Why Reiser4 is not in the Linux Kernel
"Why Reiser4 Is Not Merged" is a widespread question around forums, Slashdot, OSNews and wherever else Linux-related new appears on the web. The flame-wars on this topic have gotten to the "mine is bigger than yours" level. Hence, it's not easy to see where the real problem lies. This document tries to make clear the "official" point of view of Linux developers on this matter.
It's shocking to read some people who believe that the main reason why Reiser4 is not in the [http://en.wikipedia.org/wiki/Linux_kernel#Versions Vanilla Kernel] are "political disputes instead of technical merits". The circular reasoning behind this seems to "Reiser4 is fast/it has plugins so it can/should/must be included NOW. If you don't agree with me, then you must have an irrational hatred against Reiser4, because I'm so obviously right".
As usual, the world is not black or white, but grey. There is no "opposition force" stopping Reiser4 from making it into the main Linux tree. Reiser4 needs to attain a quality level high enough to be included. It's disturbing to listen to people saying that Linux Maintiners did not want to include a filesystems for political reasons. Linux is the kernel that supports many filesystems from its own sources. Linux even includes (being fair) a lot filesystems people consider worthless. Linux supports filesystems that are rarely used. e.g. 9p, BeFS, BFS, minixfs, ADFS, AFFS, EFS, HPFS, UFS, VXFS, qnx4fs, sysvfs, ncpfs, codafs. There are several relevant filesystem choices as well: ext3, XFS, JFS, JFFS and reiser3. Many people have chosen these and are happy with them. Does that really sounds like a group who would not merge a filesystem for political reasons? If anything, Linux Developers look more like filesystem bitches.
Since the initial request to get Reiser4 merged, it hasn't been declared ready by people who know. "It works for me on my PC" isn't exactly the way engineers should make decisions. Of course, nobody expects Reiser4 to be bug-free before getting merged. It must meet with the design tenets and quality specifications laid out by the existing maintainers. Writing a filesystem from scratch is not easy. Reiser4 in particular is attempting to break a lot of new ground. The Reiser4 authors chose that approach and it is expected that it takes a lot of time and effort: they have consistently moved forward since the first discussions of merging years ago. As for today, thanks to all the work done by the Namesys people, Reiser4 is [http://marc.theaimsgroup.com/?l=Linux-kernel&m=115265566213465&w=2 growing near to inclusion] in the main tree. It could possibly be ready as soon as 2.6.19. Reiser4 have been working on this question for two-plus years, so you can imagine what the state of readiness was when it was initially submitted. Reiser 4 is a complex filesystem, and it's taking Namesys some years to finish it. This is a good sign. In every way this points to the future success of Reiser4. You can bet that [http://www.opensolaris.org/os/community/zfs/ ZFS] has been working for a long time before being included in [http://www.opensolaris.org/os/ OpenSolaris].
So that's it: Things need to have a decent shape before being included, and Reiser4 is not there yet. Many people in this process have taken [http://en.wikipedia.org/wiki/FUD FUD] against Linux to religious zealotry. This is not the first time for difficult issues surrounding developers, maintainers and the Kernel. Egos this large sometimes have disputes. Disputes like Reiser4, and XFS before it, and the IDE layer in the 2.4.x days, don't always create this situation. For instance, take [http://oss.oracle.com/projects/ocfs2/ OCFS2] and [http://www.redhat.com/software/rha/gfs/ GFS]. These are clustering filesystems, OCFS2 made by [http://www.oracle.com/index.html Oracle] and GFS comes from [http://www.redhat.com RedHat]. Both companies/groups have submitted their filesystems for inclusion, but only OCFS2 is in today. GFS still needs some more 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 will be included, just like Reiser4 will.
Finally, before it can be merged Reiser4 code needs a review and sign-off by some kernel developer who knows how a file system must behave. This is an evolving standard and practice. It heavily affected XFS, bringing tears and whining from a number of people. Many of the same arguments for inclusion used on behalf Reiser4 were previously used for XFS. Linux Devs and Maintainers wanted to see certain qualities and changes in the XFS code. The XFS team at SGI plodded along, and after much ado, Christoph Hellwig consented to perform a final review. Some developers grew offended, some people got angry and caustic. There are not a of these people who are trusted to do this work. There are even less who are willing to do it. By the time that Christoph took up the work, it was make or break. None of the other Linux Devs were willing. Some even indicated that the job was all but impossible.
By the same token, there were Linux Devs and maintainers who did early reviews of Reiser4 and were verbally abused by Namesys. Lots of people have weighed-in for those messes. Some Maintainers are unwilling to work with Reiser4 anymore. They don't oppose the filesystem. However, someone else must review and sign off on it. Egos aside, this just makes the process slow.
FAQ and Frequently Given Answers
Q1. "But just include it as experimental code regardless of everything, Namesys programmers will fix all the problems eventually!"
Well, no and yes. As said, nobody expects Reiser4 to be bug-free, but there are some important issues that need to be fixed. The problem is that the Namesys developer team is still working on 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 good reason, etc. Every piece of code submitted needs to meet quality standards - requesting developers to fix severe issues before getting it into the main tree helps to have better code. If you ask people to fix those issues "in the future", they may be lazy and so there may be critical issues around all the time - this has happened with Linux in the past. Quality is important, especially in a stable development phase. Linux is already being criticized 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 how important Reiser4 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.
Q2. "But by not including Reiser4, you aren't allowing Reiser4 to be tested and it will never get into a decent shape!"
Wrong. Read [#includeexp question #1 above]. And we live in a Open Source land. Nobody stops people from testing Reiser4 now, or including it in the next Ubuntu/Fedora/whatever release. In fact, when Reiser4 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.
BTW, in the kernel world it's a quite common practice to get a feature into one or two distros before merging it into the baseline kernel. 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 depend on kernel developers - if a feature gets included in several distros, that means that people want it and that you can check its quality in the distributor's bug tracking systems. In the particular case of Reiser4, [http://kerneltrap.org/node/6681 Andrew Morton has even said] that Uptake by a vendor or two would be good. If you want to help Reiser4 to get developed faster, bug your particular Linux distribution and ask them to include it. Distributions 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.
Q3. "Kernel developers hate Hans Reiser, that's why they haven't merged Reiser 4"
Well, no and yes. As said, there are the technical issues that are stopping it, not anything else. Now, it is true that Hans Reiser has not helped a lot to solve those issues. Telling Linux developers that Reiser4 is stable enough and that it should get merged because they would fix all the issues later does not help a lot (see [#includeexp question #1 above], Hans Reiser has been claiming that Reiser4 is ready for years). It doesn't help either to mention how much VFS maintainers hate you and how much smarter your coders are compared to them. It doesn't help to say how closed-minded they are because they're asking Reiser4 to disable some advanced Reiser4 features (features that happen to clash with the VFS and that Reiser4 has ended up disabling anyway). It doesn't help to mention repeatedly how Linux is doomed against winfs because they're not allowing Reiser4 to be merged today - despite that other people are free to disagree with Hans's predictions - and how evil people are for asking him to fix bugs instead of helping him to do "The Right Thing". It doesn't really help to suggest to kernel developers that they should replace the Linux's VFS with Reiser4's plugin layer just because Reiser4 is faster than ext3.
Attacking people when they disagree with you is not the right way of talking within a Open Source community (or any community for that matter), neither is it helpful to try to convince kernel developers to merge Reiser4 talking to them with marketing speak. That however will not stop Reiser4 from getting into the kernel, but it certainly makes things more difficult. Some people have said that Hans is not the right person to deal with the rest of the kernel community. I think that almost everyone agrees. Reiser4 programmers are certainly more friendly, and like most of normal programmers they focus on technical discussions and they know to say "OK, you're Linux developers, so you probably know better than me how a Linux filesystem must be written".
Q4. "I've been using it for years and it rocks, how can't it be ready?"
You are not a kernel programmer, aren't you?