Updated docs. ALL_O_DIRECT was listed as XtraDB only, but made the versioning clearer. fdatasync became fsync sometime in 5.3

wlad can you clarify the Windows settings (before and after 10.2.5) so that I can document those properly? I don't have a Windows environment to test.

In MariaDB (unlike in MySQL), Windows recognizes and handles correctly all of the Unix methods, but if none specified it uses own default - unbuffered write (analog of O_DIRECT) + syncs (e.g FileFlushBuffers()) for all files. Since 5.3

The MySQL docs mention 3 Windows settings - async_unbuffered, normal, unbuffered, with the default being async_unbuffered. Are these supported in MariaDB?

This is a question to marko. He added parsing of these quite recently. the "async_unbuffered" is what I referred to as "own default". I think marko made "normal" to be fsync synonym, and unbuffered to async_unbuffered

greenman, wlad, I recently touched this code in 10.3.7:
MDEV-12218 Clean up InnoDB parameter validation

This changed innodb_change_buffering and innodb_flush_method to enums instead of strings. For Windows, there are 3 additional enum values, 2 of which are aliases to the common values:

#ifdef _WIN32

	"unbuffered",

	"async_unbuffered" /* alias for "unbuffered" */,

	"normal" /* alias for "fsync" */,

#endif

The aliases are mapped on startup (after all, this parameter is not settable at runtime):

	switch (innodb_flush_method) {

	case SRV_ALL_O_DIRECT_FSYNC + 1 /* "async_unbuffered"="unbuffered" */:

		innodb_flush_method = SRV_ALL_O_DIRECT_FSYNC;

		break;

	case SRV_ALL_O_DIRECT_FSYNC + 2 /* "normal"="fsync" */:

		innodb_flush_method = SRV_FSYNC;

		break;

	default:

		ut_ad(innodb_flush_method <= SRV_ALL_O_DIRECT_FSYNC);

	}

The default value on Windows is "unbuffered", also known as "async_unbuffered". The value "normal" is an alias for "fsync". There should be no functional change (other than allowing numeric values corresponding to the enum names) between 10.2 and 10.3.