> "Magnus Hagander" <mha@sollentuna.net> writes:
> > Is there actually a reason why we don't use O_DIRECT on Unix?
>
> Portability, or rather the complete lack of it. Stuff that isn't in
the
> Single Unix Spec is a hard sell.
Well, how about this (ok, maybe I'm way out in left field):
Change fsync option from on/off to on/off/O_SYNC. On win32 we treat
O_SYNC as opened with FILE_FLAG_WRITE_THROUGH. When we are in O_SYNC
mode, all files, WAL or otherwise, are assumed to be synced when written
and are therefore not synced during pg_fsync(). WAL syncing may of
course be overridden using alternate sync methods in postgresql.conf.
I suspect that this will drastically alter windows performance,
especially on raid systems. What is TBD is the safety aspect. What I
like about this that now are not dealing with a win32-only hack, any
unix system now has another performance setting top play with. We also
don't touch the O_DIRECT flag (on win32: FILE_FLAG_WRITE_THROUGH |
FILE_FLAG_NO_BUFFERING) leaving that can of worms for another day.
Under normal situations, we would expect O_SYNCing everything all the
time to slow stuff down, especially during checkpoints, but it might
actually help on a caching raid controller. On win32, it will help
because the performance of fsync() sucks so horribly, even or raid.
Merlin
