sync_relay_log :
If the value of this variable is greater than 0, the MySQL server synchronizes its relay log to disk (using fdatasync()) after every sync_relay_log events are written to the
relay log.
Setting sync_relay_log to 0 causes no synchronization to be done to disk; in this case, the server relies on the operating system to flush the relay log's contents from time to
time as for any other file.
Prior to MySQL 5.6.6, 0 was the default for this variable. In MySQL 5.6. and later, the default is 10000.
A value of 1 is the safest choice because in the event of a crash you lose at most one event from the relay log. However, it is also the slowest choice (unless the disk has a
battery-backed cache, which makes synchronization very fast).
如果值>0, MySQL SERVER 同步它的relay log 到磁盘(写入中继日志,使用fdatasync()) 在every sync_relay_log events are written to the relay log.）
设置sync_relay_log to 0 导致不同步到磁盘,在这种情况下,server 依赖OS来刷新
当设置为1时，slave的I/O线程每次接收到master发送过来的binlog日志都要写入系统缓冲区，然后刷入relay log中继日志里，这样是最安全的，因为在崩溃的时候，你最多会丢失一个事务，但会造成
磁盘的大量I/O。当设置为0时，并不是马上就刷入中继日志里，而是由操作系统决定何时来写入，虽然安全性降低了，但减少了大量的磁盘I/O操作。