Re: pgsql: Track last_inactive_time in pg_replication_slots.

On Wed, Mar 27, 2024 at 10:33:28AM -0400, Robert Haas wrote:
> FWIW, I thought the time-based one sounded more useful. I think it
> would be poor planning to say "well, if the slot reaches an XID age of
> a billion, kill it so we don't wrap around," because while that likely
> will prevent me from getting into wraparound trouble, my database is
> likely to become horribly bloated long before the cutoff is reached. I
> thought it would be easier to reason in terms of time: I don't expect
> a slave to ever be down for more than X period of time, say an hour or
> whatever, so if it is, forget about it. Or alternatively, I know that
> if a slave does go down for more than X period of time, I start to get
> bloat, so cut it off at that point and I'll rebuild it later. I feel
> like these are things where people's intuition is going to be much
> stronger when reckoning in units of wall-clock time, which everyone
> deals with every day in one way or another, rather than in XID-based
> units that are, at least in my view, just a lot less intuitive.

I don't disagree with this point in the context of a user who is managing a
single server or just a handful of servers.  They are going to understand
their workload best and can reason about the right value for the timeout.
I think they'd still benefit from having an XID-based setting as a backstop
in case the timeout is still not sufficient to prevent wraparound, but it's
not nearly as important in that case.

IMHO the use-case where this doesn't work so well is when you have many,
many servers to administer (e.g., a cloud provider).  In those cases,
picking a default timeout to try to prevent wraparound is going to be much
less accurate, as any reasonable value you pick is still going to be
insufficient in some cases.  I think the XID-based parameter would be
better here; if the server is at imminent risk of an outage due to
wraparound, invalidating the slots is probably a reasonable course of
action.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com