[Bug 210222] 19.4.7.2. -- Add warning about ZFS silently masking datasets with the same mountpoints when recursively sending/receiving the root dataset

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210222

            Bug ID: 210222
           Summary: 19.4.7.2. -- Add warning about ZFS silently masking
                    datasets with the same mountpoints when recursively
                    sending/receiving the root dataset
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: Documentation
          Assignee: freebsd-doc at FreeBSD.org
          Reporter: b.manas.88 at gmail.com

I was following the handbook in trying to create a backup of the whole
filesystem of a system that I had installed & configured. I ran into an issue
when trying to send/receive the snapshot of the datasets.

**

If a user takes a recursive snapshot of an entire pool, such as with

test-system1# zfs snapshot -r zroot at TEST

and then sends that to a second ZFS system, such as with

test-system1# zfs send -vR zroot at TEST | ssh test-system2 zfs recv -Fdvu
zroot/TEST/test-system1_zroot


the resulting datasets on the receiving side will have mountpoints that are
exactly the same as some of the datasets that already exist. ZFS does not warn
about this and upon a reboot, important datasets will be masked. This requires
booting into a livecd and manually changing the mountpoints so that they are no
duplicates. This has happened to me before and I think there should be at least
a warning in the handbook so that users who are not familiar with ZFS are aware
of this danger.

I spoke to Allan Jude about this at BSDCan 2016 and he responded positively to
making this change. I am happy to submit a patch, I have attended 2 of the Doc
Sprints at BSDCan 2016 and I will be familiarizing myself with the process of
submitting a patch for documentation.

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