Re: [PATCH 0/9] overlay filesystem: request for inclusion (v17)

From: Al Viro
Date: Fri Mar 15 2013 - 16:30:35 EST

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...
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