In message <Pine.LNX.4.58.0409021230450.2295@ppc970.osdl.org>, Linus Torvalds writes:
[...]
quoted text
> That said, I think you can make stackable filesystems as a _separate_
> filesystem (aka "view" or whatever), which is slightly different from
> supporting them in the VFS layer itself: make it a filesystem of its own,
> rather than supporting it on a VFS layer as a merge of two different
> filesystems. That way you still have totally arbitrary semantics for how
> the stacking works, but at least it's not codified by the VFS layer - it's
> codified by nothing more special than one more random filesystem
> implementation, and you could in fact have _different_ semantics depending
> on what the user wants.
> 
> Same end result, different "mindset". I think this is what FiST actually 
> does, but I could be wrong.
> 
> What's the difference? If the VFS layer supports it (like the VFS supports 
> regular "opaque" overmounting), you'd just do
> 
> 	mount -o stack /dir /otherdir
> 
> the same way you do "bind" mounts. In contrast, if you do it as a 
> third-party "view" filesystem, it would be more like
> 
> 	mount -t stackfs -o under=/usr,over=/usr/local /mnt
> 
> where /usr and /usr/local would be the same old underlying filesystems, 
> and you'd see a totally new filesystem at /mnt.
[...]

Yes, that's exactly how our stacked f/s's work: not touching or modifying
VFS or lower f/s's, but of course we use the VFS the way other f/s's use it
(calling vfs_* functions and such).  Each of our stackable f/s is a separate
f/s in its own.

And they're a nice way to augment the functionality of any other f/s w/o the
hassles of changing a lower-level f/s format or the VFS.  We recently did an
example f/s that adds EA+ACL functionality on top of anything else, using
the lower file system's EA/ACL support if it's there, and for f/s's that
don't support EA/ACL natively, we support the ea/acl f/s calls and store the
actual ea/acl data in a small database.  We've even demonstrated that it
worked on v/fat (yes, we know, vfat+ACLs is an an oxymoron :-)

We also found out over the years that our stackable file systems are an
interesting way to prototype features that might eventually move up to the
VFS itself; this is b/c each stacked f/s has to appear to the lower f/s as
the VFS itself, essentially emulating most of the VFS's functionality while
at the same time appearing to the VFS as a lower-level f/s.  It's not easy
maintaining this split personality. :-)

Erez.
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