this post was submitted on 24 Jul 2024
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Immutable in this context refers to an OS that can't be changed while running. Steam deck does something like that. Basically the all of the OS system files are read only, so that the user or some malware can't Bork the system. The only parts that are writable are the users profile directory and the logs.
You can still receive updates and install apps. It's just that that's handled a bit differently than with a standard OS.
E.g. it could be that the OS provider only issues complete updates, and then you either have to reboot. This is the case with steam os on the steam deck. The System portion of the OS is mounted read only during use.
Another prominent example is Android. Sure system apps can be upgraded individually -- by storing the new version in a restricted part of the 'user' partition -- but otherwise the system files are strictly read only until a new 'image' is 'flashed' to it by the update system or a power user with debugging tools. In the past, a common use of root capabilities was to remount the system partition as read/write and then change files on it directly. It's more complex now.
That's also why system apps can be rolled back to the stock version, and can sometimes be disabled, but can't be directly uninstalled like user apps. Only the updated version on the user partition (if there is one) can be removed.
Aren't apps better compareable to something like flatpack and this is the reason why they are updateable during runtime?
Sure they are, but system apps are still installed in the immutable space initially, which is the important thing, that updates to it can't go there.
I don't know how desktop immutable systems deal with that.
You can update the whole base image. Vanilla OS and SteamOS have an A/B partition that holds the currently-in-use image and can also hold a to-be-used image.
Updating works by adding the to-be-used image, setting a configuration option that tells the system to boot that one, and on the next boot it'll check if the new one is bootable, then either boot it and mark it as working, or boot into the old one and display an error about how out wasn't able to boot into the new one.
There's smart things going on like maybe hard linking files that didn't change between the two images and therefore saving space and copying time.
The result is that you never have a broken system, but you can still frequently update the base image.