"Why Reiser 4 is not in" is a widespread question around forums, slashdot, osnews, etc. when a linux-related new appears in the web. The flamewars on this topic have got to the "mine is bigger than yours" level and hence it's just not possible to know what the real problem is. This document tries to make clear the "official" POV of Linux developers is on this matter.
It's shocking to see some people writting that the main reason why reiser4 is not in is "politics instead of technical merits". The wonder reasoning behind this reasoning is "it's fast/it has plugins so it can be included NOW. If you don't agree with that you've an irrational hatred against reiser4".
BR As usually, the world is not black or white, but grey. Nobody oposes to getting reiser4 into the main linux tree, once it gets a quality level good enougth to be included. It's kind of shocking to listen people say that Linux doesn't want to include a filesystems for political reasons, being Linux the operative system that more filesystems supports from its own sources, including (being fair) a lot of worthless ones that nobody cares about. Linux supports filesystems that people rarely uses, like 9p, BeFS, BFS, minixfs, ADFS, AFFS, EFS, HPFS, UFS, VXFS, qnx4fs, sysvfs, ncpfs, codafs. As for revelant filesystems others than ext3, linux does support XFS, JFS, JFFS, reiser3, and everybody is happy with them, nobody is planning to kill them.
BR Does that really sounds like a operative system that would not merge a filesystem for political reasons?
BR In all the months that people has tried to get reiser 4 in, reiser4 just was not ready. Nobody sane expects reiser4 to be bug-free, but before getting it in it must have a decent shape. Writting a filesystem from scratch is not easy at all (specially if it's so big and complex and it features new "paradigms" that are not very common, like reiser4 does). The reiser 4 guys chose to do that and it's comprehensible that it takes a lot of efforts, they have been working on fixing those issues for years, to the point that reiser4 is [http://marc.theaimsgroup.com/?l=linux-kernel&m=115265566213465&w=2 more near] of being included in the main tree (it may be already for 2.6.19, who knows)
So thats it: Things need to have a decent shape before being included, and reiser 4 was not, so people can't understand all those FUDs against linux because of this question (or maybe it's too easy too understand). If it's not ready, they will be rejected. Just for comparison, take OCFS2 and GFS. They're clustering filesystems, OCFS2 is made by Oracle and GFS cames from Red Hat (who bought it to Sixtina). Both filesystems have submitted their filesystems for inclusion, but only OCFS2 is in (since 2.6.16 and after lots of reviews). GFS apparently needs some work, but once all the [http://marc.theaimsgroup.com/?l=linux-kernel&w=2&r=1&s=GFS&q=b issues are settled down] it'll be included, just like reiser4.
Furthermore, in order to ensure that it doesn't have any major problems it needs to be reviewed by some kernel developer guru that knows how a FS must behave. There're not a lof of them, and the ones that have tried to review it have got verbally abused by Hans Reiser so hard that some of them just don't want to review it again. They don't oppose to get reiser 4 merged if someone else reviews it, they just are not willing to be the ones that do that job, and since reviews are done by volunteers its harder to find one that is willing and has the time to review it, and all this just makes things go slower.
BR Now, a small FAQ:
"But just include it as experimental code regardless of everything, reiser programmers will fix all the problems eventually!"
Well, no and yes. As said, nobody expects reiser 4 to be bug-free, but there're some important issues that need to be fixed, the problems is that reiser 4 is still working in the important ones. Some of the issues fixed in the past included severe design issues, BTW. Others are about being well integrated with Linux: duplication of kernel's own functionality for no reason, etc. Every piece of code submitted needs to have some quality. Specially under a stable development phase. Linux is already being critized a lot for merging new features during this stable phase - that criticism happens with the current quality control. Imagine what would happen if linux started to merge things without caring a bit about what gets merged. Also, consider what Reiser 4 is. It's a filesystem, once it gets included in the kernel many people WILL use it and will DEPEND on it (your disk format is reiser4): Linux needs to ensure that things don't blow up everything.
"But by not including Reiser 4, you aren't allowing Reiser 4 to be tested and it will never get into a decent shape!"
Wrong. We live in a opensource land. Nobody stops people from testing reiser 4 now, or including it in the next ubuntu/fedora/whatever release. In fact, when reiser 4 is included it will be merged as "experimental" code, and there's no real difference between that and patching and compiling their own kernels, it's just a psychological thing, people will think it's more stable when it's included even if marked as experimental, for no real reason.
In the kernel world it's a quite common practice to get a feature in one or two distros before merging it. Being included in distros is valued a lot by linux maintainers, because it gets a lot of real-world testing and bug fixing. And that's something that doesn't really depends on kernel developers - if a feature gets included in several distros it means it will be more stable. In the particular case of Reiser 4, Andrew Morton has even [http://kerneltrap.org/node/6681 said] that Uptake by a vendor or two would be good. If you want to help reiser 4 to get developed faster, bugzilla your particular distro asking to include it. Distros can allow to merge unstable code (and they do that with several drivers that are not in mainline, like ndiswrapper), but it's not so easy for the kernel.
"Kernel developers hate Hans Reiser, that's why they haven't merged Reiser 4"
Well, no and yes. As said, they're technical issues what stop it. Now, it's true that Hans Reiser hasn't helped a lot. Telling linux developers that reiser 4 is stable enought and that it should get merged because they'll fix all the issues in the future doesn't helps a lot (see FAQ 1, he has been saying that reiser 4 is ready since years). It doesn't helps either to mention how much VFS maintainers hate you and how much smarter your coders are compared with them, how closed-minded they're for asking disable some advanced Reiser 4 features (features that happen to clash with the VFS), or how Linux is doomed because of the ignorance of their developers becayse they're not allowing reiser 4 to be merged today. It doesn't really helps to tell kernel developers that because reiser 4 is faster than ext3 they should replace the Linux's VFS with Reiser 4's plugin layer. Attacking people when they disagree with you is not the right way of talking into a open source community, neither it helps to try to convince kernel developers to merge reiser 4 talking to them like a manager. That however won't stop reiser 4 from getting into the kernel, but it certainly does things more difficult.