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

Josh Berkus wrote:
> On 12/6/10 6:10 PM, Tom Lane wrote:
> > Robert Haas <robertmhaas@gmail.com> writes:
> >> On Mon, Dec 6, 2010 at 9:04 PM, Josh Berkus <josh@agliodbs.com> wrote:
> >>> Actually, on OSX 10.5.8, o_dsync and fdatasync aren't even available.
> >>> From my run, it looks like even so regular fsync might be better than
> >>> open_sync.
> > 
> >> But I think you need to use fsync_writethrough if you actually want durability.
> > 
> > Yeah.  Unless your laptop contains an SSD, those numbers are garbage on
> > their face.  So that's another problem with test_fsync: it omits
> > fsync_writethrough.
> 
> Yeah, the issue with test_fsync appears to be that it's designed to work
> without os-specific switches no matter what, not to accurately reflect
> how we access wal.

I have now modified pg_test_fsync to use O_DIRECT for O_SYNC/O_FSYNC,
and O_DSYNC, if supported, so it now matches how we use WAL (except we
don't use O_DIRECT when in 'archive' and 'hot standby' mode).  Applied
patch attached.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +