Chapter 2, Database Hardware, touched on the importance of setting up your server to avoid volatile write-back caches. One purpose of wal_sync_method is to tune such caching behavior.
The default behavior here is somewhat different from most of the options. When the server source code is compiled, a series of possible ways to write are considered. The one believed most efficient then becomes the compiled-in default. This value is not written to the postgresql.conf file at initdb time though, making it different from other auto-detected, platform-specific values such as shared_buffers.
Before adjusting anything, you should check what your platform detected as the fastest safe method using show; the following is a Linux example: ...
