Re: Early locking option to parallel backup

On 2017-11-05 22:43:34 -0500, Tom Lane wrote:
> > 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.
>
> I wonder if we couldn't somehow repurpose the work that was done for
> parallel workers' locks. Lots of security-type issues to be handled
> if we're to open that up to clients, but maybe it's solvable. For
> instance, maybe only allowing it to clients sharing the same snapshot
> would help.

Yea, I'd been wondering the same.

I'm slightly worried that somehow tying multiple clients into parallel
mode would cause a bunch of problems - that's not really the purpose of
the code and a number of its assumptions aren't quite right for that.

I'm not sure it really buys us much in contrast to just allowing a
locker to specify that it's allowed to jump the lock queue for an ASL if
it has 'backup' rights or such. Or actually, just allow it as a general
option to LOCK, there's plenty other operational cases where the current
"fair" behaviour is really annoying, e.g. when executing operational
DDL/DML and such.

Greetings,

Andres Freund