loader zfs should check all labels
The current loader code does read pool data only from first label on disk. While in general it should be enough, still there is an chance we will hit disk issue or missing updates on label.
The first take on issue is https://www.illumos.org/rb/r/250/
For test and verification, I had debug printouts to verify the txg numbers on all 4 label instances, also for UB scan - those printfs are removed from patch posted in reviewboard.
Note there is also parallel work at fbsd side, and using slightly different approach: https://reviews.freebsd.org/D8464.