Hi,
synchronous_commit description in document[1] :
When off, there can be a delay between when success is reported to the client and when the transaction is really guaranteed to be safe against a server crash.
If I am not misunderstood, I guess it should be *When on* instead of *When off* here?
[1] Doc link : http://www.postgresql.org/docs/<9.5,9.4,9.3,9.2,9.1,9.0,8.4>/static/runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT
Regards,
Amul Sul