[Bug 249280] Issues migrating from 12-STABLE to 13-CURRENT regarding filesystem layout changes

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=249280

            Bug ID: 249280
           Summary: Issues migrating from 12-STABLE to 13-CURRENT
                    regarding filesystem layout changes
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: misc
          Assignee: bugs at FreeBSD.org
          Reporter: doctorwhoguy at gmail.com

I recently decided to make the jump from 12-STABLE to 13-CURRENT on my home
server. For the most part, the upgrade went smoothly. However, I did run into a
couple of issues so far that seem to be due to a change in the filesystem
layout and possibly adding certain directories as ZFS datasets.

The first issue I ran into was with /usr/src. Since I had sources for
12.1-RELEASE, 12-STABLE, and HEAD in /usr, I used a symlink pointing in /usr
pointing from src -> head-src during the upgrade. After each reboot during the
upgrade process, my source code in /usr/src seemingly disappeared, and I had to
fetch the source again. I was baffled until I realized that /usr/src was now a
ZFS dataset. I unmounted the dataset, and my data was still there. I was
subsequently able to move my data to a temporary directory, remount the
dataset, and put the data into /usr/src where it belongs.

That was yesterday. Today I realized that /home is now a symlink to /usr/home
which is a ZFS dataset, and all the data I had in my home directory has
seemingly disappeared. I'm sure that my data is still there, but I can't
unmount this dataset on a running system because I get a "device busy" error
when trying to unmount it.

So there seems to be two separate issues that need to be addressed.

1) The problem of "disappearing data" where the contents of a directory seem to
be gone when a ZFS dataset is mounted at the same location as a preexisting
directory.

2) There needs to be a clear way to upgrade a system to this new filesystem
layout which can avoid the above issue. I didn't see anything in UPDATING which
addresses this problem.

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