Re: Document that vacuum can't truncate if old_snapshot_threshold >= 0

On Wed, Jul 13, 2016 at 4:14 PM, Andres Freund <andres@anarazel.de> wrote:

> Currently, if old_snapshot_threshold is enabled, vacuum is prevented
> from truncating tables:
> static bool
> should_attempt_truncation(LVRelStats *vacrelstats)
> {
>         BlockNumber possibly_freeable;
>
>         possibly_freeable = vacrelstats->rel_pages - vacrelstats->nonempty_pages;
>         if (possibly_freeable > 0 &&
>                 (possibly_freeable >= REL_TRUNCATE_MINIMUM ||
>           possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION) &&
>                 old_snapshot_threshold < 0)
>                 return true;
>         else
>                 return false;
> }
>
> (note the old_snapshot_threshold < 0 condition).
>
> That appears to not be mentioned in a comment, the commit message or the
> the docs. I think this definitely needs to be prominently documented.
>
> FWIW, afaics that's required because new pages don't have an LSN, so we
> can't necessarily detect that a truncated and re-extended relation,
> wouldn't be valid. Although I do wonder if there isn't a less invasive
> way to do that.

There would be no problem on re-extended pages because they would
have a new enough LSN to cause a "snapshot too old" error; it is
when they are missing that the problem exists.  The possible
alternative that looked best to me was to leave a "guard page" to
cover sequential scans, with LSN set to (at least) the latest of
itself or any truncated page.  I think WAL would need to be
generated to cover the LSN update.  If you combined that with
treating an index leaf tuple pointing to a missing page as
"snapshot too old" I think things would work, but it's not clear to
me that it's worth the complexity.

Attached pushed.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company