Re: Enabling Checksums

On Fri, 2013-01-25 at 15:29 -0500, Robert Haas wrote:
> I thought Simon had the idea, at some stage, of writing a WAL record
> to cover hint-bit changes only at the time we *write* the buffer and
> only if no FPI had already been emitted that checkpoint cycle. I'm
> not sure whether that approach was sound, but if so it seems more
> efficient than this approach.

My patch is based on his original idea; although I've made quite a lot
of changes, I believe that I have stuck to his same basic design w.r.t.
WAL.

This patch does not cause a new FPI to be emitted if one has already
been emitted this cycle. It also does not emit a WAL record at all if an
FPI has already been emitted.

If we were to try to defer writing the WAL until the page was being
written, the most it would possibly save is the small XLOG_HINT WAL
record; it would not save any FPIs.

At first glance, it seems sound as long as the WAL FPI makes it to disk
before the data. But to meet that requirement, it seems like we'd need
to write an FPI and then immediately flush WAL before cleaning a page,
and that doesn't seem like a win. Do you (or Simon) see an opportunity
here that I'm missing?

By the way, the approach I took was to add the heap buffer to the WAL
chain of the XLOG_HEAP2_VISIBLE wal record when doing log_heap_visible.
It seemed simpler to understand than trying to add a bunch of options to
MarkBufferDirty.

Regards,
Jeff Davis