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Subjectmulti-part document (flip side, is it a user level only issue?)
Turning the discussion around: ACLs need to be invisible in the
filesystem, except to the backend and VFS and system calls for
accessing the ACLs without modifying the data part. Users shouldn't
see these in a directory listing. They are only a part different from the
data part in implementation semantics of a filesystem, and the VFS could
make explicit allowance for ACLs without generalized multi-part storage

The multi-partness of a complex document is a user level issue, it's
the user that associates the different parts with a model of a single,
complex storage object. So why isn't this an application concern instead
of a kernel concern? It would be if the documents were all stored and
loaded by apps running on traditional unix systems with traditional unix
file systems. The app would just read/write individual files for as many
parts as necessary, or embed them all into different sections
of a single file (like a tar file), and embed paths (or URLs) to
parts that are shared by multiple documents. It doesn't require kernel
support for multi-part files to do this at application level.

Leaving the kernel filesystem interface that way is not unreasonable,
it's a much simpler design (ie the one we have now).

What may be unreasonable is leaving it that way and wanting maintainable
support for legacy filesystems that moved the multi-part support out of
the user application and into the filesystem implementation.

The thinking behind multi-part files in the filesystem was probably
"solving the same problem uniformly for any application that needs
multiple part storage objects at user level, instead of having each of
them roll their own multi-part abstract storage objects in userspace".
That this has the unfortunate side-effects of kernel bloat and slowing
down access to simple, single-part files was apparently a non-issue.
A user space library would have been sufficient.

But, echoing Linus, that's water under the bridge.

Clayton Weaver

"Everybody's ignorant, just in different subjects." Will Rogers

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