/tmp is not a tmpfs on NixOS

The other day I realized that /tmp/ on my NixOS installations is not a tmpfs as I'm used to from other distros. Instead, NixOS relies on this systemd timer that cleans up old files from /tmp/. This is not NixOS specific, other systemd based distros also run this timer.

Looking at the NixOS boot.tmp related options, we see there's an option (cleanOnBoot) that clears files on boot and is disabled by default. I enabled it for my setup because I'm used to /tmp/ being cleared on boot and I think it makes sense. Moreover, there's a note about why it's not a tmpfs by default, it's used during nix builds and it's possible that a build will fail if the tmpfs runs out of space due to memory constraints.