Recently my pfSense running on the same hardware for almost 3 years, died. I tried rebooting it and removing RAM, cards etc, with no luck. So decided to bring it back from configuration backup onto new drive. But after few days I stared investigating this matter and I got some temporary solution to start it back.

Here is how kernel panic looks like. It says: “Attempt to query device size failed” and “zfs: freeing free segment”. The latter is the cause of the problem with system starting up.

First, select “3” to escape to loader prompt:

Then set:

vfs.zfs.recover=1
vfs.zfs.debug=1
boot

And you are in. However, now you need to verify if data is present, not corrupted and finally fix ZFS pool/sets as it is only a temporary fix and most probably still requires some manual work to be done.

Be prepared and have recent configuration backup.