Re: PageIsAllVisible()'s trustworthiness in Hot Standby

On Tue, Dec 4, 2012 at 9:00 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> Youre right, it currently seems to be possible, there's no LSN interlock
> prohibiting this as far as I can see.

Yeah, there certainly isn't that.  Now you could perhaps make an
argument that no operation that can propagate a set bit from master to
standby can arrive until after the standby's xmin horizon is
sufficiently current, but that does feel a little fragile to me...
even if it's true now, new WAL record types might break it, for
example.

> in lazy_scan_heap we do:
>
>                         if (!PageIsAllVisible(page))
>                         {
>                                 PageSetAllVisible(page);
>                                 MarkBufferDirty(buf);
>                                 visibilitymap_set(onerel, blkno, InvalidXLogRecPtr, vmbuffer,
>                                                                   visibility_cutoff_xid);
>                         }
>
> So buf can be written out independently from the xlog record that
> visibilitymap_set emits. ISTM we should set the LSN of the heap page as
> well as the one of the visibilitymap page. Not sure about the
> performance impact of that.

It would result in a massive increase in WAL traffic when vacuuming an
insert-only table; that's why we didn't do it originally.

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