I have read that document but I do not
see any discussion of how to handle sparse zones ; I am particularly
concerned about the zones which have their own /usr . Will their /usr
partitions be kept in sync and if so *HOW*?

There are 2 parts to this question: sparse vs whole root and how live upgrade
does it's thing.

For inherit-pkg-dirs the packaging system knows that the directories are readonly. As such the actual update to those directories doesn't happen, but the packaging metadata in /var/sadm is properly updated. The inherit-pkg-dirs are actually updated when the patch/update is applied to the global zone, but the pkg metadata
in each zone needs to be updated to reflect the new contents as well as any
changes not in an inherit-pkg-dir (such as package scripts, local data, etc).
This is the exact same thing that happens when zones are installed.

The mechanism for whole root zones is very similar, which is why much of the
documentation doesn't draw the distinction. For whole root zones the changes
are actually made when the patch/update is applied to the zone.

Live upgrade will mount the zones in the alternate boot environment when it
needs to make changes (patchadd or pkgadd/pkgrm).   What you will see it do
is mount the alternate boot environment and make all of the updates to it.
Then it will mount each non-global zone from the alternate boot environment
and update each one individually.

It really does work.   Although to be honest I am chasing my tail on some
ZFS zoneroots, but it's very likely that it is my fault.   But if not, a bug
report will be entered :-)

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