In a mailing list post last week, Torvalds said he needed an undertaking from Oracle's main legal counsel or owner Larry Ellison that said it was alright to do so.

Torvalds' stance can be understood because ZFS, which was developed by the now defunct Sun Microsystems and is rated very highly by experienced Linux systems administrators, is under a licence known as the Common Development and Distribution License that is not compatible with the General Public Licence which covers the Linux kernel.

Responding to a user who complained about the fact that the ZFS module being broken due to a kernel change, Torvalds said: "If somebody adds a kernel module like ZFS, they are on their own. I can't maintain it, and I cannot be bound by other people's kernel changes.

"And honestly, there is no way I can merge any of the ZFS efforts until I get an official letter from Oracle that is signed by their main legal counsel or preferably by Larry Ellison himself that says that yes, it's OK to do so and treat the end result as Gold."

A package that is using the GPL should make all its code available as source, in the event that it is distributed. This requirement is not satisfied by the CDDL.

"Other people think it can be OK to merge ZFS code into the kernel and that the module interface makes it OK, and that's their decision," Torvalds added. "But considering Oracle's litigious nature, and the questions over licensing, there's no way I can feel safe in ever doing so."

Oracle has been involved in an 11-year case with Google over the latter's use of Java APIs in the Android mobile operating system; a final verdict from the US Supreme Court is expected later this year to bring the matter to an end.

Said Torvalds: "And I'm not at all interested in some 'ZFS shim layer' thing either that some people seem to think would isolate the two projects. That adds no value to our side, and given Oracle's interface copyright suits (see Java), I don't think it's any real licensing win either.

"Don't use ZFS. It's that simple. It was always more of a buzzword than anything else, I feel, and the licensing issues just make it a non-starter for me.

"The benchmarks I've seen do not make ZFS look all that great. And as far as I can tell, it has no real maintenance behind it either any more, so from a long-term stability standpoint, why would you ever want to use it in the first place?"