innodb_flush_method的幾個典型取值

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.

如何取值，mysql官方文檔是這么建議的

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

也就是說，具體的取值跟硬件配置和工作負載相關，最好做一次壓測來決定。不過通常來說，linux環境下具有raid控制器和write-back寫策略，o_direct是比較好的選擇；如果存儲介質是SAN，那么使用默認fsync或者osync或許更好一些。

通常來說，貌似絕大部分人都取值o_direct，底層有raid卡，讀寫策略設置為write-back。在使用sysbench壓測oltp類型時，我發現o_direct確實比fsync性能優秀一些，看來適用于大部分場景，但是最近碰到一個這樣的sql，客戶反饋很慢，而在相同內存的情況下，它自己搭建的云主機執行相對快很多，后來我發現主要就是innodb_flush_method的設置值不同帶來的巨大性能差異。

測試場景1

innodb_flush_method為默認值，即fsync，緩存池512M，表數據量1.2G，排除緩存池影響，穩定后的結果

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)

測試場景2

innodb_flush_method改為o_direct，排除緩存池影響，穩定后的結果

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)

結果比較：

兩者執行計劃一摸一樣，性能卻差距很大。在數據庫第一次啟動時的查詢結果也差距很大，o_direct也差很多（測試結果略）。不是很懂為啥這種情況下多了一層操作系統緩存，讀取效率就高了很多，生產環境設置一定要以壓測結果為準，實際效果為準，不能盲目信任經驗值。

改進措施：

不改變innodb_flush_method的情況下，其實這條sql還可以進一步優化，通過添加組合索引(account_id,outcome,income)，使得走覆蓋索引掃描，可大大地減少響應時間

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