So in my last blog post, I mentioned that I was trying to upgrade the base operating system on my NAS boxes, with an eye toward getting off of FreeBSD 13.x specifically, and getting off of EoL versions in a broader sense. So I broke the project down in to bite-size tasks. Stage one is getting a NAS onto 15.1. Unfortunately, my first attempt resulted in a faceplant.
I was following this article from vermaden's site, created the boot environment in beadm, mounted it under /var/tmp, and manually populated it. Initially, was getting hung at the mountroot> prompt because it was not able to find the boot pool.
I posted to the forums and they pointed me in the right direction. It turns out that the kernel module for ZFS in FreeBSD 13 is (was) openzfs.ko. However, in modern FreeBSD (14/15?), the module is simply zfs.ko. Thus changing openzfs_load="YES" to zfs_load="YES" allowed me to boot the system onto 15.1. At this point, stage 1 complete.
Stage 2 will consist of coming up with a package list, copying required files over to the BE (e.g /etc/exports, /etc/hosts, /etc/rc.conf, etc) and completing the setup. I will keep everyone in the loop as that progresses. But I am taking a victory lap over getting it to boot. Was a simple fix, but very subtle to detect.