FS hang when creating snapshots on a UFS SU+J setup

[Kirk McKusick]

Hi all,

With a gentle reminder from Peter Holm that he has a test that
demonstrates this problem and the additional examples provided
by several of you, I can now reliably reproduce the problem.

Having spent a day trying to get to the bottom of it, I have
decided that it is a `hard problem' which is going to require
some time to figure out. I am hopeful that Jeff will come up
with some time to help out here, but know that he is rather
busy on other projects at the moment.

My conclusion is that as an interim solution, we disable the
ability to request snapshots on filesystems that are using SU+J.
Notably, if a snapshot is attempted, the request fail with a
message that reports that snapshots are currently supported
only on systems running with SU. I think that this would be
far preferable to the current situation. Does this seem like
a reasonable approach?

	Kirk McKusick