The documented configuration changes for NixOS appear to
be making zfs available for boot. However, these changes
are also required just to make the zfs.ko module available
to modprobe even for users who don't need ZFS available at
boot time. Also, the kernel module does not appear until
after a reboot, regardless of 'nixos-rebuild switch'.
(a more knowledgable NixOS user might know how to modprobe
without a reboot, but I don't)
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