Some more on the RHEL 5.5 system I'm helping to setup. Some benchmarking using pgbench appeared to suggest that wal_sync_method=open_sync was a little faster than fdatasync [1]. Now I recall some discussion about this enabling direct io and the general flakiness of this on Linux, so is the option regarded as safe?
Performance peaked around 2500 tps @32 clients using open_sync and 2200 with fdatasync. However the disk arrays are on a SAN and I suspect that when testing with fdatasync later in the day there may have been workload 'leakage' from other hosts hitting the SAN.