Re: Is it worth adding ReplicationSlot active_pid to ReplicationSlotPersistentData?

On Wed, Dec 1, 2021 at 9:50 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
>
> On 2021-Dec-01, Bharath Rupireddy wrote:
>
> > The active_pid of ReplicationSlot structure, which tells whether a
> > replication slot is active or inactive, isn't persisted to the disk
> > i.e has no entry in ReplicationSlotPersistentData structure. Isn't it
> > better if we add that info to ReplicationSlotPersistentData structure
> > and persist to the disk? This will help to know what were the inactive
> > replication slots in case the server goes down or crashes for some
> > reason. Currently, we don't have a way to interpret the replication
> > slot info in the disk but there's a patch for pg_replslotdata tool at
> > [1]. This way, one can figure out the reasons for the server
> > down/crash and figure out which replication slots to remove to bring
> > the server up and running without touching the other replication
> > slots.
>
> I think the PIDs are log-worthy for sure, but it's not clear to me that
> it is desirable to write them to the persistent state file. In case of
> crashes, the log should serve just fine to aid root cause investigation
> -- in fact even better than the persistent file, where the data would be
> lost as soon as the next client acquires that slot.

Thanks. +1 to log a message at LOG level whenever a replication slot
becomes active (gets assigned a valid pid to active_pid) and becomes
inactive(gets assigned 0 to active_pid). Having said that, isn't it
also helpful if we write a bool (1 byte character) whenever the slot
becomes active and inactive to the disk?

Regards,
Bharath Rupireddy.