Re: PATCH: Exclude unlogged tables from base backups

On 2017-12-12 18:18:09 -0500, David Steele wrote:
> On 12/12/17 6:07 PM, Andres Freund wrote:
> > >
> > > I don't see this as any different than what happens during recovery. The
> > > unlogged forks are cleaned / re-inited before replay starts which is the
> > > same thing we are doing here.
> >
> > It's quite different - in the recovery case there's no other write
> > activity going on. But on a normally running cluster the persistence of
> > existing tables can get changed, and oids can get recycled. What
> > guarantees that between the time you checked for the init fork the table
> > hasn't been dropped, the oid reused and now a permanent relation is in
> > its place?
>
> Well, that's a good point!
>
> How about rechecking the presence of the init fork after a main/other fork
> has been found? Is it possible for an init fork to still be lying around
> after an oid has been recycled? Seems like it could be...

I don't see how that'd help. You could just have gone through this cycle
multiple times by the time you get to rechecking. All not very likely,
but I don't want us to rely on luck here...

If we had a way to prevent relfilenode reuse across multiple checkpoints
this'd be easier, although ALTER TABLE SET UNLOGGED still'd complicate.
I guess we could have the basebackup create placeholder files that
prevent relfilenode reuse, but that seems darned ugly.

Greetings,

Andres Freund