Re: Early locking option to parallel backup

On 2017-11-05 17:38:39 -0500, Robert Haas wrote:
> On Sun, Nov 5, 2017 at 5:17 AM, Lucas <lucas75(at)gmail(dot)com> wrote:
> > The patch creates a "--lock-early" option which will make pg_dump to issue
> > shared locks on all tables on the backup TOC on each parallel worker start.
> > That way, the backup has a very small chance of failing. When it does,
> > happen in the first few seconds of the backup job. My backup scripts (not
> > included here) are aware of that and retries the backup in case of failure.
>
> I wonder why we don't do this already ... and by default.

Well, the current approach afaics requires #relations * 2 locks, whereas
acquiring them in every worker would scale that with the number of
workers. IIUC the problem here is that even though a lock is already
held by the main backend an independent locker's request will prevent
the on-demand lock by the dump worker from being granted. It seems to
me the correct fix here would be to somehow avoid the fairness logic in
the parallel dump case - although I don't quite know how to best do so.

Greetings,

Andres Freund