I'm running nix copy in runInLinuxVM, and notice that for any nontrivial closures, the VM will run out of memory during the copying process. I left it set at the default 512 megabytes. I could obviously increase the amount of memory the VM is given, but that doesn't scale for copying complex derivations with many dependencies.

I suggest adding an option to only load and copy the contents of the paths one at a time, or even better, a way to specify an upper bound on the memory to be used while copying.