulaos/system/migration-backups/. Your configuration is not part of that - it is not replaced, so it does not need restoring.

The one thing that does erase settings is a clean NebulaOS reset, and it says so before it runs.

Next step

New to NebulaOS? Installing NebulaOS.

Related

• Installing NebulaOS
• Updating NebulaOS
• Going back to your old firmware
• Recovery - when the printer will not start