Re: O_DIRECT setting

Mark Wong <markw@osdl.org> writes:
> I talked to Jan a little about this during OSCon since Linux filesystems
> (ext2, ext3, etc) let you use O_DIRECT.  He felt the only place where
> PostgreSQL may benefit from this now, without managing its own buffer first,
> would be with the log writer.  I'm probably going to get this wrong, but
> he thought it would be interesting to try an experiment by taking X number
> of pages to be flushed, sort them (by age? where they go on disk?) and
> write them out.

Hmm.  Most of the time the log writer has little choice about page write
order --- certainly if all your transactions are small it's not going to
have any choice.  I think this would mainly be equivalent to O_SYNC with
the extra feature of stopping the kernel from buffering the WAL data in
its own buffer cache.  Which is probably useful, but I doubt it's going
to make a huge difference.

            regards, tom lane