[HEADSUP] zfs root pool mounting

Recently some changes were made to how a root pool is opened for root filesystem
mounting.  Previously the root pool had to be present in zpool.cache.  Now it is
automatically discovered by probing available GEOM providers.
The new scheme is believed to be more flexible.  For example, it allows to prepare
a new root pool at one system, then export it and then boot from it on a new
system without doing any extra/magical steps with zpool.cache.  It could also be
convenient after zpool split and in some other situations.

The change was introduced via multiple commits, the latest relevant revision in
head is r243502.  The changes are partially MFC-ed, the remaining parts are
scheduled to be MFC-ed soon.

I have received a report that the change caused a problem with booting on at least
one system.  The problem has been identified as an issue in local environment and
has been fixed.  Please read on to see if you might be affected when you upgrade,
so that you can avoid any unnecessary surprises.

You might be affected if you ever had a pool named the same as your current root
pool.  And you still have any disks connected to your system that belonged to that
pool (in whole or via some partitions).  And that pool was never properly
destroyed using zpool destroy, but merely abandoned (its disks
re-purposed/re-partitioned/reused).

If all of the above are true, then I recommend that you run 'zdb -l <disk>' for
all suspect disks and their partitions (or just all disks and partitions).  If
this command reports at least one valid ZFS label for a disk or a partition that
do not belong to any current pool, then the problem may affect you.

The best course is to remove the offending labels.

If you are affected, please follow up to this email.

-- 
Andriy Gapon