一、问题来源

有一个朋友@水米田 问我，基于POSITION的主从。他做了如下的操作

将备份的一些binlog文件加入到了目录中
修改index文件，加入了这些binlog文件
flush binary logs
然后整个主从环境大量延迟。

二、朋友的测试

下面是另外一个朋友@徐晨亮的测试：

master上：(root:db1@xucl:10:30:22)[(none)]> show binary logs;+---------------------+-----------+| Log_name | File_size |+---------------------+-----------+| mysql-binlog.000031 | 1019 || mysql-binlog.000032 | 424 || mysql-binlog.000033 | 244 || mysql-binlog.000034 | 2332 || mysql-binlog.000035 | 2134 || mysql-binlog.000036 | 845915 || mysql-binlog.000037 | 11735 || mysql-binlog.000038 | 284 || mysql-binlog.000039 | 284 || mysql-binlog.000040 | 284 || mysql-binlog.000041 | 284 || mysql-binlog.000042 | 234 |+---------------------+-----------+12 rows in set (0.00 sec)(root:db1@xucl:10:30:34)[(none)]> purge binary logs to 'mysql-binlog.000039';Query OK, 0 rows affected (0.00 sec)(root:db1@xucl:10:30:49)[(none)]> show binary logs;+---------------------+-----------+| Log_name | File_size |+---------------------+-----------+| mysql-binlog.000039 | 284 || mysql-binlog.000040 | 284 || mysql-binlog.000041 | 284 || mysql-binlog.000042 | 234 |+---------------------+-----------+4 rows in set (0.00 sec)执行插入数据(root:db1@xucl:10:31:23)[xucl]> insert into t values(9,9);将备份的binlog拷贝回原先的目录并修改index文件进行注册[root@izbp12nspj47ypto9t6vyez logs]# ll总用量 884-rw-r----- 1 mysql mysql 1019 5月 20 22:03 mysql-binlog.000031-rw-r----- 1 mysql mysql 424 5月 20 22:03 mysql-binlog.000032-rw-r----- 1 mysql mysql 244 5月 20 22:03 mysql-binlog.000033-rw-r----- 1 mysql mysql 2332 5月 20 22:03 mysql-binlog.000034-rw-r----- 1 mysql mysql 2134 5月 20 22:03 mysql-binlog.000035-rw-r----- 1 mysql mysql 845915 5月 20 22:03 mysql-binlog.000036-rw-r----- 1 mysql mysql 11735 5月 20 22:05 mysql-binlog.000037-rw-r----- 1 mysql mysql 284 5月 20 22:06 mysql-binlog.000038-rw-r----- 1 mysql mysql 284 5月 21 10:28 mysql-binlog.000039-rw-r----- 1 mysql mysql 284 5月 21 10:28 mysql-binlog.000040-rw-r----- 1 mysql mysql 284 5月 21 10:28 mysql-binlog.000041-rw-r----- 1 mysql mysql 491 5月 21 10:31 mysql-binlog.000042-rw-r----- 1 mysql mysql 204 5月 21 10:30 mysql-binlog.index主库flush binary logs(root:db1@xucl:10:32:51)[(none)]> flush binary logs;Query OK, 0 rows affected (0.01 sec)(root:db1@xucl:10:32:57)[(none)]> show binary logs;+---------------------+-----------+| Log_name | File_size |+---------------------+-----------+| mysql-binlog.000031 | 1019 || mysql-binlog.000032 | 424 || mysql-binlog.000033 | 244 || mysql-binlog.000034 | 2332 || mysql-binlog.000035 | 2134 || mysql-binlog.000036 | 845915 || mysql-binlog.000037 | 11735 || mysql-binlog.000038 | 284 || mysql-binlog.000039 | 284 || mysql-binlog.000040 | 284 || mysql-binlog.000041 | 284 || mysql-binlog.000042 | 541 || mysql-binlog.000043 | 234 |+---------------------+-----------+13 rows in set (0.00 sec)此时，slave报错如下：(root:db1@xucl:10:31:05)[(none)]> show slave status\G*************************** 1. row *************************** Slave_IO_State: Master_Host: 127.0.0.1 Master_User: repl Master_Port: 3306 Connect_Retry: 60 Master_Log_File: mysql-binlog.000035 Read_Master_Log_Pos: 194 Relay_Log_File: izbp12nspj47ypto9t6vyez-relay-bin.000011 Relay_Log_Pos: 373 Relay_Master_Log_File: mysql-binlog.000035 Slave_IO_Running: No Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 194 Relay_Log_Space: 648 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Master_SSL_Allowed: No Master_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: NULLMaster_SSL_Verify_Server_Cert: No Last_IO_Errno: 1236 Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'Cannot replicate GTID-transaction when @@GLOBAL.GTID_MODE = OFF, at file /storage/single/mysql3306/logs/mysql-binlog.000035, position 194.; the first event 'mysql-binlog.000039' at 234, the last event read from '/storage/single/mysql3306/logs/mysql-binlog.000035' at 259, the last byte read from '/storage/single/mysql33' Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 3306 Master_UUID: e8bdf01a-c79b-11e8-82b3-00163e088352 Master_Info_File: mysql.slave_master_info SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates Master_Retry_Count: 86400 Master_Bind: Last_IO_Error_Timestamp: 190521 10:32:57 Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: 4c423515-6661-11e9-b767-00163e088352:1-7,e8bdf01a-c79b-11e8-82b3-00163e088352:1-57192 Auto_Position: 0 Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version:1 row in set (0.00 sec)从slave上的报错来看，在主库flush binary logs后，从库又读取注册的binlog并且又apply了

三、现象说明

从整个测试来看，MySQL视乎将手动注册的文件进行了传输和应用。报错是因为这个库以前是GITD_MODE=ON的，但是测试的时候已经关闭了GTID_MODE，改为了POSITION的模式，这个报错是因为DUMP线程会进行检测：

这个图来自我新写的一个系列（暂时还没发布，大概年底才能写好）。不管怎么说，DUMP线程已经在开始传输老的binlog文件了。那么原因是什么呢？下面我们将进行解释。

四、flush binary logs对binlog的操作

flush binary logs 将包含如下操作：

• 获取新的binlog文件名字
• 关闭旧的binlog
• 关闭index file
• 打开index file
• 打开新的binlog
• 将新的binlog加入到indexfile
  

具体可以参考函数MYSQL_BIN_LOG::new_file_impl。其中要说明一下获取新的binlog文件名字是通过函数find_uniq_filename实现的，其中包含如下代码：

file_info= dir_info->dir_entry; for (i= dir_info->number_off_files ; i-- ; file_info++) { if (strncmp(file_info->name, start, length) == 0 && is_number(file_info->name+length, &number,0)) { set_if_bigger(max_found, number); } }... *next= (need_next || max_found == 0) ? max_found + 1 : max_found;

大概意思就是扫描index file文件里面的binlog文件，获取其序号最高的一个，然后加1。栈帧如下：

#0 find_uniq_filename (name=0x7fffec5ec6d0 "/mysqldata/mysql3340/log/binlog", next=0x7fffec5ec678, need_next=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:3679#1 0x000000000187d208 in generate_new_log_name (new_name=0x7fffec5ec6d0 "/mysqldata/mysql3340/log/binlog", new_ext=0x0, log_name=0x7ffedc011520 "/mysqldata/mysql3340/log/binlog", is_binlog=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:3767#2 0x0000000001884fdb in MYSQL_BIN_LOG::new_file_impl (this=0x2e83640, need_lock_log=false, extra_description_event=0x0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:7175#3 0x0000000001884cbb in MYSQL_BIN_LOG::new_file_without_locking (this=0x2e83640, extra_description_event=0x0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:7099#4 0x0000000001886b6b in MYSQL_BIN_LOG::rotate (this=0x2e83640, force_rotate=true, check_purge=0x7fffec5ecbfb) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:7775#5 0x0000000001886d53 in MYSQL_BIN_LOG::rotate_and_purge (this=0x2e83640, thd=0x7ffedc000b90, force_rotate=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:7846

因此即便我们手动修改了index file，flush binary logs却不会有问题，因为它实际扫描了index file文件。
同时我们也看到flush binary logs重新加载了index file，这个时候手动修改的index file就生效了，使用show binary logs就能查看到你加入的文件了。

五、主从问题的产生

binlog切换后，DUMP线程也需要读取下一个binlog文件。我们来看看它是怎么确认读取哪一个文件的。

在函数sender.run()中可以找到循环每个binlog文件的代码。下面一句是寻找下一个binlog文件：

int error= mysql_bin_log.find_next_log(&m_linfo, 0);

mysql_bin_log.find_next_log 包含的代码：

my_b_seek(&index_file, linfo->index_file_offset);//进行偏移量偏移 linfo->index_file_start_offset= linfo->index_file_offset; length=my_b_gets(&index_file, fname, FN_REFLEN));//读取文件名字... if(normalize_binlog_name(full_fname, fname, is_relay_log))... linfo->index_file_offset= my_b_tell(&index_file);//偏移量从新记录以备下一次使用

我们能够看到DUMP线程并没有实际扫描整个index文件，而是通过一个index文件的偏移量进行读取。如果手动修改index文件那么偏移量就出现了错乱。因此DUMP发送的下一个文件将是不确定的。因此出现了发送手动注册的binlog文件给从库的现象，这种情况下可能出现下面情况：

如果是GTID_MODE 关闭，使用POSITION那么这种情况一定报错，比如重复的行。

如果是GTID_MODE 和AUTO_POSITION=1，那么DUMP线程会进行GTID的过滤不发送，因为Event不发送所以延迟会持续一段时间为0。

如果是GTID_MODE 和AUTO_POSITION=0，那么SQL线程应用GITD_EVENT的时候会进行过滤，延迟可能出现很大的情况。

尽管GTID可能可以屏蔽这种问题，但是DUMP线程已经在考虑发送老的binlog文件了，这是不合适的。

六、purge binary logs能够维护这个偏移量

为什么purge binary logs不会出现问题呢，因为在purge binary logs的语句下，会维护这个偏移量如下：

virtual void operator()(THD *thd) { LOG_INFO* linfo; mysql_mutex_lock(&thd->LOCK_thd_data); if ((linfo= thd->current_linfo))//b binlog.cc:2829 { if (linfo->index_file_offset < m_purge_offset) linfo->fatal = (linfo->index_file_offset != 0); else linfo->index_file_offset -= m_purge_offset; } mysql_mutex_unlock(&thd->LOCK_thd_data);

我们可以看到linfo->index_file_offset -= m_purge_offset;这样一个语句。下面是栈帧：

#0 Adjust_offset::operator() (this=0x7fffec5ec720, thd=0x7ffedc000be0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:2831#1 0x0000000000eef320 in Do_THD::operator() (this=0x7fffec5ec6a0, thd=0x7ffedc000be0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/mysqld_thd_manager.cc:46#2 0x0000000000eefa0f in std::for_each<THD**, Do_THD> (__first=0x3003358, __last=0x3003368, __f=...) at /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algo.h:4200#3 0x0000000000eeefc0 in Global_THD_manager::do_for_all_thd (this=0x3003340, func=0x7fffec5ec720) at /mysqldata/percona-server-locks-detail-5.7.22/sql/mysqld_thd_manager.cc:273#4 0x000000000187ae0a in adjust_linfo_offsets (purge_offset=0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:2873#5 0x0000000001883239 in MYSQL_BIN_LOG::remove_logs_from_index (this=0x2e83640, log_info=0x7fffec5ec7d0, need_update_threads=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:6352#6 0x0000000001883676 in MYSQL_BIN_LOG::purge_logs (this=0x2e83640, to_log=0x7fffec5eca80 "/mysqldata/mysql3340/log/binlog.000001", included=false, need_lock_index=true, need_update_threads=true, decrease_log_space=0x0, auto_purge=false) at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:6469#7 0x000000000187b4b5 in purge_master_logs (thd=0x7ffee0000c00, to_log=0x7ffee0006600 "binlog.000001") at /mysqldata/percona-server-locks-detail-5.7.22/sql/binlog.cc:3127

七、测试POSITION模式下的报错

1、 环境

mysql> show binary logs;+---------------+-----------+| Log_name | File_size |+---------------+-----------+| binlog.000001 | 198 || binlog.000002 | 154 |+---------------+-----------+2 rows in set (0.00 sec)mysql> show create table testcp \G;*************************** 1. row *************************** Table: testcpCreate Table: CREATE TABLE `testcp` ( `id` int(11) NOT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec)ERROR: No query specified

2、执行一个语句

主库：

mysql> insert into testcp values(20);Query OK, 1 row affected (0.43 sec)mysql> select * from testcp;+----+| id |+----+| 10 || 20 |+----+2 rows in set (0.01 sec)

从库：

mysql> show slave status \G;*************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 192.168.99.41 Master_User: repl Master_Port: 3340 Connect_Retry: 60 Master_Log_File: binlog.000002 Read_Master_Log_Pos: 417 Relay_Log_File: relay.000004 Relay_Log_Pos: 624 Relay_Master_Log_File: binlog.000002 Slave_IO_Running: Yes Slave_SQL_Running: Yes...mysql> select * from testcp;+----+| id |+----+| 10 || 20 |+----+

这个时候DUMP线程index file偏移指针如下：

3、主库执行purge binary logs

做之前拷贝原有binlog.000001为binlog.000001bak因为等会要拷贝回去

mysql> purge binary logs to 'binlog.000002';Query OK, 0 rows affected (3.14 sec)mysql> show binary logs;+---------------+-----------+| Log_name | File_size |+---------------+-----------+| binlog.000002 | 417 |+---------------+-----------+1 row in set (0.00 sec)

因为purge binary logs命令会维护偏移指针，这个时候DUMP线程的index file偏移指针如下：

4、手动更改

将binlog.000001bak拷贝为binlog.000001，然后修改index file将binlog.000001加入回去，然后flush binary logs如下：

mysql> flush binary logs;Query OK, 0 rows affected (0.15 sec)mysql> show binary logs;+---------------+-----------+| Log_name | File_size |+---------------+-----------+| binlog.000001 | 198 || binlog.000002 | 461 |+---------------+-----------+2 rows in set (0.00 sec)

手动完成这个工作并不会维护DUMP线程的index file偏移指针，因此如下：

就这样DUMP线程重新发送了一次binlog.000002的内容，POSITION的从库只能报错了如下：

mysql> select * from replication_applier_status_by_worker \G*************************** 1. row *************************** CHANNEL_NAME: WORKER_ID: 1 THREAD_ID: NULL SERVICE_STATE: OFFLAST_SEEN_TRANSACTION: ANONYMOUS LAST_ERROR_NUMBER: 1062 LAST_ERROR_MESSAGE: Worker 1 failed executing transaction 'ANONYMOUS' at master log binlog.000002, end_log_pos 386; Could not execute Write_rows event on table testmts.testcp; Duplicate entry '20' for key 'PRIMARY', Error_code: 1062; handler error HA_ERR_FOUND_DUPP_KEY; the event's master log binlog.000002, end_log_pos 386 LAST_ERROR_TIMESTAMP: 2019-05-21 14:46:26

八、总结

这种操作非常不规范，如果实在要这么做考虑如下步骤：

• 关闭全部的从库
• 手动注册binlog文件，修改index 文件
• flush binary logs
• 启动从库

这种情况下将会重新初始化DUMP的偏移量指针，应该是没有问题的。还是尽量不要考虑这么做。

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