MDEV-13674: Deprecate innodb_use_mtflush and innodb_mtflush_threads

These parameters and associated code is to be removed in 10.3.
Users can use innodb-page-cleaners > 1 instead.

mysql-test/suite/sys_vars/r/sysvars_innodb.result

@@ -2029,7 +2029,7 @@ GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE	8
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BIGINT
-VARIABLE_COMMENT	Number of multi-threaded flush threads
+VARIABLE_COMMENT	DEPRECATED. Number of multi-threaded flush threads
 NUMERIC_MIN_VALUE	1
 NUMERIC_MAX_VALUE	64
 NUMERIC_BLOCK_SIZE	0
@@ -3037,7 +3037,7 @@ GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE	OFF
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	BOOLEAN
-VARIABLE_COMMENT	Use multi-threaded flush. Default FALSE.
+VARIABLE_COMMENT	DEPRECATED. Use multi-threaded flush. Default FALSE.
 NUMERIC_MIN_VALUE	NULL
 NUMERIC_MAX_VALUE	NULL
 NUMERIC_BLOCK_SIZE	NULL

storage/innobase/handler/ha_innodb.cc

@@ -3751,6 +3751,16 @@ static const char*	deprecated_instrument_semaphores
 	= "Using innodb_instrument_semaphores is deprecated"
 	" and the parameter will be removed in MariaDB 10.3.";
 
+static const char*	deprecated_use_mtflush
+	= "Using innodb_use_mtflush is deprecated"
+	" and the parameter will be removed in MariaDB 10.3."
+	" Use innodb-page-cleaners instead. ";
+
+static const char*	deprecated_mtflush_threads
+	= "Using innodb_mtflush_threads is deprecated"
+	" and the parameter will be removed in MariaDB 10.3."
+	" Use innodb-page-cleaners instead. ";
+
 static my_bool innodb_instrument_semaphores;
 
 /** Update log_checksum_algorithm_ptr with a pointer to the function
@@ -4111,6 +4121,14 @@ innobase_init(
 		ib::warn() << deprecated_instrument_semaphores;
 	}
 
+	if (srv_use_mtflush) {
+		ib::warn() << deprecated_use_mtflush;
+	}
+
+	if (srv_use_mtflush && srv_mtflush_threads != MTFLUSH_DEFAULT_WORKER) {
+		ib::warn() << deprecated_mtflush_threads;
+	}
+
 	/* Validate the file format by animal name */
 	if (innobase_file_format_name != NULL) {
 
@@ -21615,7 +21633,7 @@ static MYSQL_SYSVAR_ENUM(compression_algorithm, innodb_compression_algorithm,
 
 static MYSQL_SYSVAR_LONG(mtflush_threads, srv_mtflush_threads,
   PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
-  "Number of multi-threaded flush threads",
+  "DEPRECATED. Number of multi-threaded flush threads",
   NULL, NULL,
   MTFLUSH_DEFAULT_WORKER, /* Default setting */
   1,                      /* Minimum setting */
@@ -21624,7 +21642,7 @@ static MYSQL_SYSVAR_LONG(mtflush_threads, srv_mtflush_threads,
 
 static MYSQL_SYSVAR_BOOL(use_mtflush, srv_use_mtflush,
   PLUGIN_VAR_NOCMDARG | PLUGIN_VAR_READONLY,
-  "Use multi-threaded flush. Default FALSE.",
+  "DEPRECATED. Use multi-threaded flush. Default FALSE.",
   NULL, NULL, FALSE);
 
 static MYSQL_SYSVAR_ULONG(fatal_semaphore_wait_threshold, srv_fatal_semaphore_wait_threshold,