The ZFS filesystem is a robust, modern filesystem originially developed on Sun Solaris that contains many advanced features and is being used (among other things) on the largest computer in the world, LLNL Sequoia, which is running Linux.
ZFS is licensed under the Sun CDDL, which is an OSS-approved license. As ZFS was originally developed for Solaris, it is not a derived work of Linux or other GPL software. There is little hope of getting the ZFS copyright owner (Oracle) to relicense it under GPL. Since open source software is intended to increase users' freedom instead of restrict it, there is still a broader community of users would like to make ZFS on Linux available to the masses as part of easily-used Linux distributions.
It seems relatively clear that combining ZFS and Linux source code and compiling it on your own is permissible under the GPL if one does not redistribute the combined work, but there is uncertainty about whether it is legally safe to distribute ZFS and the Linux kernel together in either source or binary form.
Unlike issues with binary kernel modules that have proprietary licenses and/or are closed source, in this case the ZFS code is open source and has none of the objections that traditionally surround binary kernel modules, and it is in fact the GPL license that prevents distributing two open source components together if they do not both use the GPL license.
Under some interpretation of the GPL, ZFS is an independent work and can function on its own without the Linux kernel (there is a userspace component that can be used to run regression tests on the code independent of any kernel), but the Linux-compiled ZFS kernel module itself is not useful to users without the kernel.
Would you consider distributing ZFS binary Linux kernel modules (together or separately from the kernel) a violation of the GPL? Would the binary ZFS kernel module be considered "not based" on the Linux kernel per the GPLv2 section 2, the last paragraph that allows "mere aggregation" of another work packaged independently on the same media or download site? Would it be permissible if the ZFS code were distributed as a source package together with the binary kernel and compiled on the end-user system at installation time? Failing that, would the FSF be willing to make a special exemption to the GPL to allow ZFS to be bundled with Linux?