I finally got ZFS installed on my Helios4 using Armbian 19.11.3 Buster with Linux 4.19.84, but I had to stick with ZFS version 0.7.12, as advise by @SvenHz in this post. Loading the ZFS 0.8 module would fail with the same error.
@Steven Keuchel Thanks again for your input on the kernel/ZFS memory settings. I had a more detailed look at your settings and noticed that you set `zfs_arc_max=805306368` (i.e. to 768 MB), which corresponds to the maximum `vmalloc` you seem to use. Or were those ZFS settings with `CONFIG_VMSPLIT_2G` and you use in fact an even higher value for `vmalloc`? Which ZFS version did you use, 0.6 or 0.7?
As @qstaq stated, the default for `zfs_arc_max` is 50% of the available memory. There is however a pull request to increase this even further to 3/4 of all memory or all but 1GB, whichever is greater. This change is enabled due to the revised memory allocation for the ARC buffers in ZFS version 0.7. In fact, this change makes me wonder if those settings for `vmalloc` and `zfs_arc_max` still make sense for ZFS >= 0.7.
The fundamental issue with the extensive use of the virtual address space in ZFS was outlined rather well by one of the developers in this ZOL issue. This was addressed however in version 0.7 with the introduction of the ARC Buffer Data (ABD), as also mentioned in the ZFS 0.7 release notes.
It appears that the issues with the virtual address space are limited to ZFS versions 0.6 and older. Or is my understanding of this incorrect? Any advice or comment in this regard would be greatly appreciated.