Reiser4 not in Linux kernel
 

I read that Reiser4 is not in the Linux kernel. I wonder what that means.

It means that it's not in the Linux kernel. ReiserFS is, Reiser4 isn't. You can easily find out why by searching the web.

Yes, because Reiser murdered his wife. I wanted to know what are the repercussions of Reiser4 not being in the Linux kernel. Slower performance? Lower reliability? Or what?

If I remember correctly, no-one liked Reiser 4 but Reiser. There were only about three people working on it anyway, so I presume they're maintaining version 3. But as Cynwulf said, the answer is out there...

My understanding is that is not the reason.

Firstly, you have to understand that Hans Reiser was a fairly disputatious person, and was hard to get on with. This, you might comment, does not make him unique in the tech world, but it does push him out towards an extreme percentile of the population.

Second, some aspects of Reiser 4 required different interfaces from the other file systems out there. This causes a bifurcation of possible paths; one would be for Linux/Linus/the developers to 'fix' Linux for Reiser's requirements and the other would be for Hans Reiser/the Reiser team to fix Reiser 4 for Linux.

My bet is that you won't have much difficulty guessing who took which position.

Of course, if you are arguing 'but we can't get on with Hans', the fact that he has been convicted of murder doesn't do your argument much harm. Neither does the fact that he would be kept away from a computer 24 hours a day.

At the time (5 - 10 years ago) with ext3 an established and stable system and ext4 just being introduced and not yet being stable, there were use cases in which Reiser 4 was faster than the obviously competing file systems. Given the nature of benchmarking, you also have to point out that there would have been tests for which Reiser 4 was slower than all of the obviously competing filesystems, so it isn't as simple as saying 'this one is faster, that one is slower'.

Now, given the limited development that Resier 4 has had in the interim, it is probably more difficult to make a clear case for Reiser 4 above the competing systems, except for very niche applications. And, some of the newer systems have taken on-board some of the innovations that might have been Reiser 3's USP, back in the day.

(But, at the end of the day. it is a choice, and choice is good, right?)

From a technical point of view, file systems such as ZFS and BTRFS now have capabilities that are beyond those of Reiser 4 (it is a very different question whether that makes them 'better' or not, but, if you were choosing on which ticks most feature boxes...), so it is more difficult than it once was to argue that Reiser 4 would bring something otherwise completely unavailable (and desirable) to the table.

@DavidMcCann
Reiser 4 did do well in some testing, back in the day, so it wasn't a total write-off. I remember using a version of Xandros on which it was an option and it certainly didn't do anything obviously bad, and, if it hadn't been for completely unrelated things to do with Xandros, it might have been my distro of choice, simply due to the availability of Reiser 4.

Whatever the Reiser team are doing these days, it doesn't seem to be maintaining Reiser 3. Any bug reports are allegedly met with the comment 'We aren't fixing it, you should be using Reiser 4'. The response, 'but I can't' (for whatever reason) doesn't cut much ice with them, as they are trying to get everyone on to Reiser 4.