Re: Introduce XID age and inactive timeout based replication slot invalidation

On Tue, Feb 11, 2025 at 03:22:49PM +0100, Álvaro Herrera wrote:
> I find this proposed patch a bit strange and I feel it needs more
> explanation.
>
> When this thread started, Bharath justified his patches saying that a
> slot that's inactive for a very long time could be problematic because
> of XID wraparound. Fine, that sounds a reasonable feature. If you
> wanted to invalidate slots whose xmins were too old, I would support
> that. He submitted that as his 0004 patch then.
>
> However, he also chose to submit 0003 with invalidation based on a
> timeout. This is far less convincing a feature to me. The
> justification for the time out seems to be that ... it's difficult to
> have a one-size-fits-all value because size of disks vary. (???)
> Or something like that. Really? I mean -- yes, this will prevent
> problems in toy databases when run in developer's laptops. It will not
> prevent any problems in production databases. Do we really want a
> setting that is only useful for toy situations rather than production?
>
>
> Anyway, the thread is way too long, but after some initial pieces were
> committed, Nisha took over and submitting patches derived from Bharath's
> 0003, and at some point the initial 0004 was dropped. But 0004 was the
> more useful one, I thought, so what's going on?
>
> I'm baffled.

I agree, and I am also baffled because I think this discussion has happened
at least once already on this thread. I still feel like the XID-based
parameter makes more sense. For replication slots, two primary concerns
are 1) storage, for which we have max_slot_wal_keep_size and 2) XID
wraparound, for which we don't really have anything today. A timeout might
be useful in some contexts, but if the goal is to prevent wraparound, why
not target that directly?

--
nathan