MDEV-24854: Change innodb_flush_method=O_DIRECT by default

We have innodb_use_native_aio=ON by default since the introduction of
that parameter in commit 2f9fb41b
(MySQL 5.5 and MariaDB 5.5).

However, to really benefit from the setting, the files should be
opened in O_DIRECT mode, to bypass the file system cache.
In this way, the reads and writes can be submitted with DMA, using
the InnoDB buffer pool directly, and no processor cycles need to be
used for copying data. The use of O_DIRECT benefits not only the
current libaio implementation, but also liburing.

os_file_set_nocache(): Test innodb_flush_method in the function,
not in the callers.

extra/mariabackup/xtrabackup.cc

@@ -1506,7 +1506,7 @@ struct my_option xb_server_options[] =
    "With which method to flush data.",
    &srv_file_flush_method, &srv_file_flush_method,
    &innodb_flush_method_typelib, GET_ENUM, REQUIRED_ARG,
-   IF_WIN(SRV_ALL_O_DIRECT_FSYNC, SRV_FSYNC), 0, 0, 0, 0, 0},
+   IF_WIN(SRV_ALL_O_DIRECT_FSYNC, SRV_O_DIRECT), 0, 0, 0, 0, 0},
 
   {"innodb_log_buffer_size", OPT_INNODB_LOG_BUFFER_SIZE,
    "The size of the buffer which InnoDB uses to write log to the log files on disk.",

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

 call mtr.add_suppression("InnoDB: Failed to set .*DIRECT");
 select @@innodb_flush_method;
 @@innodb_flush_method
-fsync
+O_DIRECT
 create table t(a serial) engine=innodb;
 FLUSH TABLES;
 # restart: --innodb-flush-method=5

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

@@ -743,7 +743,7 @@ READ_ONLY	NO
 COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	INNODB_FLUSH_METHOD
 SESSION_VALUE	NULL
-DEFAULT_VALUE	fsync
+DEFAULT_VALUE	O_DIRECT
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	ENUM
 VARIABLE_COMMENT	With which method to flush data.

sql-bench/example

@@ -6,9 +6,9 @@ machine="Linux-x64"
 
 # InnoDB tests
 
-./run-all-tests --suffix=-innodb --comments="Engine=InnoDB --innodb_buffer_pool_size=256M --innodb_log_file_size=2000M --innodb_log_buffer_size=16M --innodb_lock_wait_timeout=50 --innodb_flush_log_at_trx_commit=1 --innodb_flush_method=O_DIRECT --innodb_log_files_in_group=1 --skip-innodb-doublewrite" --create-options="ENGINE=InnoDB" --hw="$hw" --optimization="$optimization" --machine="$machine" --log
+./run-all-tests --suffix=-innodb --comments="Engine=InnoDB --innodb_buffer_pool_size=256M --innodb_log_file_size=2000M --innodb_log_buffer_size=16M --innodb_lock_wait_timeout=50 --innodb_flush_log_at_trx_commit=1 --create-options="ENGINE=InnoDB" --hw="$hw" --optimization="$optimization" --machine="$machine" --log
 
-./run-all-tests --suffix=_fast-innodb --comments="Engine=InnoDB --innodb_buffer_pool_size=256M --innodb_log_file_size=2000M --innodb_log_buffer_size=16M --innodb_lock_wait_timeout=50 --innodb_flush_log_at_trx_commit=1 --innodb_flush_method=O_DIRECT --innodb_log_files_in_group=1 --skip-innodb-doublewrite" --create-options="ENGINE=InnoDB" --hw="$hw" --optimization="$optimization" --machine="$machine" --fast --log
+./run-all-tests --suffix=_fast-innodb --comments="Engine=InnoDB --innodb_buffer_pool_size=256M --innodb_log_file_size=2000M --innodb_log_buffer_size=16M --innodb_lock_wait_timeout=50 --innodb_flush_log_at_trx_commit=1 --create-options="ENGINE=InnoDB" --hw="$hw" --optimization="$optimization" --machine="$machine" --fast --log
 
 # MyISAM tests
 

storage/innobase/handler/ha_innodb.cc

@@ -18308,7 +18308,7 @@ static MYSQL_SYSVAR_ULONG(flush_log_at_trx_commit, srv_flush_log_at_trx_commit,
 static MYSQL_SYSVAR_ENUM(flush_method, innodb_flush_method,
   PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,
   "With which method to flush data.",
-  NULL, NULL, IF_WIN(SRV_ALL_O_DIRECT_FSYNC, SRV_FSYNC),
+  NULL, NULL, IF_WIN(SRV_ALL_O_DIRECT_FSYNC, SRV_O_DIRECT),
   &innodb_flush_method_typelib);
 
 static MYSQL_SYSVAR_BOOL(force_load_corrupted, srv_load_corrupted,

storage/innobase/include/fil0fil.h

@@ -61,13 +61,11 @@ enum srv_flush_t
   /** do not flush after writing */
   SRV_NOSYNC,
   /** invoke os_file_set_nocache() on data files. This implies using
-  non-buffered IO but still using fsync, the reason for which is that
-  some FS do not flush meta-data when unbuffered IO happens */
+  unbuffered I/O but still fdatasync(), because some filesystems might
+  not flush meta-data on write completion */
   SRV_O_DIRECT,
-  /** do not use fsync() when using direct IO i.e.: it can be set to
-  avoid the fsync() call that we make when using SRV_UNIX_O_DIRECT.
-  However, in this case user/DBA should be sure about the integrity of
-  the meta-data */
+  /** Like O_DIRECT, but skip fdatasync(), assuming that the data is
+  durable on write completion */
   SRV_O_DIRECT_NO_FSYNC
 #ifdef _WIN32
   /** Traditional Windows appoach to open all files without caching,

storage/innobase/os/os0file.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1995, 2019, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2009, Percona Inc.
-Copyright (c) 2013, 2020, MariaDB Corporation.
+Copyright (c) 2013, 2021, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted
 by Percona Inc.. Those modifications are
@@ -1112,12 +1112,8 @@ os_file_create_simple_func(
 	/* This function is always called for data files, we should disable
 	OS caching (O_DIRECT) here as we do in os_file_create_func(), so
 	we open the same file in the same mode, see man page of open(2). */
-       if (!srv_read_only_mode
-	   && *success
-	   && (srv_file_flush_method == SRV_O_DIRECT
-	       || srv_file_flush_method == SRV_O_DIRECT_NO_FSYNC)) {
-
-	       os_file_set_nocache(file, name, mode_str);
+	if (!srv_read_only_mode && *success) {
+		os_file_set_nocache(file, name, mode_str);
 	}
 
 #ifdef USE_FILE_LOCK
@@ -1426,11 +1422,8 @@ os_file_create_func(
 	if (!read_only
 	    && *success
 	    && type != OS_LOG_FILE && type != OS_DATA_TEMP_FILE
-	    && type != OS_DATA_FILE_NO_O_DIRECT
-	    && (srv_file_flush_method == SRV_O_DIRECT
-		|| srv_file_flush_method == SRV_O_DIRECT_NO_FSYNC)) {
-
-	       os_file_set_nocache(file, name, mode_str);
+	    && type != OS_DATA_FILE_NO_O_DIRECT) {
+		os_file_set_nocache(file, name, mode_str);
 	}
 
 #ifdef USE_FILE_LOCK
@@ -3484,6 +3477,15 @@ os_file_set_nocache(
 	const char*	file_name	MY_ATTRIBUTE((unused)),
 	const char*	operation_name	MY_ATTRIBUTE((unused)))
 {
+	const auto innodb_flush_method = srv_file_flush_method;
+	switch (innodb_flush_method) {
+	case SRV_O_DIRECT:
+	case SRV_O_DIRECT_NO_FSYNC:
+		break;
+	default:
+		return;
+	}
+
 	/* some versions of Solaris may not have DIRECTIO_ON */
 #if defined(UNIV_SOLARIS) && defined(DIRECTIO_ON)
 	if (directio(fd, DIRECTIO_ON) == -1) {
@@ -3502,23 +3504,11 @@ os_file_set_nocache(
 		if (errno_save == EINVAL) {
 			if (!warning_message_printed) {
 				warning_message_printed = true;
-# ifdef UNIV_LINUX
-				ib::warn()
-					<< "Failed to set O_DIRECT on file"
-					<< file_name << "; " << operation_name
-					<< ": " << strerror(errno_save) << ", "
-					"continuing anyway. O_DIRECT is "
-					"known to result in 'Invalid argument' "
-					"on Linux on tmpfs, "
-					"see MySQL Bug#26662.";
-# else /* UNIV_LINUX */
-				goto short_warning;
-# endif /* UNIV_LINUX */
+				ib::info()
+					<< "Setting O_DIRECT on file "
+					<< file_name << " failed";
 			}
 		} else {
-# ifndef UNIV_LINUX
-short_warning:
-# endif
 			ib::warn()
 				<< "Failed to set O_DIRECT on file "
 				<< file_name << "; " << operation_name