rpl_semi_sync_master_wait_no_slave 參數研究實驗

最近在研究MySQL，剛學到半同步。

半同步的配置中，關於這兩個參數：

　　rpl_semi_sync_master_wait_no_slave

　　rpl_semi_sync_master_wait_for_slave_count

發現很不好搞懂，請教了一些老師，也做了一些資料搜索，每個人給我的答案都不同：

　　

中文網絡上一些典型的說法，這些都是錯誤的！！！：
rpl_semi_sync_master_wait_for_slave_count：
　　控制slave應答的數量，默認是1，表示master接收到幾個slave應答后才commit。
　　 rpl_semi_sync_master_wait_no_slave ：
　　1.需要等待幾個slave節點的ACK，否則一直waiting。
　　2.當一個事務被提交，但是Master沒有Slave連接，這時M不可能收到任何確認信息，但M會在時間限制范圍內繼續等待。如果沒有Slave鏈接，會切換到異步復制。是否允許master每個事務提交后都要等待slave的接收確認信號。默認為on，每一個事務都會等待。如果為off，則slave追趕上后，也不會開啟半同步復制模式，需要手工開啟。

 

好奇心驅使我通過實驗來驗證大家的說法，可惜的是——都是錯的。通過官方文檔，看的雲里霧里的，只好自己探索。

 

探索內容

　　半同步參數

• rpl_semi_sync_master_wait_no_slave（重點）
• rpl_semi_sync_master_wait_for_slave_count

環境信息

role	ip	port	hostname	etc
master	192.168.188.101	4306	mysqlvm1	提示符為mysql>
slave	192.168.188.201	4306	mysqlvm1-1	提示符為mysql4306>
		5306		提示符為mysql5306>
		6306		提示符為mysql6306>
		7306		提示符為mysql7306>

 

MySQL版本

　　5.7.26

 

前置條件

　　已配置好主從復制。

 

配置增強半同步

　　1.加載lib，所有主從節點都要配置。

　　　　主庫&從庫：

install plugin rpl_semi_sync_master soname 'semisync_master.so';

install plugin rpl_semi_sync_slave soname 'semisync_slave.so';   

 

      

　　2.查看，確保所有節點都成功加載。

　　　　mysql> show plugins;

　　　　　　| rpl_semi_sync_master       | ACTIVE   | REPLICATION        | semisync_master.so | GPL     |

　　　　　　| rpl_semi_sync_slave        | ACTIVE   | REPLICATION        | semisync_slave.so  | GPL     |

 

 

　　3.啟用半同步

　　　　1.先啟用從庫上的參數，最后啟用主庫的參數。

　　　　從庫：

set global rpl_semi_sync_slave_enabled = 1;   # 1：啟用，0：禁止

 

　　　　主庫：

　　　　　　　　

set global rpl_semi_sync_master_enabled = 1;   # 1：啟用，0：禁止

set global rpl_semi_sync_master_timeout = 60000;       # 60秒，時間長些便於實驗

 

　　　　2.從庫重啟io_thread

　　　　stop slave io_thread;

　　　　start slave io_thread;

 

　　　　查看主庫參數

　　　　　　

mysql> show global variables like "%sync%"; show global status like "%sync%";

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| binlog_group_commit_sync_delay            | 100        |
| binlog_group_commit_sync_no_delay_count   | 10         |
| innodb_flush_sync                         | ON         |
| innodb_sync_array_size                    | 1          |
| innodb_sync_spin_loops                    | 30         |
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 60000      |
| rpl_semi_sync_master_trace_level          | 32         |
| rpl_semi_sync_master_wait_for_slave_count | 3          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
| rpl_semi_sync_slave_trace_level           | 32         |
| sync_binlog                               | 1          |
| sync_frm                                  | ON         |
| sync_master_info                          | 10000      |
| sync_relay_log                            | 10000      |
| sync_relay_log_info                       | 10000      |
+-------------------------------------------+------------+
18 rows in set (0.00 sec)

 

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Innodb_data_fsyncs                         | 664      |
| Innodb_data_pending_fsyncs                 | 0        |
| Innodb_os_log_fsyncs                       | 413      |
| Innodb_os_log_pending_fsyncs               | 0        |
| Rpl_semi_sync_master_clients               | 4        |
| Rpl_semi_sync_master_net_avg_wait_time     | 0        |
| Rpl_semi_sync_master_net_wait_time         | 0        |
| Rpl_semi_sync_master_net_waits             | 199      |
| Rpl_semi_sync_master_no_times              | 21       |
| Rpl_semi_sync_master_no_tx                 | 48       |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_master_timefunc_failures     | 0        |
| Rpl_semi_sync_master_tx_avg_wait_time      | 3280008  |
| Rpl_semi_sync_master_tx_wait_time          | 72160195 |
| Rpl_semi_sync_master_tx_waits              | 22       |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0        |
| Rpl_semi_sync_master_wait_sessions         | 0        |
| Rpl_semi_sync_master_yes_tx                | 20       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+

19 rows in set (0.00 sec)

 

 

============================我是分割線=====================================

 

 

實驗1：

場景：

　　rpl_semi_sync_master_wait_no_slave=ON

　　rpl_semi_sync_master_wait_for_slave_count=3

　　slave存活1，其他slave停止，會發生什么？

步驟：

　　1.只保留一個slave，停止其他3個slave

mysql5306> stop slave;
mysql6306> stop slave;
mysql7306> stop slave;

 

　　2.立即查詢master

　　　　為減少信息干擾，只截取了需要關注的數據

mysql> show global variables like "%sync%"; show global status like "%sync%";

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 60000      |
| rpl_semi_sync_master_wait_for_slave_count | 3          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
+-------------------------------------------+------------+
18 rows in set (0.00 sec)

 

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 4        |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

　　3.等待一分鍾，再查詢master

mysql> show global variables like "%sync%"; show global status like "%sync%";

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 60000      |
| rpl_semi_sync_master_wait_for_slave_count | 3          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
+-------------------------------------------+------------+
18 rows in set (0.00 sec)

 

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 1        |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

　　4.另外開啟一個master會話，開始一個事務並提交

mysql> insert into new values(4);
[掛起……]

 

　　5.查看master

　　　　發現無變化

mysql> show global variables like "%sync%"; show global status like "%sync%";

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 60000      |
| rpl_semi_sync_master_wait_for_slave_count | 3          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
+-------------------------------------------+------------+
18 rows in set (0.00 sec)

 

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 1        |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

　　6.查看存活的slave

　　　　發現事務已經被應用

mysql> select * from new;

+------+
| name |
+------+
|    2 |
+------+
1 rows in set (0.00 sec)

 

 

　　7.等待master事務超時后完成。

　　 　　這里master提交到收到結果，用了1分鍾0.01秒,該事務因slave的ack應答不夠，發生了等待。

mysql> insert into new values(4);
Query OK, 1 row affected (1 min 0.01 sec)

 

 

　　8.查看master

　　　　可以發現，事務超時后，master已經轉為異步復制（Rpl_semi_sync_master_status=OFF）

 

mysql> show global variables like "%sync%"; show global status like "%sync%";

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 60000      |
| rpl_semi_sync_master_wait_for_slave_count | 3          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
+-------------------------------------------+------------+
18 rows in set (0.00 sec)

 

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 1        |
| Rpl_semi_sync_master_status                | OFF      |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

　　9.依次啟動已經停止的slave，每啟動一個，便立即查看一次master狀態

　　　　可以看到，啟動slave的動作會立即被master接收到，並且master會自動切換回半同步模式（Rpl_semi_sync_master_status=ON）

 

mysql5306> start slave;

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 2        |
| Rpl_semi_sync_master_status                | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

mysql6306> start slave;

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 3        |
| Rpl_semi_sync_master_status                | ON       |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

mysql7306> start slave;

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 4        |
| Rpl_semi_sync_master_status                | ON       |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

============================我是分割線=====================================

 

實驗2：

場景：

　　rpl_semi_sync_master_wait_no_slave=OFF

　　rpl_semi_sync_master_wait_for_slave_count=3

　　slave存活1，其他slave停止，會發生什么？

步驟：

　　1.繼續實驗1的環境，首先設置參數並查看master狀態

　　　　為減少信息干擾，只截取了需要關注的數據 

mysql> set global rpl_semi_sync_master_wait_no_slave=OFF;
Query OK, 0 rows affected (0.00 sec)

mysql> show global variables like "%sync%"; show global status like "%sync%";

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 60000      |
| rpl_semi_sync_master_wait_for_slave_count | 3          |
| rpl_semi_sync_master_wait_no_slave        | OFF        |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
+-------------------------------------------+------------+
18 rows in set (0.00 sec)

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 4        |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

　　2.只保留一個slave，依次停止其他3個slave

mysql5306> stop slave;
mysql6306> stop slave;
mysql7306> stop slave;

 

 

　　3.等待一分鍾，查看master狀態。（在等待期間可以通過反復查看master狀態，來觀察master數據的變化）

　　　　master已經轉為異步模式復制。 

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 1        |
| Rpl_semi_sync_master_status                | OFF      |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

　　4.使用另一個master的會話，開始一個事務並提交。 這里master提交到收到結果，該事務沒有發生等待。

mysql> insert into new values(5);
Query OK, 1 row affected (0.02 sec)

 

 

　　5.立即查看存活的slave

　　　　　事務已經被應用。

mysql> select * from new;

+------+
| name |
+------+
|    4 |
|    5 |
+------+
2 rows in set (0.00 sec)

 

 

　　6.依次啟動其他slave，每啟動一個便立即查看master狀態

　　　　可以看到，啟動slave的動作會立即被master接收到，並且master會自動切換回半同步模式（Rpl_semi_sync_master_status=ON）

mysql5306> start slave;

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 2        |
| Rpl_semi_sync_master_status                | OFF      |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

mysql6306> start slave;

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 3        |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

mysql7306> start slave;

mysql> show global status like "%sync%";

+--------------------------------------------+----------+
| Variable_name                              | Value    |
+--------------------------------------------+----------+
| Rpl_semi_sync_master_clients               | 4        |
| Rpl_semi_sync_master_status                | ON       |
| Rpl_semi_sync_slave_status                 | OFF      |
+--------------------------------------------+----------+
19 rows in set (0.00 sec)

 

 

============================我是分割線=====================================

結論：

使用直白簡短好理解的語言：

 

【rpl_semi_sync_master_wait_for_slave_count】 ，

　　1.master提交后所需的應答數量！如果slave clients數量大於等於這個值，那么master會一路暢行無阻；如果低於這個值，master可能會在事務提交階段發生一次超時等待，當等待超過參數（rpl_semi_sync_master_timeout）設定時，master就轉為異步模式（原理見下一個參數）。

　　2.master將這個參數值作為標桿，用來和【Rpl_semi_sync_master_clients】參數做比較。

【rpl_semi_sync_master_wait_no_slave】

　　1.為OFF時，只要master發現【Rpl_semi_sync_master_clients】小於【rpl_semi_sync_master_wait_for_slave_count】，則master立即轉為異步模式。

　　2.為ON時，空閑時間（無事務提交）里，即使master發現【Rpl_semi_sync_master_clients】小於【rpl_semi_sync_master_wait_for_slave_count】，也不會做任何調整。只要保證在事務超時之前，master收到大於等於【rpl_semi_sync_master_wait_for_slave_count】值的ACK應答數量，master就一直保持在半同步模式；如果在事務提交階段（master等待ACK）超時，master才會轉為異步模式。

 

無論【rpl_semi_sync_master_wait_no_slave】為ON還是OFF，當slave上線到【rpl_semi_sync_master_wait_for_slave_count】值時，master都會自動由異步模式轉為半同步模式。

 

粗略看來，設置為OFF時，master不會因為slave的離線而造成事務等待，這似乎是一個更合適的選擇，但是為什么5.7中默認參數為ON呢？目前我還不得而知。

另外，本次實驗建立在空閑場景（或者說極微小負載場景）下，在高並發的場景下，這個參數又會導致怎樣的結果，這塊也尚需后續探索。

 

 

 ============================我是分割線=====================================

官方文檔關於參數rpl_semi_sync_master_wait_no_slave的解釋:

 

Controls whether the master waits for the timeout period configured by rpl_semi_sync_master_timeout to expire, even if the slave count drops to less than the number of slaves configured by pl_semi_sync_master_wait_for_slave_count during the timeout period.

 

When the value of rpl_semi_sync_master_wait_no_slave is ON (the default), it is permissible for the slave count to drop to less than rpl_semi_sync_master_wait_for_slave_count during the timeout period. As long as enough slaves acknowledge the transaction before the timeout period expires, semisynchronous replication continues.

 

When the value of rpl_semi_sync_master_wait_no_slave is OFF, if the slave count drops to less than the number configured in rpl_semi_sync_master_wait_for_slave_count at any time during the timeout period configured by rpl_semi_sync_master_timeout, the master reverts to normal replication.

 

This variable is available only if the master-side semisynchronous replication plugin is installed.

 

官方文檔關於參數rpl_semi_sync_master_wait_for_slave_count的解釋:

 

The number of slave acknowledgments the master must receive per transaction before proceeding. By default rpl_semi_sync_master_wait_for_slave_count is 1, meaning that semisynchronous replication proceeds after receiving a single slave acknowledgment. Performance is best for small values of this variable.

 

For example, if rpl_semi_sync_master_wait_for_slave_count is 2, then 2 slaves must acknowledge receipt of the transaction before the timeout period configured by rpl_semi_sync_master_timeout for semisynchronous replication to proceed. If less slaves acknowledge receipt of the transaction during the timeout period, the master reverts to normal replication.

 

 

============================我是結束線=====================================

EOF