DB Journalling?

Hi all,

Can I safely set up a system with two separate computers
sharing one NAS disk array, with a postgres database on it?
A co-worker expressed concern over the possibilities of
both sharing and failover, unless postgres supports journalling.

Example:

Two Linux Boxes. Each runs Postgres, and both are connected
to the same Network Attached Storage system. For simplicity, lets
assume that they're using NFS to talk to the NAS box, which internally
contains a decent disk array.

There are two scenarios I am interested in:
1) both machines can operate the database at the same time.
2) one machine operates the database, and the other operates
in hot standby mode. When the primary fails, the secondary
takes over management of the database.

I suppose one can't do the former without some sort of atomic
locking mechanism. I would like to think one could do the latter,
but if the first machine fails mid-write, how does the database
recover, without some nifty journalling capability?

If these aren't available, (specifically the second option) is there any idea
when it might be available? Is there anything financially or otherwise
that my company can do to help?

Thanks
jb

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johnbr(at)incanta(dot)net