We really ought to do something about O_DIRECT and data=journalled on ext4

Hackers,

Some of you might already be aware that this combination produces a
fatal startup crash in PostgreSQL:

1. Create an Ext3 or Ext4 partition and mount it with data=journal on a
server with linux kernel 2.6.30 or later.
2. Initdb a PGDATA on that partition
3. Start PostgreSQL with the default config from that PGDATA

This was reported a ways back:
https://bugzilla.redhat.com/show_bug.cgi?format=multiple&id=567113

To explain: calling O_DIRECT on an ext3 or ext4 partition with
data=journalled causes a crash.  However, recent Linux kernels now
report support for O_DIRECT when we compile PostgreSQL, so we use it by
default.  This results in a "crash by default" situation with new
Linuxes if anyone sets data=journal.

We just encountered this again with another user.  With RHEL6 out now,
this seems likely to become a fairly common crash report.

Apparently, testing for O_DIRECT at compile time isn't adequate.  Ideas?

--                                  -- Josh Berkus                                    PostgreSQL Experts Inc.
                        http://www.pgexperts.com