On Wed, Apr 25, 2012 at 5:22 PM, Richard Elling
<richard.ell...@gmail.com> wrote:
> Unified namespace doesn't relieve you of 240 cross-mounts (or equivalents).
> automounters were invented 20+ years ago to handle this in a nearly seamless
> manner.
> Today, we have DFS from Microsoft and NFS referrals that almost eliminate
> the need
> for automounter-like solutions.

I disagree vehemently.  automount is a disaster because you need to
synchronize changes with all those clients.  That's not realistic.
I've built a large automount-based namespace, replete with a
distributed configuration system for setting the environment variables
available to the automounter.  I can tell you this: the automounter
does not scale, and it certainly does not avoid the need for outages
when storage migrates.

With server-side, referral-based namespace construction that problem
goes away, and the whole thing can be transparent w.r.t. migrations.

For my money the key features a DFS must have are:

 - server-driven namespace construction
 - data migration without having to restart clients,
   reconfigure them, or do anything at all to them
 - aggressive caching

 - striping of file data for HPC and media environments

 - semantics that ultimately allow multiple processes
   on disparate clients to cooperate (i.e., byte range
   locking), but I don't think full POSIX semantics are

   (that said, I think O_EXCL is necessary, and it'd be
   very nice to have O_APPEND, though the latter is
   particularly difficult to implement and painful when
   there's contention if you stripe file data across
   multiple servers)

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