With so much diversity within open source, it was inevitable that, one day, a great open source project would be ruined by license incompatibility.
It's an attempt to port the file system of Open Solaris into a version of Linux, and was created by the good people at the Lawrence Livermore Lab.
The problem, as Brian Behlendorf noted at Github, is that the licenses are incompatible. ZFS must be offered under Sun's CDDL. Linux, of course, is licensed under the GPL. You can't combine the two.
It would be like, as the late Richard Pryor noted in one of his best monologues, trying to mix regular milk with low-fat. It would explode.
There are some kludgy work-arounds, Behlendorf noted. You can implement ZFS in a user space with FUSE, making it a derived work. Or you can modify and build it separately from the Linux, then build the combination yourself. But this is very hard.
The work was done before the Oracle-Sun merger, and was originally meant for the Lustre project. It was part of a technology Sun acquired in 2007. The usual solution in cases like this is to dual-license the code. The CDDL is based on the Mozilla license, and I have yet to see a problem like this involving Firefox.
Behlendorf says Oracle was contacted about the incompatibility but his group has not had any luck on the license issue, which led to the decision to move the problem to a public forum.
That decision, in turn, just adds more public pressure on Oracle. They publicly stated a desire to cooperate with open source in the run-up to the Sun merger. Few believed them at the time.
The skeptics appear to have been right. Which means open source advocates who supported the merger, as well as Oracle, may have some explaining to do.