"because of the monolithic nature of ZFS that created problems with the Redox microkernel design"

Anyone have an idea what this actually means and what problems they were having in practice?

I can only speculate, but maybe they're referring to the same thing Andrew Morton meant when he described ZFS as a rampant layering violation.

ie ZFS isn't just a file system. It's a volume manager, raid and file system rolled into one holistic system vs for example LVM + MD + ext4.

And (again I'm only speculating) in their micro kernel design want to have individual components running separately to layer together a complete solution.

Good question. I don't know about other microkernels, but NetBSD is a small kernel that supports ZFS. The support has been there since the 4.0.5 and 5.3[0], possibly earlier too. I'm not adept at navigating the mailing lists here, but I imagine a good place to learn about the challenges of porting ZFS to a smaller kernel would be the NetBSD and ZFS lists from that era (2008-2009). What NetBSD does today is use a 'zfs' modlue that depends on a 'solaris' kernel modile. The dependency of Solaris primitives is probably one of the major challenges with porting ZFS to any kernel. FWIW, somehow a ZFS port for the "hybrid" kernel in Windows also exists[1].

[0] https://vermaden.wordpress.com/2022/03/25/zfs-compatibility/

[1] https://github.com/openzfsonwindows/openzfs

I don’t think it’s microkernels in general but their microkernel design which wants as much as possible in userspace. They want each component to have its own memory space. ZFS blurs the layers between filesystem and the volume management. This kinda bothers layers of abstraction model folks. And I assume combined with their posix like model it just sorta clashes with what they want to do. Not impossible to integrate, but they want something a little different.

That seems odd to me too. It seems like they could have put all of ZFS (and SPL) in a single system service.

The main selling point of ZFS is it being monolithic. Because of that it can optimize many things that are impossible in a layered approach.

If I had to guess, it is because ZFS likes to insert itself into things beyond just being a filesystem. It is one of the reasons ZFS notoriously works poorly with database engines, which have a tendency to bypass or ignore the filesystem (for good reason). It is a design choice on the part of ZFS.