MDEV-27734 Set innodb_change_buffering=none by default

The aim of the InnoDB change buffer is to avoid delays when a leaf page
of a secondary index is not present in the buffer pool, and a record needs
to be inserted, delete-marked, or purged. Instead of reading the page into
the buffer pool for making such a modification, we may insert a record to
the change buffer (a special index tree in the InnoDB system tablespace).
The buffered changes are guaranteed to be merged if the index page
actually needs to be read later.

The change buffer could be useful when the database is stored on a
rotational medium (hard disk) where random seeks are slower than
sequential reads or writes.

Obviously, the change buffer will cause write amplification, due to
potentially large amount of metadata that is being written to the
change buffer. We will have to write redo log records for modifying
the change buffer tree as well as the user tablespace. Furthermore,
in the user tablespace, we must maintain a change buffer bitmap page
that uses 2 bits for estimating the amount of free space in pages,
and 1 bit to specify whether buffered changes exist. This bitmap needs
to be updated on every operation, which could reduce performance.

Even if the change buffer were free of bugs such as MDEV-24449
(potentially causing the corruption of any page in the system tablespace)
or MDEV-26977 (corruption of secondary indexes due to a currently
unknown reason), it will make diagnosis of other data corruption harder.

Because of all this, it is best to disable the change buffer by default.

6 files changed, 12 insertions, 8 deletions

diff --git a/mysql-test/suite/innodb/r/ibuf_not_empty.result b/mysql-test/suite/innodb/r/ibuf_not_empty.result
index d1b8203b063..f2da89990b0 100644
--- a/mysql-test/suite/innodb/r/ibuf_not_empty.result
+++ b/mysql-test/suite/innodb/r/ibuf_not_empty.result
@@ -5,6 +5,7 @@ c INT,
 INDEX(b))
 ENGINE=InnoDB STATS_PERSISTENT=0;
 SET GLOBAL innodb_change_buffering_debug = 1;
+SET GLOBAL innodb_change_buffering=all;
 INSERT INTO t1 SELECT 0,'x',1 FROM seq_1_to_1024;
 # restart: --innodb-force-recovery=6 --innodb-change-buffer-dump
 check table t1;
diff --git a/mysql-test/suite/innodb/r/innodb-change-buffer-recovery.result b/mysql-test/suite/innodb/r/innodb-change-buffer-recovery.result
index 678c8c67be5..f676d15b134 100644
--- a/mysql-test/suite/innodb/r/innodb-change-buffer-recovery.result
+++ b/mysql-test/suite/innodb/r/innodb-change-buffer-recovery.result
@@ -13,6 +13,7 @@ c INT,
 INDEX(b))
 ENGINE=InnoDB STATS_PERSISTENT=0;
 SET GLOBAL innodb_change_buffering_debug = 1;
+SET GLOBAL innodb_change_buffering = all;
 INSERT INTO t1 SELECT 0,'x',1 FROM seq_1_to_8192;
 BEGIN;
 SELECT b FROM t1 LIMIT 3;
diff --git a/mysql-test/suite/innodb/t/ibuf_not_empty.test b/mysql-test/suite/innodb/t/ibuf_not_empty.test
index 545a78c887e..96ceb81ac00 100644
--- a/mysql-test/suite/innodb/t/ibuf_not_empty.test
+++ b/mysql-test/suite/innodb/t/ibuf_not_empty.test
@@ -24,6 +24,7 @@ ENGINE=InnoDB STATS_PERSISTENT=0;
 # change buffering is possible, so that the change buffer will be used
 # whenever possible.
 SET GLOBAL innodb_change_buffering_debug = 1;
+SET GLOBAL innodb_change_buffering=all;
 
 # Create enough rows for the table, so that the change buffer will be
 # used for modifying the secondary index page. There must be multiple
diff --git a/mysql-test/suite/innodb/t/innodb-change-buffer-recovery.test b/mysql-test/suite/innodb/t/innodb-change-buffer-recovery.test
index a12ca43cec1..129037e783b 100644
--- a/mysql-test/suite/innodb/t/innodb-change-buffer-recovery.test
+++ b/mysql-test/suite/innodb/t/innodb-change-buffer-recovery.test
@@ -33,6 +33,7 @@ ENGINE=InnoDB STATS_PERSISTENT=0;
 # change buffering is possible, so that the change buffer will be used
 # whenever possible.
 SET GLOBAL innodb_change_buffering_debug = 1;
+SET GLOBAL innodb_change_buffering = all;
 let SEARCH_FILE = $MYSQLTEST_VARDIR/log/mysqld.1.err;
 
 # Create enough rows for the table, so that the change buffer will be
diff --git a/mysql-test/suite/sys_vars/r/innodb_change_buffering_basic.result b/mysql-test/suite/sys_vars/r/innodb_change_buffering_basic.result
index 92e22c6aa34..c11f4ee617c 100644
--- a/mysql-test/suite/sys_vars/r/innodb_change_buffering_basic.result
+++ b/mysql-test/suite/sys_vars/r/innodb_change_buffering_basic.result
@@ -1,28 +1,28 @@
 SET @start_global_value = @@global.innodb_change_buffering;
 SELECT @start_global_value;
 @start_global_value
-all
+none
 Valid values are 'all', 'deletes', 'changes', 'inserts', 'none', 'purges'
 select @@global.innodb_change_buffering in ('all', 'deletes', 'changes', 'inserts', 'none', 'purges');
 @@global.innodb_change_buffering in ('all', 'deletes', 'changes', 'inserts', 'none', 'purges')
 1
 select @@global.innodb_change_buffering;
 @@global.innodb_change_buffering
-all
+none
 select @@session.innodb_change_buffering;
 ERROR HY000: Variable 'innodb_change_buffering' is a GLOBAL variable
 show global variables like 'innodb_change_buffering';
 Variable_name	Value
-innodb_change_buffering	all
+innodb_change_buffering	none
 show session variables like 'innodb_change_buffering';
 Variable_name	Value
-innodb_change_buffering	all
+innodb_change_buffering	none
 select * from information_schema.global_variables where variable_name='innodb_change_buffering';
 VARIABLE_NAME	VARIABLE_VALUE
-INNODB_CHANGE_BUFFERING	all
+INNODB_CHANGE_BUFFERING	none
 select * from information_schema.session_variables where variable_name='innodb_change_buffering';
 VARIABLE_NAME	VARIABLE_VALUE
-INNODB_CHANGE_BUFFERING	all
+INNODB_CHANGE_BUFFERING	none
 set global innodb_change_buffering='none';
 select @@global.innodb_change_buffering;
 @@global.innodb_change_buffering
@@ -62,4 +62,4 @@ ERROR 42000: Variable 'innodb_change_buffering' can't be set to the value of 'so
 SET @@global.innodb_change_buffering = @start_global_value;
 SELECT @@global.innodb_change_buffering;
 @@global.innodb_change_buffering
-all
+none
diff --git a/mysql-test/suite/sys_vars/r/sysvars_innodb.result b/mysql-test/suite/sys_vars/r/sysvars_innodb.result
index ecd107f67c8..c333b56fd2e 100644
--- a/mysql-test/suite/sys_vars/r/sysvars_innodb.result
+++ b/mysql-test/suite/sys_vars/r/sysvars_innodb.result
@@ -311,7 +311,7 @@ READ_ONLY	NO
 COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	INNODB_CHANGE_BUFFERING
 SESSION_VALUE	NULL
-DEFAULT_VALUE	all
+DEFAULT_VALUE	none
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	ENUM
 VARIABLE_COMMENT	Buffer changes to secondary indexes.