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Message-ID: <6e30082e-041b-4e31-9633-95a66de76f5d@oss.nttdata.com>
Date: Wed, 3 Jul 2024 23:08:48 +0900
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To: pgsql-hackers@lists.postgresql.org
Content-Language: en-US
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Subject: Add a GUC check hook to ensure summarize_wal cannot be enabled when
 wal_level is minimal
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Hi,

The documentation states that "WAL summarization cannot be enabled when wal_level is set to minimal." Therefore, at startup, the postmaster checks these settings and exits with an error if they are not configured properly.

However, I found that summarize_wal can still be enabled while the server is running with wal_level=minimal. Please see the following example to cause this situation. I think this is a bug.


=# SHOW wal_level;
  wal_level
-----------
  minimal
(1 row)

=# SELECT * FROM pg_get_wal_summarizer_state();
  summarized_tli | summarized_lsn | pending_lsn | summarizer_pid
----------------+----------------+-------------+----------------
               0 | 0/0            | 0/0         |         (null)
(1 row)

=# ALTER SYSTEM SET summarize_wal TO on;
ALTER SYSTEM

=# SELECT pg_reload_conf();
  pg_reload_conf
----------------
  t
(1 row)

=# SELECT * FROM pg_get_wal_summarizer_state();
  summarized_tli | summarized_lsn | pending_lsn | summarizer_pid
----------------+----------------+-------------+----------------
               1 | 0/1492D80      | 0/1492DF8   |          12228
(1 row)


The attached patch adds a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal, fixing the issue.

Regards,

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