Re: [HACKERS] WIP: long transactions on hot standby feedback replica/ proof of concept

On Fri, Aug 17, 2018 at 9:55 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Alexander Korotkov <a.korotkov@postgrespro.ru> writes:
> > On Fri, Aug 17, 2018 at 8:38 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> Alexander Korotkov <a.korotkov@postgrespro.ru> writes:
> >>> Yes, that's correct.  On standby read-only queries can tolerate
> >>> concurrent heap truncation.
>
> >> Uh, what???
>
> > VACUUM truncates heap relation only after deletion of all the tuples
> > from tail pages.
>
> Right; the problem I'm worried about is possible accesses to
> already-removed pages by concurrent scans.
>
> > And in md.c we already have logic to return zeroed pages, when trying
> > to read past relation in recovery.
>
> But then you are injecting bad pages into the shared buffer arena.
> In any case, depending on that behavior seems like a bad idea, because
> it's a pretty questionable kluge in itself.
>
> Another point is that the truncation code attempts to remove all
> to-be-truncated-away pages from the shared buffer arena, but that only
> works if nobody else is loading such pages into shared buffers
> concurrently.  In the presence of concurrent scans, we might be left
> with valid-looking buffers for pages that have been truncated away
> on-disk.  That could cause all sorts of fun later.  Yeah, the buffers
> should contain only dead tuples ... but, for example, they might not
> be hinted dead.  If somebody sets one of those hint bits and then
> writes the buffer back out to disk, you've got real problems.
>
> Perhaps there's some gold to be mined by treating truncation locks
> differently from other AELs, but I don't think you can just ignore
> them on the standby, any more than they can be ignored on the master.

Thank you for the explanation.  I see that injecting past OEF pages
into shared buffers doesn't look good.  I start thinking about letting
caller of ReadBuffer() (or its variation) handle past OEF situation.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company