今天学习了下mysqldump原理，具体的结论如下： 1、mysqldump在不加任何参数进行备份的时候： 如果单独备份一个表会对备份表上读锁，直到备份结束unlock，如果备份的整个库，那么会同时lock 这个库下的所有的表，最后在unlock tables，如果备份的是整个实例(加参数--all-databases 或者-A会备份除了performance_schema和performance_schema这俩库之外的所有的库），那么是一个库一库的去备份，也就是说先备份库A，把库A下的所有的表上读锁，备份完库A，unlock tables,然后再备份库B,把库B下的所有的表上读锁，备份完库B，unlock tables，那么所以在使用mysqldump备份某个表或者某个库的时候，没有加任何的参数，会上读锁，并且备份出来的数据是一致性的，但是如果备份的是整个实例，那么库和库之间的数据的一致性就不能保证了； 2、参数--single-transaction ； 针对innodb的引擎，可以加上参数 --single-transaction来保证备份的一致性，并且是借助的修改隔离级别为REPEATABLE READ+START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */开启快照读事务共同来保证一致性的，所以不需要加read lock;注意该参数仅仅对innodb引擎起作用，对于myisam引擎，虽然添加了--single-transaction参数的myisam表备份处理过程和innodb的过程完全一致，但是因为myisam不支持事务，在整个dump过程中无法保证可重复读，无法得到一致性的备份。 3、参数--master-data； --master-data指定为2指的是会在备份文件中生成CHANGE MASTER的注释。如下所示： -- CHANGE MASTER TO MASTER_LOG_FILE='mysql2-bin.000049', MASTER_LOG_POS=587; 如果该值设置为1，则生成的是CHANGE MASTER的命令，而不是注释。如下所示： CHANGE MASTER TO MASTER_LOG_FILE='mysql2-bin.000049', MASTER_LOG_POS=587; 当加上这个参数的时候，为了得到准确的binlog的位置状态信息，会通过FLUSH TABLES WITH READLOCK来保证，备份开始到结束，是不允许别的事务修改的，同时也就保证了一致性； 4、参数--single-transaction和参数参数--master-data一起使用； 也会执行 FLUSH TABLES WITH READ LOCK，但是在还没有开始备份时，也就是在 SHOW MASTER STATUS显示了主库的binlog状态之后就unlock tables了，也会 SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ，之后和单独加--single-transaction过程是一样的。 5、参数--lock-all-tables，通过给整个实例所有表都加read lock来保证一致性备份； 加上--lock-all-tables和不加上--lock-all-tables得区别就在于前者是FLUSH TABLES WITH READ LOCK对整个实例所有的表都上读锁，后者只针对要备份的表加读锁（LOCK TABLES `liu` READ）；并且前者并没有显现的unlock tables，因为整个过程中数据库是不能写的，并且FLUSH TABLES WITH READ LOCK这个命令一旦这个会话结束，相应的读锁也就不存在了，而后者只是锁了一个表，显现的unlock解锁了，其实后者也是会话结束就释放对表的读锁了，也可以不加unlock， 6、START TRANSACTION和START TRANSACTION WITH CONSISTENT SNAPSHOT并不一样; START TRANSACTION WITH CONSISTENT SNAPSHOT相当于在执行完START TRANSACTION后对每个Innodb表执行了SELECT操作，在隔离级别为REPEATABLE READ时，并不是当start transaction 就能保证之后的查询内容是一样，而是当你发出第一个query的时候，才会开启快照读取，之后再有相同的sql查出来的结果是一样的。 在mysqldump加上参数--single-transaction的时候使用的是START TRANSACTION WITH CONSISTENT SNAPSHOT，而不是START TRANSACTION来保证一致性的，是因为每个表的备份时间并不相同，如果使用START TRANSACTION，在对第一张表进行备份的期间，别的事务对第二个表进行了insert数据A，那么在开始对第二张表备份时,是可以看到数据A的，那么第一个表和第二个表就不是一致性的了，所以START TRANSACTION无法实现当一个库下有多个表的时候的一致性。. 综上所述： 在使用mysqldump进行数据备份的时候，尽量在业务量比较小的时候执行，并且根据是不是innodb引擎来选择不同的参数，如果是innodb的引擎可以使用--single-transaction参数来保证一致性，并且还不用上read lock；但是如果想保证整个实例的一致性（既有innodb又有myisam的表）最好还是使用参数--lock-all-tables，当然为了实现point to point恢复或者不停master服务来添加slave的目的，最好还是加上参数--master-data，同时也就能保证一致性，因为加上参数--master-data会执行FLUSH TABLES WITH READLOCK； 下面是具体的验证过程： 一：打开general log，便于分析mysqldump具体执行了什么操作 mysql> set global general_log=on; 其中，general log的存放路径可通过以下命令查看 mysql> show variables like '%general_log_file%'; 二：执行MySQLdump导出表实验如下： 2.1首先什么参数都不加的情况： bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe liuhe blocks_infos_opensearch> /mysql/blocks_infos_opensearch.sql 查看相应的general_log： bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 14:12:22 17 Quit 180429 14:12:55 18 Connect root@localhost on 18 Query /*!40100 SET @@SQL_MODE='' */ 18 Query /*!40103 SET TIME_ZONE='+00:00' */ 18 Query SHOW VARIABLES LIKE 'gtid/_mode' 18 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENG INE ORDER BY LOGFILE_GROUP_NAME 18 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NA ME 18 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 18 Init DB liuhe 18 Query SHOW TABLES LIKE 'blocks/_infos/_opensearch' 18 Query LOCK TABLES `blocks_infos_opensearch` READ /*!32311 LOCAL */ 18 Query show table status like 'blocks/_infos/_opensearch' 18 Query SET SQL_QUOTE_SHOW_CREATE=1 18 Query SET SESSION character_set_results = 'binary' 18 Query show create table `blocks_infos_opensearch` 18 Query SET SESSION character_set_results = 'utf8' 18 Query show fields from `blocks_infos_opensearch` 18 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos_opensearch` 18 Query SET SESSION character_set_results = 'binary' 18 Query use `liuhe` 18 Query select @@collation_database 18 Query SHOW TRIGGERS LIKE 'blocks/_infos/_opensearch' 18 Query SET SESSION character_set_results = 'utf8' 18 Query UNLOCK TABLES 18 Quit 2.2:加上参数--single-transaction bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe --single-transaction liuhe blocks_infos_opensearch> /mysql/blocks_infos_opensearch.sql
bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 14:20:41 21 Quit 180429 14:20:47 22 Connect root@localhost on 22 Query /*!40100 SET @@SQL_MODE='' */ 22 Query /*!40103 SET TIME_ZONE='+00:00' */ 22 Query SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ 22 Query START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ #开启快照读 22 Query SHOW VARIABLES LIKE 'gtid/_mode' 22 Query UNLOCK TABLES 22 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENG INE ORDER BY LOGFILE_GROUP_NAME 22 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NA ME 22 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 22 Init DB liuhe 22 Query SHOW TABLES LIKE 'blocks/_infos/_opensearch' 22 Query SAVEPOINT sp 22 Query show table status like 'blocks/_infos/_opensearch' 22 Query SET SQL_QUOTE_SHOW_CREATE=1 22 Query SET SESSION character_set_results = 'binary' 22 Query show create table `blocks_infos_opensearch` 22 Query SET SESSION character_set_results = 'utf8' 22 Query show fields from `blocks_infos_opensearch` 22 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos_opensearch` 22 Query SET SESSION character_set_results = 'binary'
22 Query use `liuhe` 22 Query select @@collation_database 22 Query SHOW TRIGGERS LIKE 'blocks/_infos/_opensearch' 22 Query SET SESSION character_set_results = 'utf8' 22 Query ROLLBACK TO SAVEPOINT sp #把事务回退到这个点 sp 22 Query RELEASE SAVEPOINT sp #放弃保存点，需要注意的是一旦rollback或者commit，那么之前创建的savepoint就会失效； 180429 14:20:48 22 Quit 通过.1和3.2可以看出来加上参数--single-transaction，可以保证mysqldump的时候不需要LOCK TABLES `blocks_infos_opensearch` READ ;并且使用参数--single-transaction，需要修改MySQL的隔离界别为 REPEATABLE READ来保证各个事务之间互相不影响对方，保证在执行MySQLdump的会话始终读取不到别的事务的操作，进而保证了MySQLdump出来的数据的一致性；并且为了能获得准确的pos点，需要START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ 来开启快照读的事务，因为如果只START TRANSACTION ，并没有生成快照，而是在执行第一个select的时候，才会生成快照，也就是说如果START TRANSACTION之后，另一个事务insert了数据A，然后你再select，是可以看到的数据A的，这样就不能得到精确的pos值了。 2.3加上参数--master-data 具体如下： bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe --master-data=2 liuhe blocks_infos_opensearch> /mysql/blocks_infos_opensearch.sql bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 18:01:27 35 Quit 180429 18:02:15 36 Connect root@localhost on 36 Query /*!40100 SET @@SQL_MODE='' */ 36 Query /*!40103 SET TIME_ZONE='+00:00' */ 36 Query FLUSH /*!40101 LOCAL */ TABLES 36 Query FLUSH TABLES WITH READ LOCK 36 Query SHOW VARIABLES LIKE 'gtid/_mode' 36 Query SHOW MASTER STATUS 36 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENG INE ORDER BY LOGFILE_GROUP_NAME 36 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NA ME 36 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 36 Init DB liuhe 36 Query SHOW TABLES LIKE 'blocks/_infos/_opensearch' 36 Query show table status like 'blocks/_infos/_opensearch' 36 Query SET SQL_QUOTE_SHOW_CREATE=1 36 Query SET SESSION character_set_results = 'binary' 36 Query show create table `blocks_infos_opensearch` 36 Query SET SESSION character_set_results = 'utf8' 36 Query show fields from `blocks_infos_opensearch` 36 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos_opensearch` 180429 18:02:16 36 Query SET SESSION character_set_results = 'binary' 36 Query use `liuhe` 36 Query select @@collation_database 36 Query SHOW TRIGGERS LIKE 'blocks/_infos/_opensearch' 36 Query SET SESSION character_set_results = 'utf8' 36 Quit 通过2.1和2.2可以看出来，通过 SHOW MASTER STATUS来显示当时binlog的位置，通过FLUSH TABLES WITH READ LOCK，来保证一致性，注意尽管只是备份一个表，由于这个binlog的位置是可以在不停主库的前提下添加从库时直接可以使用的位置，所以需要锁住整个实例的所有的表（ FLUSH TABLES WITH READ LOCK），来保证这个位置在备份开始的时候，不再有任何dml操作，也就是这个位置就不再增大； 2.4同时添加上参数--master-data和--single-transaction bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe --single-transaction --master-data=2 liuhe blocks_infos_opensearch> /mysql/blocks_infos_opensearch.sql Warning: Using a password on the command line interface can be insecure. bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 18:50:06 37 Quit 180429 18:50:36 38 Connect root@localhost on 38 Query /*!40100 SET @@SQL_MODE='' */ 38 Query /*!40103 SET TIME_ZONE='+00:00' */ 38 Query FLUSH /*!40101 LOCAL */ TABLES 38 Query FLUSH TABLES WITH READ LOCK 38 Query SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ 38 Query START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ 38 Query SHOW VARIABLES LIKE 'gtid/_mode' 38 Query SHOW MASTER STATUS 38 Query UNLOCK TABLES 38 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENG INE ORDER BY LOGFILE_GROUP_NAME 38 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('blocks_infos_opensearch')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NA ME 38 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 38 Init DB liuhe 38 Query SHOW TABLES LIKE 'blocks/_infos/_opensearch' 38 Query SAVEPOINT sp 38 Query show table status like 'blocks/_infos/_opensearch' 38 Query SET SQL_QUOTE_SHOW_CREATE=1 38 Query SET SESSION character_set_results = 'binary' 38 Query show create table `blocks_infos_opensearch` 38 Query SET SESSION character_set_results = 'utf8' 38 Query show fields from `blocks_infos_opensearch` 38 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos_opensearch` 38 Query SET SESSION character_set_results = 'binary' 38 Query use `liuhe` 38 Query select @@collation_database 38 Query SHOW TRIGGERS LIKE 'blocks/_infos/_opensearch' 38 Query SET SESSION character_set_results = 'utf8' 38 Query ROLLBACK TO SAVEPOINT sp 38 Query RELEASE SAVEPOINT sp 38 Quit 通过2.4和2.1对比可以知道，当同时添加上参数--master-data和 --single-transaction 的时候，会执行 FLUSH TABLES WITH READ LOCK（但是还没有开始备份，在 SHOW MASTER STATUS显示了主库的binlog状态之后就unlock tables了），也会 SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ，之后和单独加--single-transaction是一样的； 2.5:如果是myisam引擎会怎么样？（创建了存储引擎为myisam的表liu） bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe --single-transaction liuhe liu>/mysql/liu.sql 查看general log发现和innodb 添加--single-transaction参数的情况是一样的执行过程 bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 19:54:28 3 Quit 180429 19:55:29 4 Connect root@localhost on 4 Query /*!40100 SET @@SQL_MODE='' */ 4 Query /*!40103 SET TIME_ZONE='+00:00' */ 4 Query SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ 4 Query START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ 4 Query SHOW VARIABLES LIKE 'gtid/_mode' 4 Query UNLOCK TABLES 4 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('liu'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENGINE ORDER BY LOGFILE _GROUP_NAME 4 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('liu')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NAME 4 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 4 Init DB liuhe 4 Query SHOW TABLES LIKE 'liu' 4 Query SAVEPOINT sp 4 Query show table status like 'liu' 4 Query SET SQL_QUOTE_SHOW_CREATE=1 4 Query SET SESSION character_set_results = 'binary' 4 Query show create table `liu` 4 Query SET SESSION character_set_results = 'utf8' 4 Query show fields from `liu` 4 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `liu` 4 Query SET SESSION character_set_results = 'binary' 4 Query use `liuhe` 4 Query select @@collation_database 4 Query SHOW TRIGGERS LIKE 'liu' 4 Query SET SESSION character_set_results = 'utf8' 4 Query ROLLBACK TO SAVEPOINT sp 4 Query RELEASE SAVEPOINT sp 4 Quit bogon:root@/mysql/data/data> 分析： 虽然添加了--single-transaction参数的myisam表处理过程和innodb的过程完全一致，但是因为myisam不支持事务，在整个dump过程中无法保证可重复读，无法得到一致性的备份。而innodb在备份过程中，虽然其他线程也在写数据，但是dump出来的数据能保证是备份开始时那个binlog pos的数据。 myisam引擎要保证得到一致性的数据的可以通过添加--lock-all-tables，这样在flush tables with read lock后，直到整个dump过程结束，断开线程后才会unlock tables释放锁(没必要主动发unlock tables指令)，整个dump过程其他线程不可写，从而保证数据的一致性; 2.6:备份myisam的时候，加上--lock-all-tables和不加该参数的不同的执行过程如下： 2.6.1加上--lock-all-tables的情况如下： bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe --lock-all-tables liuhe liu>/mysql/liu.sql bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 20:16:12 5 Quit 180429 20:18:18 6 Connect root@localhost on 6 Query /*!40100 SET @@SQL_MODE='' */ 6 Query /*!40103 SET TIME_ZONE='+00:00' */ 6 Query FLUSH TABLES 180429 20:18:19 6 Query FLUSH TABLES WITH READ LOCK 6 Query SHOW VARIABLES LIKE 'gtid/_mode' 6 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('liu'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENGINE ORDER BY LOGFILE _GROUP_NAME 6 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('liu')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NAME 6 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 6 Init DB liuhe 6 Query SHOW TABLES LIKE 'liu' 6 Query show table status like 'liu' 6 Query SET SQL_QUOTE_SHOW_CREATE=1 6 Query SET SESSION character_set_results = 'binary' 6 Query show create table `liu` 6 Query SET SESSION character_set_results = 'utf8' 6 Query show fields from `liu` 6 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `liu` 6 Query SET SESSION character_set_results = 'binary' 6 Query use `liuhe` 6 Query select @@collation_database 6 Query SHOW TRIGGERS LIKE 'liu' 6 Query SET SESSION character_set_results = 'utf8' 6 Quit 2.6.2不加上--lock-all-tables得过程如下： bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe liuhe liu>/mysql/liu.sql bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 20:25:43 7 Quit 180429 20:25:53 8 Connect root@localhost on 8 Query /*!40100 SET @@SQL_MODE='' */ 8 Query /*!40103 SET TIME_ZONE='+00:00' */ 8 Query SHOW VARIABLES LIKE 'gtid/_mode' 8 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('liu'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENGINE ORDER BY LOGFILE _GROUP_NAME 8 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA='liuhe' AND TABLE_NAME IN ('liu')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NAME 8 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 8 Init DB liuhe 8 Query SHOW TABLES LIKE 'liu' 8 Query LOCK TABLES `liu` READ /*!32311 LOCAL */ 8 Query show table status like 'liu' 8 Query SET SQL_QUOTE_SHOW_CREATE=1 8 Query SET SESSION character_set_results = 'binary' 8 Query show create table `liu` 8 Query SET SESSION character_set_results = 'utf8' 8 Query show fields from `liu` 8 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `liu` 8 Query SET SESSION character_set_results = 'binary' 8 Query use `liuhe` 8 Query select @@collation_database 8 Query SHOW TRIGGERS LIKE 'liu' 8 Query SET SESSION character_set_results = 'utf8' 8 Query UNLOCK TABLES 8 Quit 对比2.6.1和2.6.2可以知道myisam表，加上--lock-all-tables和不加上--lock-all-tables得区别就在于前者是FLUSH TABLES WITH READ LOCK对整个实例所有的表都上读锁，后者只针对要备份的表加读锁（LOCK TABLES `liu` READ）；并且前者并没有显现的unlock tables，因为整个过程中数据库是不能写的，并且FLUSH TABLES WITH READ LOCK这个命令一旦这个会话结束，相应的读锁也就不存在了，而后者只是锁了一个表，显现的unlock解锁了，其实后者也是会话结束就释放对表的读锁了，也可以不加unlock， 2.7.备份整个库时候，不加任何参数，可以看到会同时lock 这个库下的所有的表，最后在unlock bogon:root@/mysql/data/data>mysqldump -uroot -pliuwenhe liuhe > /mysql/blocks_infos_opensearch.sql
bogon:root@/mysql/data/data>more bogon.log /usr/local/mysql/bin/mysqld, Version: 5.6.38-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 21:21:44 9 Quit 180429 21:22:21 10 Connect root@localhost on 10 Query /*!40100 SET @@SQL_MODE='' */ 10 Query /*!40103 SET TIME_ZONE='+00:00' */ 10 Query SHOW VARIABLES LIKE 'gtid/_mode' 10 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHE MA.PARTITIONS WHERE TABLE_SCHEMA IN ('liuhe'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENGINE ORDER BY LOGFILE_GROUP_NAME 10 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SC HEMA.PARTITIONS WHERE TABLE_SCHEMA IN ('liuhe')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NAME 10 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 10 Init DB liuhe 10 Query show tables 10 Query LOCK TABLES `blocks_infos` READ /*!32311 LOCAL */,`blocks_infos1` READ /*!32311 LOCAL */,`blocks_inf os_opensearch` READ /*!32311 LOCAL */,`liu` READ /*!32311 LOCAL */,`test` READ /*!32311 LOCAL */ 10 Query show table status like 'blocks/_infos' 10 Query SET SQL_QUOTE_SHOW_CREATE=1 10 Query SET SESSION character_set_results = 'binary' 10 Query show create table `blocks_infos` 10 Query SET SESSION character_set_results = 'utf8' 10 Query show fields from `blocks_infos` 10 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos` 180429 21:22:23 10 Query SET SESSION character_set_results = 'binary' 10 Query use `liuhe` 10 Query select @@collation_database 10 Query SHOW TRIGGERS LIKE 'blocks/_infos' 10 Query SHOW CREATE TRIGGER `tri_insert_blocks_infos1` 10 Query SHOW CREATE TRIGGER `tri_update_blocks_infos1` 10 Query SHOW CREATE TRIGGER `tri_delete_blocks_infos1` 10 Query SHOW CREATE TRIGGER `tri_delete_blocks_infos` 10 Query SET SESSION character_set_results = 'utf8' 10 Query show table status like 'blocks/_infos1' 10 Query SET SQL_QUOTE_SHOW_CREATE=1 10 Query SET SESSION character_set_results = 'binary' 10 Query show create table `blocks_infos1` 10 Query SET SESSION character_set_results = 'utf8' 10 Query show fields from `blocks_infos1` 10 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos1` 10 Query SET SESSION character_set_results = 'binary' 10 Query use `liuhe` 10 Query select @@collation_database 10 Query SHOW TRIGGERS LIKE 'blocks/_infos1' 10 Query SET SESSION character_set_results = 'utf8' 10 Query show table status like 'blocks/_infos/_opensearch' 10 Query SET SQL_QUOTE_SHOW_CREATE=1 10 Query SET SESSION character_set_results = 'binary' 10 Query show create table `blocks_infos_opensearch` 10 Query SET SESSION character_set_results = 'utf8' 10 Query show fields from `blocks_infos_opensearch` 10 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `blocks_infos_opensearch` 180429 21:22:24 10 Query SET SESSION character_set_results = 'binary' 10 Query use `liuhe` 10 Query select @@collation_database 10 Query SHOW TRIGGERS LIKE 'blocks/_infos/_opensearch' 10 Query SET SESSION character_set_results = 'utf8' 10 Query show table status like 'liu' 10 Query SET SQL_QUOTE_SHOW_CREATE=1 10 Query SET SESSION character_set_results = 'binary' 10 Query show create table `liu` 10 Query SET SESSION character_set_results = 'utf8' 10 Query show fields from `liu` 10 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `liu` 10 Query SET SESSION character_set_results = 'binary' 10 Query use `liuhe` 10 Query select @@collation_database 10 Query SHOW TRIGGERS LIKE 'liu' 10 Query SET SESSION character_set_results = 'utf8' 10 Query show table status like 'test' 10 Query SET SQL_QUOTE_SHOW_CREATE=1 10 Query SET SESSION character_set_results = 'binary' 10 Query show create table `test` 10 Query SET SESSION character_set_results = 'utf8' 10 Query show fields from `test` 10 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `test` 10 Query SET SESSION character_set_results = 'binary' 10 Query use `liuhe` 10 Query select @@collation_database 10 Query SHOW TRIGGERS LIKE 'test' 10 Query SET SESSION character_set_results = 'utf8' 10 Query UNLOCK TABLES 10 Quit 2.8：备份整个实例的所有的库加上参数--all-databases 或者-A ，然后不加别的参数 如果是备份整个实例的所有的库，也就加上参数--all-databases 或者-A ，然后不加别的参数，如下可知，是一个库一库的去备份，也就是说先备份库A，把库A下的所有的表上读锁，备份完库A，unlock tables,然后再备份库B,把库B下的所有的表上读锁，备份完库B，unlock tables,这样就可以知道，不加任何参数的话，全实例备份时，只能保证一个库下的所有的表是一致性的，但是库和库之间却不能保证一致性； [root@oracle3 ~]# more /home/mysql/data/data/oracle3.log /usr/local/mysql/bin/mysqld, Version: 5.6.39-log (Source distribution). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time Id Command Argument 180429 21:58:24 35 Query show variables like '%general_log_file%' 180429 21:58:30 35 Quit 180429 21:58:45 36 Connect root@localhost on 36 Query /*!40100 SET @@SQL_MODE='' */ 36 Query /*!40103 SET TIME_ZONE='+00:00' */ 36 Query SHOW VARIABLES LIKE 'gtid/_mode' 36 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SC HEMA.FILES WHERE FILE_TYPE = 'UNDO LOG' AND FILE_NAME IS NOT NULL GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENGINE ORDER BY LOGFILE_GR OUP_NAME 36 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FR OM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NAME 36 Query SHOW DATABASES 36 Query SHOW VARIABLES LIKE 'ndbinfo/_version' 36 Init DB liuhe 36 Query SHOW CREATE DATABASE IF NOT EXISTS `liuhe` 36 Query show tables 36 Query UNLOCK TABLES 36 Init DB liuwenhe 36 Query SHOW CREATE DATABASE IF NOT EXISTS `liuwenhe` 36 Query show tables 36 Query LOCK TABLES `test` READ /*!32311 LOCAL */ 36 Query show table status like 'test' 36 Query SET SQL_QUOTE_SHOW_CREATE=1 36 Query SET SESSION character_set_results = 'binary' 36 Query show create table `test` 36 Query SET SESSION character_set_results = 'utf8' 36 Query show fields from `test` 36 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `test` 36 Query SET SESSION character_set_results = 'binary' 36 Query use `liuwenhe` 36 Query select @@collation_database 36 Query SHOW TRIGGERS LIKE 'test' 36 Query SET SESSION character_set_results = 'utf8' 36 Query UNLOCK TABLES 36 Init DB mysql 36 Query SHOW CREATE DATABASE IF NOT EXISTS `mysql` 36 Query show tables 36 Query LOCK TABLES `columns_priv` READ /*!32311 LOCAL */,`db` READ /*!32311 LOCAL */,`event` READ /*!32311 LOCAL */,`func` READ /*!32311 LOCAL */,`help_category` READ /*!32311 LOCAL */,`help_keyword` READ /*!32311 LOCAL */,`help_relation` READ /*!32311 LOCAL */,`help_topic` READ /*!32311 LOCAL */,`innodb_index_stats` READ /*!32311 LOCAL */,`innodb_table_stats` READ /*! 32311 LOCAL */,`ndb_binlog_index` READ /*!32311 LOCAL */,`plugin` READ /*!32311 LOCAL */,`proc` READ /*!32311 LOCAL */,`procs_priv` READ /*!32311 LOCAL */,`proxies_priv` READ /*!32311 LOCAL */,`servers` READ /*!32311 LOCAL */,`slave_master_info` READ /*!32311 LOCA L */,`slave_relay_log_info` READ /*!32311 LOCAL */,`slave_worker_info` READ /*!32311 LOCAL */,`tables_priv` READ /*!32311 LOCAL */,` time_zone` READ /*!32311 LOCAL */,`time_zone_leap_second` READ /*!32311 LOCAL */,`time_zone_name` READ /*!32311 LOCAL */,`time_zone_ transition` READ /*!32311 LOCAL */,`time_zone_transition_type` READ /*!32311 LOCAL */,`user` READ /*!32311 LOCAL */ 题外话 考虑一下，我们知道当没有添加任何参数的时候，mysqldump默认也会lock 这个需要备份的表，但是如果mysiam引擎中也添加--single-transaction参数（这样备份出来的数据就是不一致的）再用这个备份去创建从库或恢复到指定时间点，会有什么样的影响？ 我个人的理解是如果整个dump过程中只有简单的insert操作，是没有关系的，期间肯定会有很多的主键重复错误，直接跳过或忽略就好了。如果是update操作，那就要出问题了，分几种情况考虑 1） 如果是基于时间点的恢复，假设整个dump过程有update a set id=5 where id=4之类的操作，相当于重复执行两次该操作，应该问题不大 2） 如果是创建从库，遇到上面的sql从库会报错，找不到该记录，这时跳过就好 3）不管是恢复还是创建从库，如果dump过程中有update a set id=id+5 之类的操作，那就有问题，重复执行两次，数据全变了。
