Re: Should we remove vacuum_defer_cleanup_age?

On Sat, Mar 18, 2023 at 12:09 AM Andres Freund <andres@anarazel.de> wrote:

> Hi,
>
> As evidenced by the bug fixed in be504a3e974, vacuum_defer_cleanup_age is
> not
> heavily used - the bug was trivial to hit as soon as
> vacuum_defer_cleanup_age
> is set to a non-toy value. It complicates thinking about visibility
> horizons
> substantially, as vacuum_defer_cleanup_age can make them go backward
> substantially. Obviously it's also severely undertested.
>
> I started writing a test for vacuum_defer_cleanup_age while working on the
> fix
> referenced above, but now I am wondering if said energy would be better
> spent
> removing vacuum_defer_cleanup_age alltogether.
>
> vacuum_defer_cleanup_age was added as part of hot standby. Back then we did
> not yet have hot_standby_feedback. Now that that exists,
> vacuum_defer_cleanup_age doesn't seem like a good idea anymore. It's
> pessimisistic, i.e. always retains rows, even if none of the standbys has
> an
> old enough snapshot.
>
> The only benefit of vacuum_defer_cleanup_age over hot_standby_feedback is
> that
> it provides a limit of some sort. But transactionids aren't producing dead
> rows in a uniform manner, so limiting via xid isn't particularly useful.
> And
> even if there are use cases, it seems those would be served better by
> introducing a cap on how much hot_standby_feedback can hold the horizon
> back.
>
> I don't think I have the cycles to push this through in the next weeks,
> but if
> we agree removing vacuum_defer_cleanup_age is a good idea, it seems like a
> good idea to mark it as deprecated in 16?
>

+1. I haven't seen any (correct) use of this in many many years on my end
at least.

And yes, having a cap on hot_standby_feedback would also be great.

-- 
 Magnus Hagander
 Me: https://www.hagander.net/ <http://www.hagander.net/>
 Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>