pgsql: Force default wal_sync_method to be fdatasync on Linux.

Force default wal_sync_method to be fdatasync on Linux.

Recent versions of the Linux system header files cause xlogdefs.h to
believe that open_datasync should be the default sync method, whereas
formerly fdatasync was the default on Linux. open_datasync is a bad
choice, first because it doesn't actually outperform fdatasync (in fact
the reverse), and second because we try to use O_DIRECT with it, causing
failures on certain filesystems (e.g., ext4 with data=journal option).
This part of the patch is largely per a proposal from Marti Raudsepp.
More extensive changes are likely to follow in HEAD, but this is as much
change as we want to back-patch.

Also clean up confusing code and incorrect documentation surrounding the
fsync_writethrough option. Those changes shouldn't result in any actual
behavioral change, but I chose to back-patch them anyway to keep the
branches looking similar in this area.

In 9.0 and HEAD, also do some copy-editing on the WAL Reliability
documentation section.

Back-patch to all supported branches, since any of them might get used
on modern Linux versions.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=87eadd7e3d6f5581d5b4cb8083212a323050e388

Modified Files
--------------
doc/src/sgml/config.sgml | 22 +++---
doc/src/sgml/wal.sgml | 120 ++++++++++++++++---------
src/backend/storage/file/fd.c | 9 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
src/include/access/xlogdefs.h | 6 +-
src/include/port/linux.h | 8 ++
src/include/port/win32.h | 14 ++-
7 files changed, 114 insertions(+), 67 deletions(-)