Re: Unlogged relation copy is not fsync'd

On Fri, Aug 25, 2023 at 8:47 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> 1. Create an unlogged table
> 2. ALTER TABLE unlogged_tbl SET TABLESPACE ... -- This calls
> RelationCopyStorage
> 3. a checkpoint happens while the command is running
> 4. After the ALTER TABLE has finished, shut down postgres cleanly.
> 5. Lose power
>
> When you recover, the unlogged table is not reset, because it was a
> clean postgres shutdown. But the part of the file that was copied after
> the checkpoint at step 3 was never fsync'd, so part of the file is lost.
> I was able to reproduce with a VM that I kill to simulate step 5.
>
> This is the same scenario that the smgrimmedsync() call above protects
> from for WAL-logged relations. But we got the condition wrong: instead
> of worrying about the init fork, we need to call smgrimmedsync() for all
> *other* forks of an unlogged relation.
>
> Fortunately the fix is trivial, see attached.

The general rule throughout the system is that the init-fork of an
unlogged relation is treated the same as a permanent relation: it is
WAL-logged and fsyncd. But the other forks of an unlogged relation are
neither WAL-logged nor fsync'd ... except in the case of a clean
shutdown, when we fsync even that data.

In other words, somehow it feels like we ought to be trying to defer
the fsync here until a clean shutdown actually occurs, instead of
performing it immediately. Admittedly, the bookkeeping seems like a
problem, so maybe this is the best we can do, but it's clearly worse
than what we do in other cases.

--
Robert Haas
EDB: http://www.enterprisedb.com