I had a reasonable idea of what was needed, and a roadmap for what order to tackle things in, when I was working on this (briefly) before. I didn't work on it for long and stopped working on this because I changed jobs and had less time for non-work projects, and personal circumstances got in the way too so, I switched to focusing on smaller projects. This was several years ago, before HAMMER2 and, I haven't actually followed developments since that closely (I haven't been looking at the code really) so, I expect some of my conclusions are no longer valid and I need to revisit this. I have been interested in taking this up again for a few months but, hadn't actually decided to start again until a few weeks ago when someone else I know expressed interest too.

What utilities you need and, what you do in userspace, depends on what the goal of the port is somewhat I think. Also, what goes in user-space depends on what you are porting to (FUSE or what OS). My goal was (and is again now) to have first class support on Linux, i.e. you should eventually be able to do everything you can on Dragonfly with HAMMER on Linux, with as good performance as possible. Initially, performance takes a back seat to feature support (unless it were to be so bad that it made features unusable) but, good performance is an important end goal of mine. I think for my goals, an in-kernel driver and user space utilities based on the Dragonfly ones but, with some substantial differences in code, make sense.

There is a FUSE port of HAMMER that lets you read, without history I think. I don't think this is maintained anymore but, it had very different goals to mine and, a port with different goals to my own, might want to pickup this/start a new FUSE port. In particular, if you don't care about supporting every feature, you want to get things that you do plan to support working more quickly, you don't care about performance in the long-run and/or, you want to target other platforms than Linux as well/instead, FUSE might make more sense. I think there are reasons to work on a FUSE port as well as a specific port to Linux (and other OSs), they have slightly different goals and trade-offs.

To be honest, I'm not sure exactly what you want to know here so, if I haven't addressed it, please do ask (but, it is possibly the case that what I know is too out of date to be interesting).

If you're interested in working on this together (there are a few people I know already who have expressed interest in working on this, all based in London so far), hearing more about progress in the near future or, if you are planning your own Linux port or port to another OS, I'd be interested to hear. Let me know if you'd like me to provide contact details for non-public messages (there's no email on my profile right now). I am not planning to make anything I do public until at least we have read support, with no history, working and at least somewhat tested but, I'm not set in stone about that particularly. I don't see any reason to use our port over the existing FUSE one until after then either.