Re: Synchronizing slots from primary to standby

Hi,

On 11/21/23 6:16 AM, Amit Kapila wrote:
> On Mon, Nov 20, 2023 at 6:51 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.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).

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com