I was considering to write this article a while because it doesn't fit in any type of article I have published before. And it isn't my primary business to discuss various open source licensing here. The thing is, it is useful to understand the role of them but it is often quite difficult to imagine what they just want to say. Sometimes, I have a feeling you need a lawyer education to understand them.
You know the obvious questions like "why it has to be GPLed?", "why this license is not compatible with that one?" or "why it can't be part of Linux kernel?". You know the open source license ensures the availability of source code which you can modify and redistribute. The true pitfalls begin appearing when you would like to integrate two products available under two different licenses. To make things clearer I borrowed these two comprehensive schemes from chandanlog at Sun blogs. The first one presents general attitude of open source licenses and classical EULA to source code. The second one explains differences of open source licenses. They are quite minor but may have out of sight consequences.
Let's try to apply the licensing rules to the problem of releasing ZFS filesystem with Linux kernel. What's the problem? First, Sun owns some patent rights which prohibit such action. Second, as Linux kernel is GPLized anything included has to be GPLized as well. ZFS is covered with CDDL license which requires to be preserved. From here, I see the main reason of incompatibility. But if I realize there are other binary only modules like video drivers from ATI or NVIDIA which are linked with kernel via some sort of GPLized open source wrapper why we can't do it the same way with ZFS?!? The question is if it is legal.
The two practical schemes makes me to understand the topic more deeply. The example with ZFS made the situation complicated and I need to find out something what shows me that it is not. I hope you will find these graphical explanations as useful as mine. And check the chandanlog who created them!