Date: Mon, 25 Nov 2019 16:50:25 -0500
From: Noah Misch <noah@leadboat.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	Andrew Dunstan <andrew.dunstan@2ndquadrant.com>,
	hlinnaka <hlinnaka@iki.fi>, Michael Paquier <michael@paquier.xyz>
Subject: Re: [HACKERS] WAL logging problem in 9.4.3?
Message-ID: <20191125215025.GA53580@gust.leadboat.com>
References: 
 <CA+TgmoYzTPpxvPUUs_ALW1YYNxV6mCrj6zwqnaN6FqdNc1JUzw@mail.gmail.com>
 <20191105225335.GA395764@rfd.leadboat.com>
 <20191118045434.GA1173436@rfd.leadboat.com>
 <20191121.160107.1405593316918593372.horikyota.ntt@gmail.com>
 <20191123212136.GA41522@gust.leadboat.com>
 <CA+TgmoYzjPYCyU5o6XQ1B6JGhYM8MXGNGxhPpz=tFYdHEHGeGA@mail.gmail.com>
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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@leadboat.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.