mysql参数innodb_flush_log_at_trx_commit

Posted 2020-10-05 Vincent

查看mysql数据库innodb_flush_log_at_trx_commit ：

mysql>  SHOW GLOBAL VARIABLES LIKE ‘innodb_flush_log%‘;
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_timeout    | 1     |
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
2 rows in set (0.00 sec)

mysql> 

参考资料：https://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_flush_log_at_trx_commit

 

innodb_flush_log_at_trx_commit

Command-Line Format	--innodb-flush-log-at-trx-commit[=#]
System Variable	Name	innodb_flush_log_at_trx_commit
	Variable Scope	Global
	Dynamic Variable	Yes
Permitted Values	Type	enumeration
	Default	1
	Valid Values	0
		1
		2

Controls the balance between strict ACID compliance for commit operations and higher performance that is possible when commit-related I/O operations are rearranged and done in batches. You can achieve better performance by changing the default value but then you can lose up to a second of transactions in a crash.

• The default value of 1 is required for full ACID compliance. With this value, the contents of the InnoDB log buffer are written out to the log file at each transaction commit and the log file is flushed to disk.

• With a value of 0, the contents of the InnoDB log buffer are written to the log file approximately once per second and the log file is flushed to disk. No writes from the log buffer to the log file are performed at transaction commit. Once-per-second flushing is not guaranteed to happen every second due to process scheduling issues. Because the flush to disk operation only occurs approximately once per second, you can lose up to a second of transactions with any mysqld process crash.

• With a value of 2, the contents of the InnoDB log buffer are written to the log file after each transaction commit and the log file is flushed to disk approximately once per second. Once-per-second flushing is not 100% guaranteed to happen every second, due to process scheduling issues. Because the flush to disk operation only occurs approximately once per second, you can lose up to a second of transactions in an operating system crash or a power outage.

• InnoDB log flushing frequency is controlled by innodb_flush_log_at_timeout, which allows you to set log flushing frequency to N seconds (where N is 1 ... 2700, with a default value of 1). However, any mysqld process crash can erase up to N seconds of transactions.

• DDL changes and other internal InnoDB activities flush the InnoDB log independent of the innodb_flush_log_at_trx_commit setting.

• InnoDB crash recovery works regardless of the innodb_flush_log_at_trx_commit setting. Transactions are either applied entirely or erased entirely.

For durability and consistency in a replication setup that uses InnoDB with transactions:

• If binary logging is enabled, set sync_binlog=1.

• Always set innodb_flush_log_at_trx_commit=1.

Caution

Many operating systems and some disk hardware fool the flush-to-disk operation. They may tell mysqld that the flush has taken place, even though it has not. In this case, the durability of transactions is not guaranteed even with the setting 1, and in the worst case, a power outage can corrupt InnoDB data. Using a battery-backed disk cache in the SCSI disk controller or in the disk itself speeds up file flushes, and makes the operation safer. You can also try to disable the caching of disk writes in hardware caches.

 

 

命令查看：mysqld --verbose --help

可通过命令设置，也可通过my.cnf配置.

--innodb-flush-log-at-trx-commit[=#] 
                      Set to 0 (write and flush once per second), 1 (write and
                      flush at each commit) or 2 (write at commit, flush once
                      per second).