Booting from zfs snapshot

[Andreas Nilsson]

Hello,

I'm trying to wrap my head around the process of booting from a zfs
snapshot. I have hit a few roadblocks, which I hope this is the adequate
list to post to regarding those.

A short note on what I'm trying to achieve might be in order. In short: a
nanobsd system on zfs only. I want to boot from a snapshot so that when I
push out an upgrade with zfs send, I want the root filesystem to remain
unchanged.

The problems I've hit so far:
*1 Making the zpool.cache file available
*2 Having / mount via entry in fstab.

*1: The zpool.cache is needed to autoimport a pool as I understand it. Is
there a way to force the kernel to import a pool during bootup even though
no zpool.cache is around? What does this file actually contain?

I made an experiment and booted a disk with zfs root from machine a in
machine b and that worked. I did partition the disk with gpart using a gpt
scheme, and labeled the partition on which the pool resides as os, and upon
creation of the zpool used gpt/os as device. Does this mean that as long as
gpt/os is available, any machine boot this disk will have the zpool
autoimported?

*2: Having a line like
tank/root/8.2-RELEASE-p5 at ro / zfs ro 0 0
in fstab causes mount to throw an error and leave me in single user mode,
when the system is booted however mount can mount a zfs snapshot just fine.
Setting vfs.root.mountfrom in loader.conf works just fine though.


Best regards
Andreas Nilsson