fsync: InnoDB uses the fsync() system call to flush both the data and log files. fsync is the default setting.

O_DSYNC: InnoDB uses O_SYNC to open and flush the log files, and fsync() to flush the data files. InnoDB does not use O_DSYNC directly because there have been problems with it on many varieties of Unix.

O_DIRECT: InnoDB uses O_DIRECT (or directio() on Solaris) to open the data files, and uses fsync() to flush both the data and log files. This option is available on some GNU/Linux versions,FreeBSD, and Solaris.

How each settings affects performance depends on hardware configuration and workload. Benchmark
your particular configuration to decide which setting to use, or whether to keep the default setting.
Examine the Innodb_data_fsyncs status variable to see the overall number of fsync() calls for
each setting. The mix of read and write operations in your workload can affect how a setting performs.
For example, on a system with a hardware RAID controller and battery-backed write cache, O_DIRECT
can help to avoid double buffering between the InnoDB buffer pool and the operating system's file
system cache. On some systems where InnoDB data and log files are located on a SAN, the default
value or O_DSYNC might be faster for a read-heavy workload with mostly SELECT statements. Always
test this parameter with hardware and workload that reflect your production environment

mysql> show variables like '%innodb_flush_me%';
+---------------------+-------+
| Variable_name    | Value |
+---------------------+-------+
| innodb_flush_method |    |
+---------------------+-------+
1 row in set (0.00 sec)


mysql> SELECT sql_no_cache SUM(outcome)-SUM(income) FROM journal where account_id = '1c6ab4e7-main';
+--------------------------+
| SUM(outcome)-SUM(income) |
+--------------------------+
|        -191010.51 |
+--------------------------+
1 row in set (1.22 sec)


mysql> SELECT sql_no_cache SUM(outcome)-SUM(income) FROM journal where account_id = '1c6ab4e7-main';
+--------------------------+
| SUM(outcome)-SUM(income) |
+--------------------------+
|        -191010.51 |
+--------------------------+
1 row in set (1.22 sec)
mysql> explain SELECT sql_no_cache SUM(outcome)-SUM(income) FROM journal where account_id = '1c6ab4e7-main';
+----+-------------+---------+------+---------------+------------+---------+-------+--------+-----------------------+
| id | select_type | table  | type | possible_keys | key    | key_len | ref  | rows  | Extra         |
+----+-------------+---------+------+---------------+------------+---------+-------+--------+-----------------------+
| 1 | SIMPLE   | journal | ref | account_id  | account_id | 62   | const | 161638 | Using index condition |
+----+-------------+---------+------+---------------+------------+---------+-------+--------+-----------------------+
1 row in set (0.03 sec)

mysql> show variables like '%innodb_flush_me%';
+---------------------+----------+
| Variable_name    | Value  |
+---------------------+----------+
| innodb_flush_method | O_DIRECT |
+---------------------+----------+
1 row in set (0.00 sec)


mysql> SELECT sql_no_cache SUM(outcome)-SUM(income) FROM journal where account_id = '1c6ab4e7-main';
+--------------------------+
| SUM(outcome)-SUM(income) |
+--------------------------+
|        -191010.51 |
+--------------------------+
1 row in set (3.22 sec)


mysql> SELECT sql_no_cache SUM(outcome)-SUM(income) FROM journal where account_id = '1c6ab4e7-main';
+--------------------------+
| SUM(outcome)-SUM(income) |
+--------------------------+
|        -191010.51 |
+--------------------------+
1 row in set (3.02 sec)


mysql> explain SELECT sql_no_cache SUM(outcome)-SUM(income) FROM journal where account_id = '1c6ab4e7-main';
+----+-------------+---------+------+---------------+------------+---------+-------+--------+-----------------------+
| id | select_type | table  | type | possible_keys | key    | key_len | ref  | rows  | Extra         |
+----+-------------+---------+------+---------------+------------+---------+-------+--------+-----------------------+
| 1 | SIMPLE   | journal | ref | account_id  | account_id | 62   | const | 161638 | Using index condition |
+----+-------------+---------+------+---------------+------------+---------+-------+--------+-----------------------+
1 row in set (0.00 sec)