mysql數據庫讀寫分離的好處

雖然知道處理大數據量時，數據庫要做讀寫分離，但是爲什麼讀寫分離可以提高性能呢？
下面是搜來的一些解釋，看看再說！
一 什麼是讀寫分離
      
MySQL Proxy最強大的一項功能是實現“讀寫分離(Read/Write Splitting)”。基本的原理是讓主數據庫處理事務性查詢，而從數據庫處理SELECT查詢。數據庫複製被用來把事務性查詢導致的變更同步到集羣中 的從數據庫。 當然，主服務器也可以提供查詢服務。使用讀寫分離最大的作用無非是環境服務器壓力。可以看下這張圖：


 
二 讀寫分離的好處


1.增加冗餘
2.增加了機器的處理能力
3.對於讀操作爲主的應用，使用讀寫分離是最好的場景，因爲可以確保寫的服務器壓力更小，而讀又可以接受點時間上的延遲。
 
三 讀寫分離提高性能之原因
 
1.物理服務器增加，負荷增加
2.主從只負責各自的寫和讀，極大程度的緩解X鎖和S鎖爭用
3.從庫可配置myisam引擎，提升查詢性能以及節約系統開銷
4.從庫同步主庫的數據和主庫直接寫還是有區別的，通過主庫發送來的binlog恢復數據，但是，最重要區別在於主庫向從庫發送binlog是異步的，從庫恢復數據也是異步的
5.讀寫分離適用與讀遠大於寫的場景，如果只有一臺服務器，當select很多時，update和delete會被這些select訪問中的數據堵塞，等待select結束，併發性能不高。 對於寫和讀比例相近的應用，應該部署雙主相互複製
6.可以在從庫啓動是增加一些參數來提高其讀的性能，例如--skip-innodb、--skip-bdb、--low-priority-updates以及--delay-key-write=ALL。當然這些設置也是需要根據具體業務需求來定得，不一定能用上
7.分攤讀取。假如我們有1主3從，不考慮上述1中提到的從庫單方面設置，假設現在1 分鐘內有10條寫入，150條讀取。那麼，1主3從相當於共計40條寫入，而讀取總數沒變，因此平均下來每臺服務器承擔了10條寫入和50條讀取（主庫不 承擔讀取操作）。因此，雖然寫入沒變，但是讀取大大分攤了，提高了系統性能。另外，當讀取被分攤後，又間接提高了寫入的性能。所以，總體性能提高了，說白 了就是拿機器和帶寬換性能。MySQL官方文檔中有相關演算公式：官方文檔 見6.9FAQ之“MySQL複製能夠何時和多大程度提高系統性能”
8.MySQL複製另外一大功能是增加冗餘，提高可用性，當一臺數據庫服務器宕機後能通過調整另外一臺從庫來以最快的速度恢復服務，因此不能光看性能，也就是說1主1從也是可以的。
 
四 讀寫分離示意圖
  
    
 
五 讀寫分離模擬
 
實驗環境簡介
serv01：代理服務器 192.168.1.11 serv01.host.com
serv08：主服務器（主要寫數據，可讀可寫） 192.168.1.18 serv08.host.com
serv09：從服務器（主要讀數據） 192.168.1.19 serv09.host.com
 
操作系統版本
RHEL Server6.1 64位系統
 
使用到的軟件包版本
mysql-5.5.29-linux2.6-x86_64.tar.gz
 mysql-proxy-0.8.2-linux-glibc2.3-x86-64bit.tar.gz
 
第一步，搭建MySQL服務器，清空日誌。注意：代理服務器中不需要裝MySQL
 
第二步，拷貝mysql-proxy-0.8.2-linux-glibc2.3-x86-64bit.tar.gz文件，解壓文件
[root@larrywen 1005]# scp /opt/soft/ule-mysql/mysql-proxy-0.8.2-linux-glibc2.3-x86-64bit.tar.gz 192.168.1.11:/opt [root@serv01 opt]# tar -xvf mysql-proxy-0.8.2-linux-glibc2.3-x86-64bit.tar.gz -C /usr/local/ [root@serv01 opt]# cd /usr/local/ [root@serv01 local]# mv mysql-proxy-0.8.2-linux-glibc2.3-x86-64bit/ mysql-proxy [root@serv01 local]# ll mysql-proxy/ total 24 drwxr-xr-x. 2 7157 wheel 4096 Aug 17 2011 bin drwxr-xr-x. 2 7157 wheel 4096 Aug 17 2011 include drwxr-xr-x. 4 7157 wheel 4096 Aug 17 2011 lib drwxr-xr-x. 2 7157 wheel 4096 Aug 17 2011 libexec drwxr-xr-x. 3 7157 wheel 4096 Aug 17 2011 licenses drwxr-xr-x. 3 7157 wheel 4096 Aug 17 2011 share --可以查看幫助 [root@serv01 bin]# ./mysql-proxy --help-all 
 
第三步，serv08主服務器創建用戶，serv09從服務器創建用戶，注意用戶名和密碼一致
serv08 mysql> grant all on *.* to 'larry'@'192.168.1.%' identified by 'larry'; Query OK, 0 rows affected (0.00 sec) serv09 mysql> grant all on *.* to 'larry'@'192.168.1.%' identified by 'larry'; Query OK, 0 rows affected (0.00 sec) 


第四步，serv09從服務器更改設置，開啓slave，查看slave狀態。創建測試數據庫，插入測試數據
serv09 mysql> change master to master_host='192.168.1.18', master_user='larry', master_password='larry', master_port=3306, master_log_file='mysql-bin.000001', master_log_pos=107; Query OK, 0 rows affected (0.01 sec) mysql> start slave; Query OK, 0 rows affected (0.00 sec) mysql> show slave status \G; *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 192.168.1.18 Master_User: larry Master_Port: 3306 Connect_Retry: 60 Master_Log_File: mysql-bin.000001 Read_Master_Log_Pos: 107 Relay_Log_File: serv09-relay-bin.000002 Relay_Log_Pos: 253 Relay_Master_Log_File: mysql-bin.000001  Slave_IO_Running: Yes 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: 107 Relay_Log_Space: 410 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: 0 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 2 1 row in set (0.00 sec) ERROR: No query specified mysql> select user,password,host from mysql.user; +------+-------------------------------------------+-----------------+ | user | password | host | +------+-------------------------------------------+-----------------+ | root | | localhost | | root | | serv08.host.com | | root | | 127.0.0.1 | | root | | ::1 | | | | localhost | | | | serv08.host.com | | rep | *0CDC8D34246E22649D647DB04E7CCCACAB4368B6 | 192.168.1.% | +------+-------------------------------------------+-----------------+ 7 rows in set (0.00 sec) mysql> create database larrydb; Query OK, 1 row affected (0.00 sec) mysql> use larrydb; Database changed mysql> create table user(id int, name varchar(30)); Query OK, 0 rows affected (0.01 sec) mysql> insert into user values(1,'larrywen'); Query OK, 1 row affected (0.01 sec) mysql> insert into user values(2,'wentasy'); Query OK, 1 row affected (0.00 sec) mysql> select * from user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | +------+----------+ 2 rows in set (0.00 sec) serv09 mysql> select * from larrydb.user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | +------+----------+ 2 rows in set (0.00 sec) 


第五步，爲了查看現象，serv09從服務器關閉slave
mysql> stop slave;
Query OK, 0 rows affected (0.01 sec)


第六步，serv 01查看是否有MySQL用戶，修改rw-splitting.lua文件，修改如下幾個參數
[root@serv01 mysql-proxy]# id mysql uid=500(mysql) gid=500(mysql) groups=500(mysql) [root@serv01 mysql-proxy]# vim rw-splitting.lua [root@serv01 mysql-proxy]# cat rw-splitting.lua | grep -e min_idle_connections -e max_idle_connections -e is_debug min_idle_connections = 1,--最小空閒連接數，爲了測試，這裏設置爲1 max_idle_connections = 1,--最大空閒連接數，爲了測試，這裏設置爲1 is_debug = true--是否打開Debug調試，爲了查看調試信息，這裏設置爲true 
 
第七步，啓動mysql-proxy
[root@serv01 mysql-proxy]# /etc/init.d/mysql-proxy start Starting mysql-proxy: --先確定是否可以連接 [root@serv01 ~]# mysql -ularry -plarry -h 192.168.1.18 Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 6 Server version: 5.5.29-log Source distribution Copyright (c) 2000, 2012, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> exit Bye [root@serv01 ~]# mysql -ularry -plarry -h 192.168.1.19 Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 8 Server version: 5.5.29-log Source distribution Copyright (c) 2000, 2012, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> exit Bye 


第八步，查看現象
[root@serv01 ~]# /etc/init.d/mysql-proxy start Starting mysql-proxy: [root@serv01 ~]# mysql -ularry -plarry -h 192.168.1.11 [connect_server] 192.168.1.11:51054 [1].connected_clients = 0 [1].pool.cur_idle = 0 [1].pool.max_idle = 1 [1].pool.min_idle = 1 [1].type = 1 [1].state = 0 [1] idle-conns below min-idle Welcome to the MySQL monitor. Commands end with ; or \g. [read_query] 192.168.1.11:51054 current backend = 0 client default db = client username = larry query = select @@version_comment limit 1 sending to backend : 192.168.1.19:3306 is_slave : false server default db: server username : larry in_trans : false in_calc_found : false COM_QUERY : true Your MySQL connection id is 10 Server version: 5.5.29-log Source distribution Copyright (c) 2000, 2012, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> mysql> use larrydb; [read_query] 192.168.1.11:51054 current backend = 0 client default db = client username = larry query = SELECT DATABASE()  sending to backend : 192.168.1.19:3306 is_slave : false server default db: server username : larry in_trans : false in_calc_found : false COM_QUERY : true [read_query] 192.168.1.11:51054 current backend = 0 client default db = client username = larry sending to backend : 192.168.1.19:3306 is_slave : false server default db: server username : larry in_trans : false in_calc_found : false COM_QUERY : false Database changed mysql> select * from user; [read_query] 192.168.1.11:51054 current backend = 0 client default db = larrydb client username = larry query = select * from user sending to backend : 192.168.1.19:3306 is_slave : false server default db: larrydb server username : larry in_trans : false in_calc_found : false COM_QUERY : true +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | +------+----------+ 2 rows in set (0.00 sec) mysql> insert into user values(3,'jsutdb'); [read_query] 192.168.1.11:51644 current backend = 0 client default db = larrydb client username = larry query = insert into user values(3,'jsutdb')  sending to backend : 192.168.1.19:3306 is_slave : false server default db: larrydb server username : larry in_trans : false in_calc_found : false COM_QUERY : true Query OK, 1 row affected (0.00 sec) serv08 mysql> select * from user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | +------+----------+ 2 rows in set (0.00 sec) serv09 mysql> select * from larrydb.user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | | 3 | jsutdb | +------+----------+ 3 rows in set (0.00 sec) 


第九步，以上的測試雖有效果，但不是預期。排查原因，重新配置。發現proxy-read-only-backend-addresses和proxy-backend-addresses參數配置出錯，proxy-read-only-backend-addresses應該配置成從服務器的IP地址，proxy-backend-addresses應該配置成主服務器的IP地址。
[root@serv01 ~]# vim /etc/init.d/mysql-proxy [root@serv01 ~]# cat /etc/init.d/mysql-proxy #!/bin/sh # # mysql-proxy This script starts and stops the mysql-proxy daemon # # chkconfig: - 78 30 # processname: mysql-proxy # description: mysql-proxy is a proxy daemon to mysql # Source function library. . /etc/rc.d/init.d/functions #PROXY_PATH=/usr/local/bin PROXY_PATH=/usr/local/mysql-proxy/bin prog="mysql-proxy" # Source networking configuration. . /etc/sysconfig/network # Check that networking is up. [ ${NETWORKING} = "no" ] && exit 0 # Set default mysql-proxy configuration. #PROXY_OPTIONS="--daemon" PROXY_OPTIONS="--proxy-read-only-backend-addresses=192.168.1.19:3306 --proxy-backend-addresses=192.168.1.18:3306 --proxy-lua-script=/usr/local/mysql-proxy/share/doc/mysql-proxy/rw-splitting.lua" #PROXY_PID=/usr/local/mysql-proxy/run/mysql-proxy.pid PROXY_PID=/var/run/mysql-proxy.pid # Source mysql-proxy configuration. if [ -f /etc/sysconfig/mysql-proxy ]; then . /etc/sysconfig/mysql-proxy fi PATH=$PATH:/usr/bin:/usr/local/bin:$PROXY_PATH # By default it's all good RETVAL=0 # See how we were called. case "$1" in start) # Start daemon. echo -n $"Starting $prog: "  $NICELEVEL $PROXY_PATH/mysql-proxy $PROXY_OPTIONS --daemon --pid-file=$PROXY_PID --user=mysql --log-level=debug --log-file=/var/log/mysql-proxy.log --proxy-address=192.168.1.11:3306 RETVAL=$? echo if [ $RETVAL = 0 ]; then touch /var/lock/subsys/mysql-proxy fi ;; stop) # Stop daemons. echo -n $"Stopping $prog: " killproc $prog RETVAL=$? echo if [ $RETVAL = 0 ]; then rm -f /var/lock/subsys/mysql-proxy rm -f $PROXY_PID fi ;; restart) $0 stop sleep 3 $0 start ;; condrestart) [ -e /var/lock/subsys/mysql-proxy ] && $0 restart ;; status) status mysql-proxy RETVAL=$? ;; *) echo "Usage: $0 {start|stop|restart|status|condrestart}" RETVAL=1 ;; esac exit $RETVAL 
 
第十步，測試。插入數據，可以發現連接的是主服務器，查詢的時候也是主服務器。說明主服務器和從服務器均有讀的的功能。
[root@serv01 ~]# mysql -ularry -plarry -h 192.168.1.11 [connect_server] 192.168.1.11:57891 [1].connected_clients = 0 [1].pool.cur_idle = 0 [1].pool.max_idle = 1 [1].pool.min_idle = 1 [1].type = 1 [1].state = 1 [1] idle-conns below min-idle [read_query] 192.168.1.11:57891 current backend = 0 client default db = client username = larry query = select @@version_comment limit 1  sending to backend : 192.168.1.18:3306 is_slave : false server default db: server username : larry in_trans : false in_calc_found : false COM_QUERY : true mysql> insert into user values(5,'test'); Query OK, 1 row affected (0.01 sec) [read_query] 192.168.1.11:57893 current backend = 0 client default db = larrydb client username = larry query = insert into user values(5,'test')  sending to backend : 192.168.1.18:3306 is_slave : false server default db: larrydb server username : larry in_trans : false in_calc_found : false COM_QUERY : true mysql> select * from user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | | 5 | test | +------+----------+ 3 rows in set (0.00 sec) [read_query] 192.168.1.11:57893 current backend = 0 client default db = larrydb client username = larry query = select * from user  sending to backend : 192.168.1.18:3306 is_slave : false server default db: larrydb server username : larry in_trans : false in_calc_found : false COM_QUERY : true serv08主服務器查看數據，可以查詢到，說明主服務器可以寫 mysql> select * from larrydb.user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | | 5 | test | +------+----------+ 3 rows in set (0.00 sec) serv09從服務器查詢數據，發現不可查詢到，說明從服務器只讀 mysql> mysql> select * from larrydb.user; +------+----------+ | id | name | +------+----------+ | 1 | larrywen | | 2 | wentasy | | 3 | jsutdb | | 4 | db | +------+----------+ 4 rows in set (0.00 sec) 


第十一步，開啓slave。發現數據同步成功。
mysql> start slave;
Query OK, 0 rows affected (0.00 sec)


mysql> select * from larrydb.user;
+------+----------+
| id   | name     |
+------+----------+
|    1 | larrywen |
|    2 | wentasy  |
|    3 | jsutdb   |
|    4 | db       |
|    5 | test     |
+------+----------+
5 rows in set (0.00 sec)
轉自：http://blog.csdn.net/justdb/article/details/17331569 
********************下面是另一篇啦啦啦***************************


隨着一個網站的業務不斷擴展，數據不斷增加，數據庫的壓力也會越來越大，對數據庫或者SQL的基本優化可能達不到最終的效果，我們可以採用讀寫分離的策 略來改變現狀。讀寫分離現在被大量應用於很多大型網站，這個技術也不足爲奇了。ebay就做得非常好。ebay用的是oracle，聽說是用Quest Share Plex 來實現主從複製數據。
     讀寫分離簡單的說是把對數據庫讀和寫的操作分開對應不同的數據庫服務器，這樣能有效地減輕數據庫壓力，也能減輕io壓力。主數據庫提供寫操作，從數據庫提 供讀操作，其實在很多系統中，主要是讀的操作。當主數據庫進行寫操作時，數據要同步到從的數據庫，這樣纔能有效保證數據庫完整性。Quest SharePlex就是比較牛的同步數據工具，聽說比oracle本身的流複製還好，mysql也有自己的同步數據技術。mysql只要是通過二進制日誌來複制數據。通過日誌在從數據庫重複主數據庫的操作達到複製數據目的。這個複製比較好的就是通過異步方法，把數據同步到從數據庫。
      主數據庫同步到從數據庫後，從數據庫一般由多臺數據庫組成這樣才能達到減輕壓力的目的。讀的操作怎麼樣分配到從數據庫上？應該根據服務器的壓力把讀的操作分配到服務器，而不是簡單的隨機分配。mysql提供了MySQL-Proxy實現讀寫分離操作。不過MySQL-Proxy好像很久不更新了。oracle可以通過F5有效分配讀從數據庫的壓力。




 mysql的讀寫分離
       上面說的數據庫同步複製，都是在從同一種數據庫中，如果我要把oracle的數據同步到mysql中，其實要實現這種方案的理由很簡單，mysql免費，oracle太貴。好像Quest SharePlex也實現不了改功能吧。好像現在市面還沒有這個工具吧。那樣應該怎麼實現數據同步？其實我們可以考慮自己開發一套同步數據組件，通過消息，實現異步複製數據。其實這個實現起來要考慮很多方面問題，高併發的問題，失敗記錄等。其實這種方法也可以同步數據到memcache中。聽說oracle的Stream也能實現，不過沒有試過