Re: filesystem namespace regions, or making mountd less bozotic

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Wed, 5 Dec 2012 21:29:06 +0000
    From:        David Holland <dholland-tech%NetBSD.org@localhost>
    Message-ID:  <20121205212906.GC14200%netbsd.org@localhost>

  | I am tired of PR 3019 and its many duplicates, so I'd like to see a
  | scheme that allows managing arbitrary subtrees of the filesystem
  | namespace in a reasonably useful manner.

If you're going to do something like that, then you need to work out
what it means to have a file linked into different regions, and how
you'd manage to implement that.

If the answer is to forbid it (not easy when regions are very dynamic)
then what you've designed is essentially mount points, with more
flexibility (your uses for regions are more or less exactly what I
use large numbers of different filesystems for, except I lose on
the ability to move the boundaries simply, and you lose on different
mount attributes (read only, async, ...)

On the other hand, if you're going to allow cross region links, then
you need to work out how a vnode for a file that's in multiple regions
(that is, for which path that end in multiple different regions reference
the same inode) manages to reference all the regions, and exactly what
that means to whatever access controls you (like nfs exportability) that
you have depending upon regions.

kre