Re: why do hash index builds use smgrextend() for new splitpoint pages

On Fri, Feb 25, 2022 at 11:17 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Sat, Feb 26, 2022 at 3:01 AM Melanie Plageman
> <melanieplageman(at)gmail(dot)com> wrote:
> >
> > Since _hash_alloc_buckets() WAL-logs the last page of the
> > splitpoint, is it safe to skip the smgrimmedsync()? What if the last
> > page of the splitpoint doesn't end up having any tuples added to it
> > during the index build and the redo pointer is moved past the WAL for
> > this page and then later there is a crash sometime before this page
> > makes it to permanent storage. Does it matter that this page is lost? If
> > not, then why bother WAL-logging it?
> >
>
> I think we don't care if the page is lost before we update the
> meta-page in the caller because we will try to reallocate in that
> case. But we do care after meta page update (having the updated
> information about this extension via different masks) in which case we
> won't lose this last page because it would have registered the sync
> request for it via sgmrextend before meta page update.

and could it happen that during smgrextend() for the last page, a
checkpoint starts and finishes between FileWrite() and
register_dirty_segment(), then index build finishes, and then a crash
occurs before another checkpoint completes the pending fsync for that
last page?