> IIRC OpenSolaris (and other derivations) had a writable root but did ZFS snapshots on changes, Not exact, there is SUN zfs auto-snapshot service, but that's

> IIRC OpenSolaris (and other derivations) had a writable root but did ZFS snapshots on changes, Not exact, there is SUN zfs auto-snapshot service, but that's for another purpose, a time-machine alike setup at a certain point in time even integrated with nautilus with a right-click menus to see the $volRoot/.zfs/snapshots/* of a specific file/folder, the package manage do clones so you can create different roots and keep or dispose them independently as long as you want. GNU/Linux scripts to wrapper grub-install and package managers try to mimic that but that's not on feature parity at all. Yes, in OpenSolaris/IllumOS the root was/is not read-only, but if you manage the system with IPS it's nearly that, "only" lacking respect of NixOS/Guix System the declarative configuration since IPS (the package manager) was a classic manual one. NixOS/Guix System lack the zfs integration, using a network of symlinks to mount a read-only root out of /{gnu,nix}/store. Doing something like containers, even if done "the right way" (meaning without wasting resources, something not easy on x86 and AFAIK not done by anyone so far) it's pointless because it's a false "security" model typically led to far less safe systems. Even if done properly like OpenSolaris/IllumOS zones, who do not waste much resources at least for storage thanks to zfs dedup, they only waste ram/cpu, it's not a good practice. A safe system is one designed to be safe in every bit, not one were you create "shells", compartments to "keep something potentially unsafe in a sandbox". All have tried this approach have failed.