Ask the kernel to minimize caching effects for relation data and WAL files using
O_DIRECT (most Unix-like systems),
F_NOCACHE (macOS) or
May be set to an empty string (the default) to disable use of direct I/O, or a comma-separated list of operations that should use direct I/O. The valid options are
data for main data files,
wal for WAL files, and
wal_init for WAL files when being initially allocated.
Some operating systems and file systems do not support direct I/O, so non-default settings may be rejected at startup or cause errors.
Currently this feature reduces performance, and is intended for developer testing only.