my07_lock-tables与single-transaction的区别

概念描述

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mysqldump进行逻辑备份时(innodb)，为保证事务的一致性，会进行加锁，通常使用以下两个参数，

锁定一个schema对象中的所有表，默认开启，可保证一个库中所有表的状态一致
-l, --lock-tables Lock all tables for read.(Defaults to on; use --skip-lock-tables to disable.)

必须是innodb存储引擎，备份过程中不能出现DDL语句，使用该参数则自动关闭lock-tables参数

--single-transaction
Creates a consistent snapshot by dumping all tables in a single transaction. Works ONLY for tables stored in storage engines which support multiversioning (currently only InnoDB does); the dump is NOT guaranteed to beconsistent for other storage engines. While a
--single-transaction dump is in process, to ensure a valid dump file (correct table contents and binary log position), no other connection should use the following statements: ALTER TABLE, DROP TABLE, RENAME TABLE,TRUNCATE TABLE, as consistent snapshot is not isolated from them. Option automatically turns off --lock-tables.

补充说明：因为single-transaction 使用了“ a consistent snapshot”一致性快照，它除了在开始生成一致性快照时锁定所有表外（很小的一段时间，几秒或者更小），在后续备份到某个表时才会锁定某个表，因此它对线上业务影响小，为在线备份推荐选项

测试准备

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就是设计一套测试方案，来验证一下以上的观点

两台虚拟机，分别做时间校准，red6是mysql数据库（5.7.22版本），red4是备份恢复机
[root@red6 network-scripts]# /sbin/ntpdate 182.92.12.11
30 Jul 14:40:52 ntpdate[7592]: step time server 182.92.12.11 offset 182.635092 sec
[root@red4 ~]# /sbin/ntpdate 182.92.12.11
30 Jul 14:38:04 ntpdate[10222]: adjust time server 182.92.12.11 offset 0.002791 sec

创建测试用户
create database txdb character set utf8;
create user 'automng'@'%' identified by 'Automng_123';
ALTER USER 'automng'@'%' IDENTIFIED WITH mysql_native_password BY 'Automng_123';
GRANT ALL PRIVILEGES ON *.* TO 'automng'@'%' WITH GRANT OPTION;

sysbench初始化数据，50张表，每张5万数据
sysbench --test=/opt/soft/sysbench/sysbench/tests/db/parallel_prepare.lua --oltp-tables-count=50 --oltp-table-size=50000 --num-threads=2 --mysql-table-engine=innodb --mysql-user=automng --mysql-password=Automng_123 --mysql-port=3318 --mysql-host=192.168.56.201 --mysql-db=txdb run
sysbench --test=/opt/soft/sysbench/sysbench/tests/db/parallel_prepare.lua --oltp-tables-count=50 --oltp-table-size=50000 --num-threads=2 --mysql-table-engine=innodb --mysql-user=automng --mysql-password=Automng_123 --mysql-port=3318 --mysql-host=192.168.56.201 --mysql-db=txdb cleanup

lock-tables备份语句
/usr/local/mysql/bin/mysqldump -h$host -uautomng -pAutomng_123 -P$port --log-error=$bkdir/$today_bklog --set-gtid-purged=OFF --add-drop-database --add-drop-table --flush-logs --lock-tables --triggers --routines --events --flush-privileges --master-data=2 --default-character-set=$charset $db_name > $bkdir/$today_bkfile

准备好的更新测试语句
select now();
update sbtest10 set pad='wa ka ka' where id=11;
select now();
update sbtest10 set pad='1234567' where id=12;
select now();
update sbtest10 set pad='wa ka ka' where id=13;
select now();
update sbtest10 set pad='1234567' where id=14;
select now();

single-transaction备份语句
/usr/local/mysql/bin/mysqldump -h$host -uautomng -pAutomng_123 -P$port --log-error=$bkdir/$today_bklog --set-gtid-purged=OFF --add-drop-database --add-drop-table --flush-logs --single-transaction --triggers --routines --events --flush-privileges --master-data=2 --default-character-set=$charset $db_name > $bkdir/$today_bkfile
准备好的更新测试语句
select now();
update sbtest21 set pad='wa ka ka' where id=21;
select now();
update sbtest31 set pad='1234567' where id=22;
select now();
update sbtest41 set pad='wa ka ka' where id=23;
select now();
update sbtest41 set pad='1234567' where id=24;
select now();

备份过程在脚本中进行，会自动打印出备份的起始时间

lock-tables参数测试

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备份的起始时间
start backup txdb 2018-07-30 14:53:53
end backup 2018-07-30 14:54:32

DML语句的执行，手工复制所有语句，一次性粘贴到SQL命令行

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 14:53:58 |
+---------------------+
1 row in set (0.04 sec)
 
mysql> update sbtest10 set pad='wa ka ka' where id=11;
 
Query OK, 1 row affected (33.98 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 14:54:32 |
+---------------------+
1 row in set (0.00 sec)
 
mysql> update sbtest10 set pad='' where id=12;
Query OK, 1 row affected (0.17 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 14:54:32 |
+---------------------+
1 row in set (0.00 sec)
 
mysql> update sbtest10 set pad='wa ka ka' where id=13;
Query OK, 1 row affected (0.14 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 14:54:32 |
+---------------------+
1 row in set (0.00 sec)
 
mysql> update sbtest10 set pad='' where id=14;
Query OK, 1 row affected (0.11 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 14:54:32 |
+---------------------+
1 row in set (0.00 sec)
 
mysql>
mysql>
mysql> show warnings;
Empty set (0.03 sec)

由于DML每条语句的执行几乎不耗时，而第一条语句的结束时间与备份的结束时间完全吻合。印证了以上lock-tables锁全表的观点完全。

single-transaction参数测试

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备份的起止时间

start backup txdb 2018-07-30 15:20:33
end backup 2018-07-30 15:21:22

DML语句的执行，手工复制所有语句，一次性粘贴到SQL命令行

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 15:20:37 |
+---------------------+
1 row in set (0.00 sec)
 
mysql> update sbtest21 set pad='wa ka ka' where id=21;
Query OK, 1 row affected (0.35 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 15:20:38 |
+---------------------+
1 row in set (0.00 sec)
 
mysql> update sbtest31 set pad='' where id=22;
Query OK, 1 row affected (0.09 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 15:20:38 |
+---------------------+
1 row in set (0.01 sec)
 
mysql> update sbtest41 set pad='wa ka ka' where id=23;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 15:20:38 |
+---------------------+
1 row in set (0.00 sec)
 
mysql> update sbtest41 set pad='' where id=24;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0
 
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-07-30 15:20:38 |
+---------------------+
1 row in set (0.00 sec)

33秒开始备份，37秒开始执行DML，38秒时执行完毕，DML没有被事务锁阻塞。印证了single-transaction不是锁定所有表的观点。

补充说明，该验证方法是以抽样的形式（随机测试了其中一张表），从“结论的角度（lock-tables 会锁一个schema的所有表，另外一个则不会）”验证，并不严谨，但有一定参考价值。