On 2020/06/30 17:07, Fujii Masao wrote:
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:
https://www.postgresql.org/message-id/20200619121552.gh453...@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.
Sorry this is not true. That distance can be calculated without those operators.
For example,
SELECT restart_lsn - pg_current_wal_lsn() + (SELECT setting::numeric * 1024 *
1024 FROM pg_settings WHERE name = 'max_slot_wal_keep_size') distance FROM
pg_replication_slots;
If the calculated distance is small or negative value, which means that
we may lose some required WAL files. So in this case it's worth considering
to increase max_slot_wal_keep_size.
I still think it's better and more helpful to display something like
that distance in pg_replication_slots rather than making each user
calculate it...
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
