Revisited: ZFS, Btrfs and Oracle.
This entry is a continuation of one published in May of 2009. In fact it is relating to a comment made earlier today which I responded to in brief words. I am now taking the time to offer my viewpoint on the whole ZFS licensing under the CDDL and the reasoning for it.
It wasn’t until I started working with the OpenSolaris kernel and by working I mean, modifying code and going through the build process that I finally realized why OpenSolaris was licensed under the Common Development and Distribution License (CDDL). A lot of other people and companies have claim to code used within Solaris. That includes copyrighted code to which Sun does not have the authority to publish in an open source license. This is why they needed to work with a weak copyleft license such as the Mozilla Public License and modify it to their expectations. The CDDL was eventually approved by the Open Source Initiative (OSI) as a valid open source license and Sun Microsystems was then able to release code under its limitations.
Now before I continue I wish to describe 3 different open source licensing models: (1) the strong copyleft license, (2) the weak copyleft license and (3) the non-copyleft license.
The strong copyleft license is a project based license in which it requires that any derived code from the original project must remain under the original license. This method of licensing makes it nearly impossible to link with code under a non-strong copyleft license. As a result of this approach, strong copyleft licenses are often referred to as viral licenses. The most popular of these licenses is the General Public License (GPL) with 3 available versions. The Linux kernel is licensed under this and its success and growth can be attributed to it.
The weak copyleft license is similar to the strong copyleft license except that it is file-based instead of project based. This means that if there are any modifications to a file, the original license must apply; but that file can be combined in a project with code under a different license. This method makes the type of licensing non-viral. The CDDL and the MPL are categorized as weak copyleft licenses.
The third type is the non-copyleft license which offers no requirement for derived works to stay under the original license. In fact, there is also no requirement for derived code to be released under any open source license. This makes it simple for someone to take an open source project and use it as a basis for a proprietary product. A best known example is the BSD license; and Apple’s adoption of FreeBSD kernel code in their XNU kernel or NetApp and their use of FreeBSD in their customized storage appliances.
Continuing where I left off, it would not have been possible to open source the Solaris kernel for the OpenSolaris project if it weren’t for the CDDL license. In turn, ZFS would have been incompatible with the CDDL license if it were licensed under the GPL; although it has no conflict with non-copyleft licenses such as the BSD license. Because of this and now because of Oracle’s admitted support and commitment to Solaris, I doubt this licensing will change; especially to merge it into the Linux kernel. That is why we should be grateful that: (a) ZFS is available under an open source license making it impossible for it to disappear and (b) that Oracle has been committed to Btrfs and bringing an enterprise class solution into the Linux kernel.
This is why we have choices. If you want ZFS functionality, use OpenSolaris or Solaris. If you don’t necessarily need ZFS and are more comfortable with Linux, you have a lot more distributions to choose from. Or if you want ZFS and a familiar Linux environment, there is also Nexenta.