Re: Mention idle_replication_slot_timeout in pg_replication_slots docs

On Mon, Jun 30, 2025 at 6:12 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
>
>
> On 2025/06/30 20:32, Nisha Moond wrote:
> > On Fri, Jun 27, 2025 at 5:40 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>
> >>
> >>
> >> On 2025/06/27 15:32, Nisha Moond wrote:
> >>> On Thu, Jun 26, 2025 at 1:33 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2025/06/26 15:46, Nisha Moond wrote:
> >>>>> On Wed, Jun 25, 2025 at 9:56 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> The pg_replication_slots documentation mentions only max_slot_wal_keep_size
> >>>>>> as a condition under which the wal_status column can show unreserved or lost.
> >>>>>> However, since commit ac0e33136ab, idle_replication_slot_timeout can also
> >>>>>> cause this behavior when it is set. This has not been documented yet.
> >>>>>> https://www.postgresql.org/docs/devel/view-pg-replication-slots.html
> >>>>>>
> >>>>>
> >>>>> +1 to the doc update.
> >>>>
> >>>> Thanks for the review!
> >>>>
> >>>>
> >>>>>> So, how about updating the documentation to also mention
> >>>>>> idle_replication_slot_timeout as a factor that can cause wal_status to
> >>>>>> become unreserved or lost? Patch attached.
> >>>>>>
> >>>>>
> >>>>> Since idle_replication_slot_timeout can only cause wal_status to
> >>>>> become 'lost' and not 'unreserved', perhaps we can reword the sentence
> >>>>> slightly for clarity, suggestion -
> >>>>> "The last two states are seen when max_slot_wal_keep_size is
> >>>>> non-negative and, the 'lost' state may also appear when
> >>>>> idle_replication_slot_timeout is greater than zero."
> >>>>
> >>>> I was thinking that when idle_replication_slot_timeout triggers,
> >>>> the following functions are called, and that wal_status can become
> >>>> "unreserved" before ReplicationSlotRelease() runs. It's very short
> >>>> period, though. Am I wrong?
> >>>>
> >>>> ReplicationSlotMarkDirty();
> >>>> ReplicationSlotSave();
> >>>> ReplicationSlotRelease();
> >>>>
> >>>
> >>> Thank you for pointing it out.
> >>> You are correct that while the checkpointer is in the process of
> >>> invalidating a slot, it sets its PID as the slot’s active_pid. During
> >>> this short window, if a user queries pg_replication_slot, the
> >>> underlying function pg_get_replication_slots will compute the
> >>> wal_status as 'unreserved' for the invalidated slot because the slot
> >>> has a valid active_pid.
> >>>
> >>> That said, it's reasonable to mention in the doc that 'unreserved' may
> >>> appear when idle_replication_slot_timeout is greater than zero, as
> >>> this can indeed happen. So, let's retain the current description.
> >>>
> >>> However, this behavior isn’t specific to
> >>> idle_replication_slot_timeout. For example, when a slot is being
> >>> invalidated due to a different cause "wal_level_insufficient",
> >>> 'unreserved' may also briefly appear in wal_status.
> >>
> >> Yes, and "lost" can appear for various reasons, including wal_level_insufficient,
> >> so it seems odd to highlight max_slot_wal_keep_size as the cause of the "lost"
> >> status in the note. It would probably be better to remove the mention of "lost"
> >> from that note.
> >>
> >
> > +1
>
> Is this true starting from v16, when logical replication from standby was introduced?
> In other words, in v15 and earlier, only max_slot_wal_keep_size could cause
> the wal_status to become "unreserved" or "lost"? I'm wondering where to back-patch
> this fix to.
>

I also think we should back-patch this till v16, since that’s when
additional slot invalidation causes were also introduced(commit
be87200). And since then “max_slot_wal_keep_size” is no longer the
sole reason for “unreserved” or “lost” status.

--
Thanks,
Nisha