Hello,
> > Third idea:
> > I install the Slony-I, linux-ha and postgresql on the same server of
> > the two 2U servers. The web application access to the db server
> > directly and without pgpool.
AFAIK, Slony does not have failover capabilities you need in HA solution:
http://gborg.postgresql.org/project/slony1/genpage.php?howto_overview
So you will need pgpool layer (or some other connection manager +
network monitoring software) to detect failures anyway.
Regards,
Ivan Zolotukhin