FATAL: invalid value for parameter "wal_sync_method": "open_datasync"

O.S.:
SuSE SLES-8 (i386)
VERSION = 8.1

After some test with the "syncbench" posted on performance mailing-list
(http://archives.postgresql.org/pgsql-performance/2004-09/msg00080.php)
i have obtained that open() with O_DSYNC was the fastest sync method.
I want to test this for parameter "wal_sync_method" on a test workload
directly on Postgres and i have configured that on postgresql.conf file:

wal_sync_method = open_datasync

But at the server start:

 > pg_ctl -D /db10/postgres/data start
postmaster starting
 > FATAL:  invalid value for parameter "wal_sync_method": "open_datasync"

What's wrong?

Best regards.