Re: Use of O_DIRECT only for open_* sync options

On Wed, Jan 19, 2011 at 1:53 PM, Bruce Momjian <bruce@momjian.us> wrote:
> Is there a reason we only use O_DIRECT with open_* sync options?
> xlogdefs.h says:
>
> /*
>  *  Because O_DIRECT bypasses the kernel buffers, and because we never
>  *  read those buffers except during crash recovery, it is a win to use
>  *  it in all cases where we sync on each write().  We could allow O_DIRECT
>  *  with fsync(), but because skipping the kernel buffer forces writes out
>  *  quickly, it seems best just to use it for O_SYNC.  It is hard to imagine
>  *  how fsync() could be a win for O_DIRECT compared to O_SYNC and O_DIRECT.
>  *  Also, O_DIRECT is never enough to force data to the drives, it merely
>  *  tries to bypass the kernel cache, so we still need O_SYNC or fsync().
>  */
>
> This seems wrong because fsync() can win if there are two writes before
> the sync call.

Well, the comment does say "...in all cases where we sync on each
write()".  But that's certainly not true of WAL, so I dunno.

--
Robert Haas
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