Thanks, Bill and Scott, for your responses.
To summarize, turning fsync off on the master of a Slony-I cluster is
probably safe if you observe the following:
1. When failover occurs, drop all databases on the failed machine and
sync it with the new master before re-introducing it into the cluster.
Note that the failed machine must not be returned to use until this is
done.
2. Be aware that the above implies that you will lose any transactions
which did not reach the standby machine prior to failure, violating the
Durability component of ACID. This is true of any system which relies on
asynchronous replication and automatic failover.
- Joel