Re: Synchronizing slots from primary to standby

On Tue, Nov 21, 2023 at 1:13 PM Drouvot, Bertrand
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> On 11/21/23 6:16 AM, Amit Kapila wrote:
> > On Mon, Nov 20, 2023 at 6:51 PM Drouvot, Bertrand
> > <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >> As far the 'i' state here, from what I see, it is currently useful for:
> >>
> >> 1. Cascading standby to not sync slots with state = 'i' from
> >> the first standby.
> >> 2. Easily report Slots that did not catch up on the primary yet.
> >> 3. Avoid inactive slots to block "active" ones creation.
> >>
> >> So not creating those slots should not be an issue for 1. (sync are
> >> not needed on cascading standby as not created on the first standby yet)
> >> but is an issue for 2. (unless we provide another way to keep track and report
> >> such slots) and 3. (as I think we should still need to reserve WAL).
> >>
> >> I've a question: we'd still need to reserve WAL for those slots, no?
> >>
> >> If that's the case and if we don't call ReplicationSlotCreate() then ReplicationSlotReserveWal()
> >> would not work as MyReplicationSlot would be NULL.
> >>
> >
> > Yes, we need to reserve WAL to see if we can sync the slot. We are
> > currently creating an RS_EPHEMERAL slot and if we don't explicitly
> > persist it when we can't sync, then it will be dropped when we do
> > ReplicationSlotRelease() at the end of synchronize_one_slot(). So, the
> > loss is probably, the next time we again try to sync the slot, we need
> > to again create it and may need to wait for newer restart_lsn on
> > standby
>
> Yeah, and doing so we'd reduce the time window to give the slot a chance
> to catch up (as opposed to create it a single time and maintain an 'i' state).
>
> > which could be avoided if we have the slot in 'i' state from
> > the previous run.
>
> Right.
>
> > I don't deny the importance of having 'i'
> > (initialized) state but was just trying to say that it has additional
> > code complexity.
>
> Right, and I think it's worth it.
>
> > OTOH, having it may give better visibility to even
> > users about slots that are not active (say manually created slots on
> > the primary).
>
> Agree.
>
> All that being said, on my side I'm +1 on keeping the 'i' state behavior
> as it is implemented currently (would be happy to hear others' opinions too).
>

+1 for 'i' state. I feel it gives a better slot-sync functionality
(optimizing redo-effort for inactive slots, inactive not blocking
active ones) along with its usage for monitoring purposes.