Using swap for install

[pmaterer]

I'm using nixos-anywhere to install NixOS on a machine with 1GB of memory. During the Disko phase the /nix/store runs out of space (tmpfs filesystem running in memory). So I've taken to using a larger (30GB) external disk for swap. And this works, but when it gets to the reboot phase it fails. This is because I'm configuring zpools using Disko, and nixos-anywhere runs swapoff -a if it detects zpools. I understand this was added because of #420 (@TilCreator is using swap on a zpool) which makes sense... but maybe we can add a flag to disable this behavior. This flag would be useful when you are using swap for the install and not using swap on a zpool.

[pmaterer]

For clarity, I immediately get an "Out of memory" error after swap is turned off and my zpools do not get exported.

[pmaterer]

Some ideas:

• My initial idea was to add a new nixos-anywhere flag (something like --no-swapoff). If this flag is set simply skip the swapoff -a step.
• Somehow check if the swap devices that are on are using ZFS and only turn those off.
• Make users label their ZFS swap devices. Then require a flag, something like --swapoff-labels. Only turn those swap devices off.

Any thoughts?

[useranon350]

Swap on zpool isn't really supported by upstream ZFS BTW. It's known to have deadlock issues when memory pressure is high, or when certain features like hibernate and suspend are used.

https://openzfs.github.io/openzfs-docs/Project%20and%20Community/FAQ.html#using-a-zvol-for-a-swap-device-on-linux