Maybe this should be put into the changelog to not cause any surprises.

I don't think the zfs integration (i.e. direct zvol support) is widely used.

That's not a reason to disable a feature.

Presumably the extra dependencies (like zfs) only make a difference for the libvirtd daemon. What we should probably have is two packages, libvirt (with minimal dependencies), and libvirtFull (with all dependencies). Most packages should depend on libvirt (because they only use the library to communicate with the daemon). The libvirtd module should then default to the libvirtFull package for the daemon, and this package should be configurable.

If you don't want zfs, you could just set virtualisation.libvirtd.package = pkgs.libvirt.

We use this feature at mayflower, and I also use it for my personal stuff. I'd definitely appreciate it it remained enabled by default (though it wouldn't kill me if it didn't ;) )

That said, this could (and should!) probably be a runtime thing: meson only seems to use the zfs and zpool executables, which should generally be obtained from the booted system and not a "hard-coded" version, so that the tools match the kernel module version. This would simultaneously eliminate the path dependency, reducing the closure size, and make it more reliable by matching zfs's expectations more closely.

@lheckemann Is relying on /run a good idea? I doubt this will work on non-NixOS systems

Indeed it won't. Maybe a nixos-specific patch to try /run/booted-system and fall back to PATH would make sense. I don't currently have the capacity to implement that though.