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

On Mon, Mar 18, 2024 at 9:58 AM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Sat, Mar 16, 2024 at 3:55 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > > What's proposed with max_slot_xid_age is that during checkpoint we
> > > look at slot's xmin and catalog_xmin, and the current system txn id.
> > > Then, if the XID age of (xmin, catalog_xmin) and current_xid crosses
> > > max_slot_xid_age, we invalidate the slot.
> > >
> >
> > I can see that in your patch (in function
> > InvalidatePossiblyObsoleteSlot()). As per my understanding, we need
> > something similar for slot xids in ComputeXidHorizons() as we are
> > doing WAL in KeepLogSeg(). In KeepLogSeg(), we compute the minimum LSN
> > location required by slots and then adjust it for
> > 'max_slot_wal_keep_size'. On similar lines, currently in
> > ComputeXidHorizons(), we compute the minimum xid required by slots
> > (procArray->replication_slot_xmin and
> > procArray->replication_slot_catalog_xmin) but then don't adjust it for
> > 'max_slot_xid_age'. I could be missing something in this but it is
> > better to keep discussing this
>
> After invalidating slots because of max_slot_xid_age, the
> procArray->replication_slot_xmin and
> procArray->replication_slot_catalog_xmin are recomputed immediately in
> InvalidateObsoleteReplicationSlots->ReplicationSlotsComputeRequiredXmin->ProcArraySetReplicationSlotXmin.
> And, later the XID horizons in ComputeXidHorizons are computed before
> the vacuum on each table via GetOldestNonRemovableTransactionId.
> Aren't these enough?
>

IIUC, this will be delayed by one cycle in the vacuum rather than
doing it when the slot's xmin age is crossed and it can be
invalidated.

Do you want the XID horizons recomputed
> immediately, something like the below?
>

I haven't thought of the exact logic but we can try to mimic the
handling similar to WAL.

--
With Regards,
Amit Kapila.