Re: Concurrency issue in pg_rewind

Hi,

On Fri, 18 Sep 2020 at 08:59, Michael Paquier <michael(at)paquier(dot)xyz> wrote:

> If this stuff is willing to do so, you may have your reasons, but even
> if you wish to locate both pg_wal/ and the prefetch path in the same
> partition, I don't get why it is necessary to have the prefetch path
> included directly in pg_wal? You could just use different paths for
> both. Say, with a base partition at /my/path/, you can just have
> /my/path/pg_wal/ that the Postgres backend links to, and
> /my/path/wal-g/prefetch/ for the secondary path.

Well, I agree and disagree at the same time.
Yes, it would be nice not to write anything unexpected to PGDATA and
pg_wal, but this is also a usability issue.
Creating a separate directory and configuring wal-e/wal-g to use it
(now it is not even possible to configure it), requires some effort
from the administrator, while using something inside pg_wal just
works.

At the same time, pg_rewind due to such "fatal" error leaves PGDATA in
an inconsistent state with empty pg_control file, this is totally bad
and easily fixable. We want the specific file to be absent and it is
already absent, why should it be a fatal error and not warning?

Regards,
--
Alexander Kukushkin