"can't load 'kernel'" on ZFS root
peterjeremy at acm.org
Wed Sep 7 04:48:04 UTC 2011
I have a several systems with ZFS on mirrored root and have run into
problems during recent 8-stable upgrades with loader reporting it
was unable to load the kernel.
On the first system, I did an installkernel, rebooted and all came
up OK. I then did an installworld, rebooted and got (re-typed):
Unable to load a kernel!
can't load 'kernel'
and the loader prompt.
After some research, I found bin/144214 which suggested my bootblocks
were out of date. I booted from another system and installed new
bootblocks on both disks, rebooted and got the same result.
I then tried backing up and restoring all the files in /boot that were
touched by installworld. This time when I tried to boot, I was
greeted with "ZFS i/o error - all block copies unavailable" followed
by the bootblock prompt. I tried loading zroot:/boot/zfsloader.old
At this point, I gave up and rolled back to a snapshot prior to the
installworld (but after the installkernel) and everything worked.
I have since verified that the bootblocks match the newly built
pmbr and gptzfsboot and that the changes in r208892 are present
in the sourcetree.
On the second system, I was unable to boot after the installkernel
and rolled back - I haven't checked how old the bootblocks are there.
Firstly, does anyone have any ideas on how to move forward? I will
rebuild both zroots at some point but that's not a fix. (I am
reasonably confident I can reproduce the problem if someone has
a patch to test).
Secondly, I found that ZFS booting is extremely fragile - if your
default kernel & modules won't load automatically, there's no way to
boot the backup kernel because (as far as I can find) there's no way
to manually load /boot/zfs/zpool.cache - without which the kernel
can't mount the root FS. In both above cases, recovering the system
required booting from recovery media. There needs to be a documented
method for booting from a snapshot or clone.
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