ideas about a unioning file system

Erez Zadok ezk at
Fri Jun 20 09:52:37 PDT 2003

In message <20030620030712.GA85079 at>, David Schultz writes:
> On Thu, Jun 19, 2003, Mohammad Nayyer Zubair wrote:
> Unionfs in FreeBSD has a number of implementation issues.  Many of
> these stem out of locking problems and the fact that VFS layering
> doesn't really work right in FreeBSD.  These are fixable details,
> and perhaps FiST addresses some of these.

We're looking to find out general problems that people have faced with the
existing stackable union mounts in fbsd.  We'd like to know more than just
implementation problems (which presumably are "easy" to solve): are there
any fundamental design problems with union mounts that may require OS/VFS
changes or a redesign of how unioning works?

Our fan-out unionfs, for example, is the first step in exploring general
fan-out file systems and their issues (e.g., partial failures in replicated

> I think the cleanest way to do a union-like implementation is at
> the block level, with writable (``snap-off'') snapshots.  This
> approach avoids ugly situations where file or directory renames
> require massive amounts of copying.

That certainly sounds like a viable aproach, but it seems like there are
already block-level snapshotting systems out there (mostly commercial).  The
reasons we want to stick w/ stackable are several:

1. So we can mount it on top of any other file system, not just a
   block-based one.  Also avoid breaking the lower-level f/s format.

2. So we can port unionfs (or any other fan-out) f/s to the other platforms
   on which we have FiST templates (fbsd, linux, and solaris).


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