There's a lot of misinformation in this thread about softupdates. I only have so much time to reply so I'll hit a few key points. I'm the author of journaling extensions to softupdates so I have some experience in this area.
This notion that softupdates was so complex and so inhibited new features in ffs is bogus. I've seen it repeated a few times. There simply was not much pressure for these features and the filesystem metadata did not support it until ufs2. The total amount of code dedicated to extended attributes in softupdates can't be more than 100 lines. ffs sees fewer features because we have fewer developers period.
Furthermore, softupdates is just a different approach. It is no more complex than journaling. When I review a sophisticated journaling implementation such as xfs I see more lines of code dedicated to journaling and transaction management than softupdates requires for dependency tracking. I have worked on a number of production filesystems and while softdep is definitely not trivial, neither were any of the others unless you compare to synchronous ufs. I think a lot of people who are familiar with COW and Journaling are looking at this unfairly because they already know another system and forget how long it took to become comfortable with it.
In cpu benchmarks softdep costs more than async ffs, this is true. However, rollbacks are actually quite infrequent because our buffercache attempts to write buffers without dependencies first. Generally there are enough of those which satisfy dependencies on other buffers that you can keep the pipeline busy. Looking at the code size and depth in any modern filesystem it's clear that a lot of cpu is involved. Are journal blocks not consuming memory? Is the transaction tracking free? Most dependency structures are quite small compared to generating a copy of a metadata block for a jouranl write.
NetBSD abandoned softdep for something much simpler because they didn't have the resources to fix the bugs in it and they didn't incorporate fixes from FreeBSD. Their journaling implementation is similar to our gjournal which is mostly filesystem agnostic and does full block logging in a very simple fashion.
The journaled filesystem project was started simply to get rid of fsck. I think this hybrid solution is very promising. It gives us a place to issue barriers which can affect arbitrary numbers of filesystem operations. The journal write overhead is much lower than with traditional journals.
And regarding benchmarks; FreeBSD doesn't really have a comparably developed journaling filesystem to benchmark softdep against. I think it's unreasonable to compare linux with ext4 to FreeBSD with ffs+softdep for purposes of evaluating the filesystem design. Too many other factors come into play.
You can read more about softdep journaling at http://jeffr_tech.livejournal.com/