Unfortunately, Sun (now Oracle) provided the original source code under the CDDL, so re-licensing it would require Oracle's approval, which is unlikely to happen.
ZFS's license is compatible with those of FreeBSD and Illumos, which are very stable operating systems. Given that ZFS is most likely to be used for a SAN or a NAS box, you can quite easily use FreeBSD for those boxes and Linux for your application servers if you choose.
So essentially Oracle still owns the original rights on it? How does it work with derivatives? Let's say OpenZFS over the years will move far away from Oracle's ZFS. Will it sill be indirectly controlled by them as in not allowing to relicense it?
Generally it's a complicated area. Generally the only safe way to sneak out from under an existing license would be a black-box rewrite, done by people who hadn't looked at the source for the original version. Otherwise the original author could claim that it's a derivative work, and thus falls under the terms of the original license.
The CDDL in particular specifies that any modifications (changes, additions or deletions to the source code or their files) are also under the CDDL.
My understanding (although I'm not certain) is that a license to use and modify the software also implicitly grants a license to use the patents contained in the original code.
ZFS's license is compatible with those of FreeBSD and Illumos, which are very stable operating systems. Given that ZFS is most likely to be used for a SAN or a NAS box, you can quite easily use FreeBSD for those boxes and Linux for your application servers if you choose.