Re: [HACKERS] WAL logging problem in 9.4.3?

On Mon, Nov 25, 2019 at 03:58:14PM -0500, Robert Haas wrote:
> On Sat, Nov 23, 2019 at 4:21 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > I noticed an additional defect:
> >
> > BEGIN;
> > CREATE TABLE t (c) AS SELECT 1;
> > CHECKPOINT; -- write and fsync the table's one page
> > TRUNCATE t; -- no WAL
> > COMMIT; -- no FPI, just the commit record
> >
> > If we crash after the COMMIT and before the next fsync or OS-elected sync of
> > the table's file, the table will stay on disk with its pre-TRUNCATE content.
>
> Shouldn't the TRUNCATE be triggering an fsync() to happen before
> COMMIT is permitted to complete?

With wal_skip_threshold=0, you do get an fsync(). The patch tries to avoid
at-commit fsync of small files by WAL-logging file contents instead. However,
the patch doesn't WAL-log enough to handle files that decreased in size.

> You'd have the same problem if the
> TRUNCATE were replaced by INSERT, unless fsync() happens in that case.

I think an insert would be fine. You'd get an FPI record for the relation's
one page, which fully reproduces the relation.