Re: min_safe_lsn column in pg_replication_slots view

On 2020/06/26 13:45, Amit Kapila wrote:
> On Fri, Jun 26, 2020 at 4:54 AM Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>>
>> On 2020-Jun-26, Michael Paquier wrote:
>>
>>> On Thu, Jun 25, 2020 at 11:24:27AM -0400, Alvaro Herrera wrote:
>>>> I don't understand the proposal. Michael posted a patch that adds
>>>> pg_wal_oldest_lsn(), and you say we should apply the patch except the
>>>> part that adds that function -- so what part would be applying?
>>>
>>> I have sent last week a patch about only the removal of min_safe_lsn:
>>> https://www.postgresql.org/message-id/20200619121552.GH453547@paquier.xyz
>>> So this applies to this part.
>>
>> Well, I oppose that because it leaves us with no way to monitor slot
>> limits. In his opening email, Masao-san proposed to simply change the
>> value by adding 1. How you go from adding 1 to a column to removing
>> the column completely with no recourse, is beyond me.
>>
>> Let me summarize the situation and possible ways forward as I see them.
>> If I'm mistaken, please correct me.
>>
>> Problems:
>> i) pg_replication_slot.min_safe_lsn has a weird definition in that all
>> replication slots show the same value
>>
>
> It is also not clear how the user can make use of that value?
>
>> ii) min_safe_lsn cannot be used with pg_walfile_name, because it returns
>> the name of the previous segment.
>>
>> Proposed solutions:
>>
>> a) Do nothing -- keep the min_safe_lsn column as is. Warn users that
>> pg_walfile_name should not be used with this column.
>> b) Redefine min_safe_lsn to be lsn+1, so that pg_walfile_name can be used
>> and return a useful value.
>> c) Remove min_safe_lsn; add functions that expose the same value
>> d) Remove min_safe_lsn; add a new view that exposes the same value and
>> possibly others
>>
>> e) Replace min_safe_lsn with a "distance" column, which reports
>> restart_lsn - oldest valid LSN
>> (Note that you no longer have an LSN in this scenario, so you can't
>> call pg_walfile_name.)

I like (e).

>
> Can we consider an option to "Remove min_safe_lsn; document how a user
> can monitor the distance"? We have a function to get current WAL
> insert location and other things required are available either via
> view or as guc variable values. The reason I am thinking of this
> option is that it might be better to get some more feedback on what is
> the most appropriate value to display. However, I am okay if we can
> reach a consensus on one of the above options.

Yes, that's an idea. But it might not be easy to calculate that distance
manually by subtracting max_slot_wal_keep_size from the current LSN.
Because we've not supported -(pg_lsn, numeric) operator yet. I'm
proposing that operator, but it's for v14.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION