Re: Reviving lost replication slots

On Tue, Nov 8, 2022 at 12:08 PM sirisha chamarthi
<sirichamarthi22(at)gmail(dot)com> wrote:
>
> On Fri, Nov 4, 2022 at 11:02 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>
>> On Fri, Nov 4, 2022 at 1:40 PM sirisha chamarthi
>> <sirichamarthi22(at)gmail(dot)com> wrote:
>> >
>> > A replication slot can be lost when a subscriber is not able to catch up with the load on the primary and the WAL to catch up exceeds max_slot_wal_keep_size. When this happens, target has to be reseeded (pg_dump) from the scratch and this can take longer. I am investigating the options to revive a lost slot.
>> >
>>
>> Why in the first place one has to set max_slot_wal_keep_size if they
>> care for WAL more than that?
>
> Disk full is a typical use where we can't wait until the logical slots to catch up before truncating the log.
>

Ideally, in such a case the subscriber should fall back to the
physical standby of the publisher but unfortunately, we don't yet have
a functionality where subscribers can continue logical replication
from physical standby. Do you think if we had such functionality it
would serve our purpose?

>> If you have a case where you want to
>> handle this case for some particular slot (where you are okay with the
>> invalidation of other slots exceeding max_slot_wal_keep_size) then the
>> other possibility could be to have a similar variable at the slot
>> level but not sure if that is a good idea because you haven't
>> presented any such case.
>
> IIUC, ability to fetch WAL from the archive as a fall back mechanism should automatically take care of all the lost slots. Do you see a need to take care of a specific slot?
>

No, I was just trying to see if your use case can be addressed in some
other way. BTW, won't copying the WAL again back from archive can lead
to a disk full situation.

--
With Regards,
Amit Kapila.