OpenBSD chooses to not incorporate OpenZFS due to their ideology. License wise, you're allowed to use it side by side with OpenBSD's code. OpenZFS is restricted from use alongside GPL.
Not so. CDDL allows use with other non-viral copyleft, and with permissive licenses with varying patent clauses. CDDL can be used alongside MPL 2.0, MPL1.1, Apache 2.0, Ms-PL, OSL, EPL, EUPL. These can be used together in any order. Not so with the GPL, it restricts use with them, with the exception of Apache 2.0, which is only restricted from with GPL2 and LGPL2. Only GPL created these restrictions, the other licenses aren't forcing these viral restrictions. GPL2 can't even use LGPL3. What does that say?
Nonviral copyleft licenses are the right balance between BSD and GPL licenses. Perhaps, BSD licenses are best for libraries, for maximum compatibility. Though, I don't like the idea of a BSD license getting absorbed by a restrictive license, then closed off for use by programs which used it, which then causes two versions of the program to be needed.
It's called Hammer2FS made by DragonFlyBSD. Nothing wrong with ZFS. We were led to believe CDDL was wrong by Stallman. I wonder how much that harmed Sun Microsystems' business model, but there were poor business strategies by Sun Microsystems. CDDL conflicted with viral GPL licenses. A few good licenses are based off of CDDL. It took an ecosystem of it to see the value in it, and it gave examples of limiting viralness.
I would put GPL2 and GPL3 behind an emulator as an interface level only for them. So, if it's allowed, libraries below that emulator interface can remain unchanged and work together.