A HP ProLiant MicroServer Gen8 had FreeBSD 9.1 successfully installed on it with root on zfs following the instructions here http://wp.strahlert.net/wordpress/zfs-2/installing-freebsd-9-1-using-root-on-zfs-and-gpt-disks/ In short a pair of sata disks in mirror array. The server was running fine but needed to be relocated so at that point the FreeBSD-update command was run to upgrade to 9.2 On attempting to reboot the system stopped at the "OK" prompt (of one of the zfs boot loaders) stating the kernel was not found. There was also an error reported that no disk could be found to boot from and lsdev listed no devices of any sort. It appears another person was having the exact same problem http://forums.freebsd.org/showpost.php?p=234988&postcount=3 Fix: I put the bootcode from FreeBSD 9.2 on the disks "just in case" but this made no difference. I was not able to fix the problem but I was able to rescue the system by booting from a 9.2 loaded memory stick, and running FreeBSD-update rollback which successfully restored the system to 9.1 and allowed it to boot. How-To-Repeat: I have not attempted to repeat the problem.
(In reply to dms from comment #0) Unfortunately the forum page is a 404 now :(. I'm guessing you ran into a slew of errors like "boot: ZFS: i/o error - all block copies unavailable". I recently replaced a spinning rust disk with an SSD (with 4kB sectors), installed 9.3, upgraded to the latest point release, and unfortunately ran into this unbootable scenario, similar to what you've described below. I had to wipe out my root zpool and repartition the SSD. Fortunately I didn't have any critical data on it, but it was unfortunate that I had to hop through that hoop in order to get my system back in working shape..
batch change: For bugs that match the following - Status Is In progress AND - Untouched since 2018-01-01. AND - Affects Base System OR Documentation DO: Reset to open status. Note: I did a quick pass but if you are getting this email it might be worthwhile to double check to see if this bug ought to be closed.
I am afraid it's impossible to diagnose the problems now.
(In reply to Andriy Gapon from comment #3) I understand... 9.x is a little long in the tooth now. For the record that microserver is still in service and still running FreeBSD 9.1. If the disks fail before the server itself, I'll swap in an uptodate version of FreeBSD. It's a pity I've had trouble with FreeBSD and the HP Microservers (had issues with Microserver 8 & 10 re booting, 10 was at gotten round with some community advice) as they are great budget machines.