Re: O_DIRECT for WAL writes

Neil Conway <neilc(at)samurai(dot)com> writes:
> On Mon, 2005-05-30 at 02:52 -0400, Tom Lane wrote:
> Well, that claims that "data is guaranteed to have been transferred",
> but transferred to *where* is the question :)

Oh, I see what you are worried about. I think you are right: what the
doc promises is only that the DMA transfer has finished (ie, it's safe
to scribble on your buffer again). So you'd still need an fsync;
which makes O_DIRECT orthogonal to wal_sync_method rather than a
valid choice for it. (Hm, I wonder if specifying both O_DIRECT and
O_SYNC works ...)

> The other question is whether these semantics are identical among the
> various O_DIRECT implementations (e.g. Linux, FreeBSD, AIX, IRIX, and
> others).

Wouldn't count on it :-(. One thing I'm particularly worried about is
buffer cache consistency: does the kernel guarantee to flush any buffers
it has that overlap the O_DIRECT write operation? Without this, an
application reading the WAL using normal non-O_DIRECT I/O might see the
wrong data; which is bad news for PITR.

regards, tom lane