这节将介绍下 PostgreSQL HOT-Standby 的主备切换，虽然 PostgreSQL 的主备切换不太方便，没能像 Oracle DataGuard 一样提供切换命令，但是仍然有方法实现这点在官网手册中有指出，但没给出详细的指导步骤。今天在测试过程中遇到不少问题，后经德哥指点，终于完成本实验，下面是详细过程。

环境信息

PostgreSQL 版本： PostgreSQL 9.1beta3
OS ： Red Hat Enterprise Linux Server release 5.5
硬件环境 : 笔记本上的两台虚拟机
主库 IP : 192.168.1.25
主库 PORT : 1921
备库 IP : 192.168.1.26
备库 PORT : 1921

备注：这节不详细介绍 HOT-Standby 的搭建，关于搭建的内容可以参考之前的BLOG, PostgreSQL: Setting up streaming log replication (Hot Standby )

如何判断主库和备库

有时在论坛上会有人问，如何区分主库和备库，这里提供两种方法。

方法一: 根据主机进程判断

[postgres@pg1 pg_root]$ ps -ef | grep "wal"  
postgres 17715 17684 0 20:41 ? 00:00:00 postgres: wal writer process  
postgres 17746 17684 0 20:43 ? 00:00:00 postgres: wal sender process repuser 192.168.1.26(43246) streaming 0/700178A8  
postgres 17819 17590 0 21:00 pts/2 00:00:00 grep wal

备注：这里显示了wal 日志发送进程”wal sender process”，说明是主库。

1
2
3

[postgres@pgb pg_xlog]$ ps -ef | grep wal  
postgres 29436 29386 0 20:43 ? 00:00:00 postgres: wal receiver process streaming 0/700178A8  
postgres 29460 29289 0 21:00 pts/3 00:00:00 grep wal

备注：这里显示了 wal 日志接收进程 “wal receiver process” ，说明是备库；

方法二: 根据 pg_controldata 输出

1	pg_controldata 输出数据库服务的当前状态，可以根据 "Database cluster state: " 的信息来判断,

如果值为 “in production” 说明是主库，如果值为 “in archive recovery” 说明是备库。

主库的 pg_controldata 输出

[postgres@pg1 pg_root]$ pg_controldata  
pg_control version number: 903  
Catalog version number: 201105231  
Database system identifier: 5640897481082175487  
Database cluster state: in production...
备库的 pg_controldata 输出  
[postgres@pgb pg_xlog]$ pg_controldata  
pg_control version number: 903  
Catalog version number: 201105231  
Database system identifier: 5640897481082175487  
Database cluster state: in archive recovery...

Recovery.conf 文件

recovery.conf 是一个配置文件，用于主库，备库切换时的参数配置，可以从 $PGHOME/share 目录下复制一份 recovery.conf.sample 到备库 $PGDATA 目录，里面有众多参数，这里只介绍用于切换时的关键参数。

1
2
3

standby_mode = '' --标记PG为STANDBY SERVER  
primary_conninfo = '' --标识主库信息  
trigger_file = '' --标识触发器文件

主备切换

4.1 创建备库recovery.conf 文件（ On Slave ）

1	cp $PGHOME/share/recovery.conf.sample $PGDATA/recovery.conf

配置以下参数

1
2
3

standby_mode = 'on' --标记PG为STANDBY SERVER  
primary_conninfo = 'host=192.168.1.25 port=1921 user=repuser ' --标识主库信息  
trigger_file = '/opt/pgdata/pg_root/postgresql.trigger.1921' --标识触发器文件

4.2 关闭主库(on Primary)

1
2
3

[postgres@pg1 pg_root]$ pg_ctl stop -m fast -D $PGDATA  
waiting for server to shut down....... done  
server stopped

4.3 激活备库到主库状态 ( on slave )
激活备库只要创建一个文件即可，根据备库 recovery.conf 配置文件的参数 trigger_file 值，创建这个 trigger 文件即可。例如 “touch /opt/pgdata/pg_root/postgresql.trigger.1921 “

1	[postgres@pgb pg_root]$ touch /opt/pgdata/pg_root/postgresql.trigger.1921

过一会儿发现 recovery.conf 文件变成 recovery.done ，说明备库已经激活。

[postgres@pgb pg_root]$ ll  
total 176K  
-rw------- 1 postgres postgres 168 Aug 24 10:24 backup_label.old  
drwx------ 5 postgres postgres 4.0K Aug 15 10:03 base  
drwx------ 2 postgres postgres 4.0K Aug 24 20:50 global  
drwx------ 2 postgres postgres 4.0K Aug 15 10:03 pg_clog  
-rw------- 1 postgres postgres 4.5K Aug 24 10:39 pg_hba.conf  
-rw------- 1 postgres postgres 1.6K Aug 15 10:03 pg_ident.conf  
drwx------ 4 postgres postgres 4.0K Aug 15 10:03 pg_multixact  
drwx------ 2 postgres postgres 4.0K Aug 24 20:42 pg_notify  
drwx------ 2 postgres postgres 4.0K Aug 15 10:03 pg_serial  
drwx------ 2 postgres postgres 4.0K Aug 15 10:03 pg_stat_tmp  
drwx------ 2 postgres postgres 4.0K Aug 15 10:03 pg_subtrans  
drwx------ 2 postgres postgres 4.0K Aug 21 20:21 pg_tblspc  
drwx------ 2 postgres postgres 4.0K Aug 15 10:03 pg_twophase  
-rw------- 1 postgres postgres 4 Aug 15 10:03 PG_VERSION  
drwx------ 3 postgres postgres 4.0K Aug 24 21:20 pg_xlog  
-rw------- 1 postgres postgres 19K Aug 24 10:24 postgresql.conf  
-rw------- 1 postgres postgres 51 Aug 24 20:42 postmaster.opts  
-rw------- 1 postgres postgres 69 Aug 24 20:42 postmaster.pid  
-rw-r--r-- 1 postgres postgres 4.7K Aug 24 20:42 recovery.conf

查看从库CSV日志(正在激活成主库)

1
2
3

2011-08-24 21:20:55.130 CST,,,29388,,4e54f1c5.72cc,11,,2011-08-24 20:42:45 CST,1/0,0,LOG,00000,"selected new timeline ID: 6",,,,,,,,,""  
2011-08-24 21:20:58.119 CST,,,29388,,4e54f1c5.72cc,12,,2011-08-24 20:42:45 CST,1/0,0,LOG,00000,"archive recovery complete",,,,,,,,,""  
2011-08-24 21:20:58.495 CST,,,29386,,4e54f1c3.72ca,5,,2011-08-24 20:42:43 CST,,0,LOG,00000,"database system is ready to accept connections",,,,,,,,,""

说明从库已经为OPEN状态，可以进行读写操作。

4.4 激活原来的主库，让其转变成从库 (在原来的主库上执行)

创建 $PGDATA/recovery.conf 文件，配置以下参数

recovery_target_timeline = 'latest'  
standby_mode = 'on' --标记PG为STANDBY SERVER  
primary_conninfo = 'host=192.168.1.26 port=1921 user=repuser ' --标识主库信息  
trigger_file = '/opt/pgdata/pg_root/postgresql.trigger.1921' --标识触发器文件

创建密码文件 /home/postgres/.pgpass 密码文件，输入以下内容

1	192.168.1.26:1921:replication:repuser:rep123us345er

修改 pg_hba.conf (现在的主库上增加)，添加以下配置

1	host replication repuser 192.168.1.25/32 md5

将原来的主库（现在的从库）启动

1 2	[postgres@pg1 pg_root]$ pg_ctl start -D $PGDATA server starting

查看从库日志，发现大量 FATAL 错误信息

2011-08-24 21:31:59.178 CST,,,17889,,4e54fd4f.45e1,1,,2011-08-24 21:31:59 CST,,0,FATAL,XX000,"timeline 6 of the primary does not match recovery target timeline 5",,,,,,,,,""  
2011-08-24 21:32:04.208 CST,,,17891,,4e54fd54.45e3,1,,2011-08-24 21:32:04 CST,,0,FATAL,XX000,"timeline 6 of the primary does not match recovery target timeline 5",,,,,,,,,""  
2011-08-24 21:32:09.135 CST,,,17892,,4e54fd59.45e4,1,,2011-08-24 21:32:09 CST,,0,FATAL,XX000,"timeline 6 of the primary does not match recovery target timeline 5",,,,,,,,,""  
2011-08-24 21:32:14.136 CST,,,17895,,4e54fd5e.45e7,1,,2011-08-24 21:32:14 CST,,0,FATAL,XX000,"timeline 6 of the primary does not match recovery target timeline 5",,,,,,,,,""

备注：出现了大量 FATAL,XX000,”timeline 6 of the primary does not match recovery target timeline 5” 估计是时间线有问题，网上查了下资料也没啥结果，后来咨询了下德哥，只要将从库 $PGDATA/pg_xlog 一个文件考过来就行。

将主库文件 00000006.history 复制到从库

1
2
3

[postgres@pgb pg_xlog]$ scp 00000006.history [postgres@192.168.1.25:/opt/pgdata/pg_root/pg_xlog](mailto:postgres@192.168.1.25:/opt/pgdata/pg_root/pg_xlog)  
[postgres@192.168.1.25's](mailto:postgres@192.168.1.25's) password:  
00000006.history

再次查看从库日志

1
2
3

2011-08-24 21:36:04.819 CST,,,17948,,4e54fe44.461c,1,,2011-08-24 21:36:04 CST,,0,FATAL,XX000,"timeline 6 of the primary does not match recovery target timeline 5",,,,,,,,,""  
2011-08-24 21:36:09.742 CST,,,17885,,4e54fd44.45dd,5,,2011-08-24 21:31:48 CST,1/0,0,LOG,00000,"new target timeline is 6",,,,,,,,,""  
2011-08-24 21:36:09.824 CST,,,17977,,4e54fe49.4639,1,,2011-08-24 21:36:09 CST,,0,LOG,00000,"streaming replication successfully connected to primary",,,,,,,,,""

备注：根据日志信息，说明从库已经恢复正常；

验证

主库上创建一张表

1 2	skytf=> create table test_11 (id integer,name varchar(32)); CREATE TABLE

从库上查询

[postgres@pgb pg_root]$ psql  
psql (9.1beta3)  
Type "help" for help.
skytf=> /d  
 List of relations  
Schema | Name | Type | Owner  
--------+--------------------+-------+----------  
public | pg_stat_statements | view | postgres  
skytf | pgbench_accounts | table | skytf  
skytf | pgbench_branches | table | skytf  
skytf | pgbench_history | table | skytf  
skytf | pgbench_tellers | table | skytf  
skytf | test_11 | table | skytf  
skytf | test_stadnby | table | skytf

备注：可见表 test_11 迅速从主库上同步过来了，到此为止，库切换完成。

总结

Hot-Standby 切换步骤比较多，有些配置可以提前做好的，例如 .pgpass, pg_hba.conf 等;
主，备切换时，务必先将主库关闭，否则一旦从库被激活时，而主库尚未关闭，会有问题;
主，备切换可作为生产库迁移的一种方式，因为这最大限度减少了业务停机时间。

原创文章，作者：3628473679，如若转载，请注明出处：https://blog.ytso.com/tech/database/236422.html

PostgreSQL HOT-Standby 的主备切换