从XA恢复的逻辑我们可以知道，只要保证InnoDB Prepare的redo日志在写Binlog前完成write/sync即可。因此我们对Group Commit的第一个stage的逻辑做了些许修改，大概描述如下：
Step1. InnoDB Prepare，记录当前的LSN到thd中；
Step2. 进入Group Commit的flush stage；Leader搜集队列，同时算出队列中最大的LSN。
Step3. 将InnoDB的redo log write/fsync到指定的LSN (注：这一步就是redo log的组写入。因为小于等于LSN的redo log被一次性写入到ib_logfile[0|1])
Step4. 写Binlog并进行随后的工作(sync Binlog, InnoDB commit , etc)
也就是将 redo log的write/sync延迟到了 binlog group commit的 flush stage 之后，sync binlog之前。
通过延迟写redo log的方式，显式的为redo log做了一次组写入(redo log group write)，并减少了(redo log) log_sys->mutex的竞争。
也就是将 binlog group commit 对应的redo log也进行了 group write. 这样binlog 和 redo log都进行了优化。
官方MySQL在5.7.6的代码中引入了淘宝的优化，对应的Release Note如下：
When using InnoDB with binary logging enabled, concurrent transactions written in the InnoDB redo log are now grouped together before synchronizing to disk when innodb_flush_log_at_trx_commit is set to 1, which reduces the amount of synchronization operations. This can lead to improved performance.5. XA参数 innodb_support_xa
Command-Line Format
--innodb_support_xa
System Variable
Name
Variable Scope
Global, Session
Dynamic Variable
Yes
Permitted Values
Type
boolean
Default
TRUE
Enables InnoDB support for two-phase commit(2PC) in transactions, causing an extra disk flush for transaction preparation. This setting is the default. The XA mechanism is used internally and is essential for any server that has its binary log turned on and is accepting changes to its data from more than one thread. If you turn it off, transactions can be written to the binary log in a different order from the one in which the live database is committing them. This can produce different data when the binary log is replayed in disaster recovery or on a replication slave. Do not turn it off on a replication master server unless you have an unusual setup where only one thread is able to change data.
For a server that is accepting data changes from only one thread, it is safe and recommended to turn off this option to improve performance forInnoDB tables. For example, you can turn it off on replication slaves where only the replication SQL thread is changing data.
You can also turn off this option if you do not need it for safe binary logging or replication, and you also do not use an external XA transaction manager.
参数innodb_support_xa默认为true，表示启用XA，虽然它会导致一次额外的磁盘flush(prepare阶段flush redo log). 但是我们必须启用，而不能关闭它。因为关闭会导致binlog写入的顺序和实际的事务提交顺序不一致，会导致崩溃恢复和slave复制时发生数据错误。如果启用了log-bin参数，并且不止一个线程对数据库进行修改，那么就必须启用innodb_support_xa参数。