Re: [ANN] 8-CURRENT, RELENG_7 and RELENG_6 have gotten latest ?unionfs improvements

[Note: stripped excessive Cc.]

Dmitry Marakasov <amdmi3_at_amdmi3.ru> wrote:
 > Btw, I was wondering: is unionfs related in any way with -ounion
 > option of mount_*?

No.  The union mount option (-o union) is completely separate
from UNIONFS, although it can be used to achieve a somewhat
similar effect.  It depends on your requirements whether it
is sufficient or not.

 > I was told long time ago that -ounion is even
 > more broken than unionfs.

That's wrong.  The union mount option was _never_ really
broken.  I'm using it for almost as long as FreeBSD exists.

However UNIONFS was broken for a long time (along with
NULLFS and UMAPFS).  NULLFS has been fixed some time ago,
UMAPFS was abandoned apparently, i.e. nobody showed up to
fix it, and UNIONFS was pretty much completely overhauled
by Daichi GOTO and his team.  I would now regard it as
stable.

 > Though, those two features seem to do very
 > similar thing and I think that -ounion option is pretty useful.

Yes, it is useful.  The biggest differences are:

 - The union mount option newly mounts a filesystem on top
   of an arbitrary existing directory tree, while UNIONFS
   mounts another representation of one existing directory
   tree on top of another one.  That means UNIONFS does the
   same as NULLFS, but unlike NULLFS it does not hide the
   underlying directory tree.

 - When using the union mount option, only the entries in
   the root directory show through from the "lower" file
   system.  When using UIONFS, _all_ entries in _all_
   directories are visible (unless masked by an identical
   entry in the upper file system, of course).

 - The implementation of the union mount option is rather
   simple has rather low overhead.  UNIONFS is much more
   complex and has some overhead for certain operations,
   especially when files and directories have to be created
   automatically in the upper layer.

Best regards
   Oliver

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