David,
I'm qute willing to help with "What exactly is a long transaction" but
it needs a complete rewrite. It currently recommends using "onarchive"
and I'm not sure if that is still distributed. I don't even know if
informix would recommend clearing a long transaction in that way. It
also doesn't take any account of newer advances with dynamic allocation
of logs etc.

As to your comments on LTXHWM/LTXEHWM there is no reason that I know of
to set theses to anything lower than 50/60. As I understand it the
setting of 60 for EHWM allows for one rollback record for each pair of
insert/update records.

I would also disagree strongly on the sizing of logical logs.
Practical experience has shown that restores using onbar with certain
backup storage managers is better with a few large logs. It seems the
time taken to search tapes for each log can be prohibitive. That said
it is also significant not to make logs to big from the aspect of
restore point granularity, especially when using ONTAPE(no PIT
restore).
Generally I suspect we could write a complete Informix handbook on the
configuration of logs etc for the wide variety of potential disaster
recovery scenarios. And now we have Table level PIT restore. And that
changes lots of things.

regards

Malcolm