From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH 0/9] overlay filesystem: request for inclusion (v17)
Date: Fri, 15 Mar 2013 20:30:18 +0000
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Cc: James Bottomley <James.Bottomley@HansenPartnership.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Andrew Morton <akpm@linux-foundation.org>,
	torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org,
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To: "J. R. Okajima" <hooanon05@yahoo.co.jp>
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On Sat, Mar 16, 2013 at 03:57:18AM +0900, J. R. Okajima wrote:
> 
> Al Viro:
> > The trouble with such mechanisms is that they tend to end up depending on
> > fairly non-trivial properties of underlying fs.  Try aufs one on btrfs,
> > see how soon you spot the problem.  It's nice when a method turns out
> > to be really redundant and implementable in uniform way via other methods
> > present; see fh_to_dentry history for example of situation where it hadn't...
> 
> Hmm, I could not see problem around aufs using btrfs as the upper RW
> branch, tested on linux-3.9-rc2.
> Would you describe more specifically?

Sure - btrfs happens to have an interesting limit on the number of
links to the same object located in one directory.

The thing is, you are trying to retrofit a new primitive into many
filesystems and do it in the same way.  Doesn't work well...

And yes, it is an independent primitive.  What I really don't understand
is WTF is so attractive about not having to touch individual filesystems;
it's not particulary hard to do for any fs we might care about...