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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
