为什么有时候binlog文件会很大于max_binlog_size以及max_binlog_cache_size

root@test10:39:24>set global max_binlog_size=4096;

root@test10:48:23>begin;
Query OK, 0 rows affected (0.00 sec)

root@test10:48:39>insert into testb select * from testa limit 32;
Query OK, 32 rows affected (0.00 sec)
Records: 32  Duplicates: 0  Warnings: 0

root@test10:49:05>insert into testb select * from testa limit 32;
Query OK, 32 rows affected (0.00 sec)
Records: 32  Duplicates: 0  Warnings: 0

root@test10:49:10>insert into testb select * from testa limit 32;
Query OK, 32 rows affected (0.00 sec)
Records: 32  Duplicates: 0  Warnings: 0

root@test10:49:14>insert into testb select * from testa limit 32;
Query OK, 32 rows affected (0.00 sec)
Records: 32  Duplicates: 0  Warnings: 0

root@test10:49:18>commit;
Query OK, 0 rows affected (0.00 sec)

# at 3994  -- 上一个event结束的地方
#190130 10:49:14 server id 21036055 end_log_pos 4064 CRC32 0x64e8a1c6 Rows_query
# insert into testb select * from testa limit 32
# at 4064
#190130 10:49:14 server id 21036055 end_log_pos 4113 CRC32 0x7ef21b8c Table_map: `test`.`testb` mapped to number 11496
# at 4113
#190130 10:49:14 server id 21036055 end_log_pos 4692 CRC32 0xfc3f78bd Write_rows: table id 11496 flags: STMT_END_F  -- 这已经是下一个insert 语句了，在一个binlog文件中，说明binlog是以事务为单位来进行切割的，不是事务里面的单个sql语句，这也是比较好理解的，因为事务只有执行完了， 才能在内存中生成完整的binlog，才存在刷盘的操作。

root@test11:01:52>set global  max_binlog_cache_size=4096;
root@test11:15:20>insert into testb select * from testb limit 200;   --  3.9K Jan 30 11:15 mysql-bin.003691
root@test11:15:28>insert into testb select * from testb limit 460;   --  12K Jan 30 11:16 mysql-bin.003691
root@test11:17:34>insert into testb select * from testb limit 470;   --  最大就是在460到470之间   
ERROR 1197 (HY000): Multi-statement transaction required more than ‘max_binlog_cache_size‘ bytes of storage; increase this mysqld variable and try again
这个实验说明binlog文件最大应该是max_binlog_size + max_binlog_cache_size的和，但是max_binlog_cache_size在文件大小上可能比实际设置的值要大，见下问题。所有严格来讲应该是大于两者的和的。

root@test11:07:21>insert into testb select * from testb limit 400;

# at 331
#190130 11:07:32 server id 21036055 end_log_pos 402 CRC32 0x62bb0a5e Rows_query
# insert into testb select * from testb limit 400
# at 402
#190130 11:07:32 server id 21036055 end_log_pos 451 CRC32 0x3177b22e Table_map: `test`.`testb` mapped to number 11496
# at 451
#190130 11:07:32 server id 21036055 end_log_pos 7286 CRC32 0x35efff16 Write_rows: table id 11496 flags: STMT_END_F  

能正常插入证明内存中产生的binlog应该是小于4096的，但是实际在binlog中看见的确实7286，明显是比4094大的，为啥会这样呢，在从内存中把binlog落盘的过程中做了什么处理吗？

root@test11:21:01>show create table testb\G
*************************** 1. row ***************************
       Table: testb
Create Table: CREATE TABLE `testb` (
  `a` bigint(20) DEFAULT NULL,
  `b` bigint(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
1 row in set (0.00 sec)

root@test11:24:10>select version();
+---------------+
| version() |
+---------------+
| 5.7.21-21-log |
+---------------+
1 row in set (0.00 sec)

 以上是表结构以及MySQL的版本