On 2020/06/26 13:45, Amit Kapila wrote:
On Fri, Jun 26, 2020 at 4:54 AM Alvaro Herrera <alvhe...@2ndquadrant.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:
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.

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.


Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters

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