基于streaming replication搭建的PostgreSQL HA环境,如出现网络访问/硬件故障等原因导致Standby节点升级为Master节点,但旧Master节点数据库并未损坏,在排除故障后旧Master节点是否不需要通过备份方式重新搭建成为Standby节点呢?答案是肯定的,PG提供了pg_rewind这个工具实现.

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原理
在PostgreSQL HA环境中,Standby节点升级为Master节点后,时间线会切换为新的时间线,比如从1变为2.而旧Master节点的时间线仍然为原来的时间线,比如仍为1,那么使用pg_rewind工具,旧Master节点如何从新Master节点读取相关的数据成为新的Standby节点?
简单来说,有以下几步:
1.确定新Master和旧Master数据一致性的Checkpoint位置.在该位置上,新Master和旧Master数据完全一致.这可以通过读取新旧Master节点时间线历史文件可以获得,该文件位于$PGDATA/pg_wal/目录下,文件名称为XX.history
2.旧Master节点根据上一步获取的Checkpoint读取本机日志文件WAL Record,获取在此Checkpoint之后出现变化的Block,并以链表的方式存储Block编号等信息
3.根据第2步获取的Block信息从新Master节点拷贝相应的Block,替换旧Master节点相应的Block
4.拷贝新Master节点上除数据文件外的所有其他文件,包括配置文件等(如果拷贝数据文件,与备份方式搭建区别不大)
5.旧Master启动数据库,应用从Checkpoint开始后的WAL Record.

示例
考虑两节点的HA环境,旧Master节点IP为192.168.26.25,Standby节点(新Master)IP为192.168.26.26,模拟主库宕机,备库升级为主库,然后通过pg_rewind把旧Master节点切换为Standby节点.
切换前,26.25节点相关信息

testdb=# select * from pg_stat_replication;
-[ RECORD 1 ]----+------------------------------
pid              | 1537
usesysid         | 90113
usename          | replicator
application_name | standby_26
client_addr      | 192.168.26.26
client_hostname  | 
client_port      | 53164
backend_start    | 2019-03-27 15:19:09.254987+08
backend_xmin     | 654
state            | streaming
sent_lsn         | 0/6B000060
write_lsn        | 0/6B000060
flush_lsn        | 0/6B000060
replay_lsn       | 0/6B000060
write_lag        | 00:00:00.156457
flush_lag        | 00:00:00.158792
replay_lag       | 00:00:00.158815
sync_priority    | 0
sync_state       | async
testdb=# 
testdb=# select pg_is_in_recovery();
-[ RECORD 1 ]-----+--
pg_is_in_recovery | f

插入测试数据

testdb=# create table t_oldmaster(id int);
CREATE TABLE
testdb=# insert into t_oldmaster select generate_series(1,1000000);
INSERT 0 1000000
testdb=#

切换前26.26节点信息,数据已复制,角色为Standby

testdb=# select count(*) from t_oldmaster;
  count  
---------
 1000000
(1 row)
testdb=# select pg_is_in_recovery();
 pg_is_in_recovery 
-------------------
 t
(1 row)

模拟Master节点宕机

[xdb@localhost testdb]$ ps -ef|grep postgres
xdb       1353     1  0 14:46 pts/1    00:00:00 /appdb/xdb/pg11.2/bin/postgres
xdb       1354  1353  0 14:46 ?        00:00:00 postgres: logger   
xdb       1356  1353  0 14:46 ?        00:00:00 postgres: checkpointer   
xdb       1357  1353  0 14:46 ?        00:00:00 postgres: background writer   
xdb       1358  1353  0 14:46 ?        00:00:00 postgres: walwriter   
xdb       1359  1353  0 14:46 ?        00:00:00 postgres: autovacuum launcher  
xdb       1360  1353  0 14:46 ?        00:00:00 postgres: archiver   last was 00000010000000000000006D
xdb       1361  1353  0 14:46 ?        00:00:00 postgres: stats collector   
xdb       1362  1353  0 14:46 ?        00:00:00 postgres: logical replication launcher  
xdb       1418  1353  0 15:11 ?        00:00:02 postgres: xdb testdb [local] idle
xdb       1537  1353  0 15:19 ?        00:00:00 postgres: walsender replicator 192.168.26.26(53164) streaming 0/6ED4FDA8
xdb       1555  1317  0 15:25 pts/1    00:00:00 grep --color=auto postgres
[xdb@localhost testdb]$ kill -9 1353
[xdb@localhost testdb]$ ps -ef|grep postgres
xdb       1557  1317  0 15:25 pts/1    00:00:00 grep --color=auto postgres
[xdb@localhost testdb]$

Standby节点执行切换
编辑recovery.conf文件,添加trigger_file和recovery_target_timeline

[xdb@localhost testdb]$ cat recovery.conf 
standby_mode = 'on'
primary_conninfo = 'host=192.168.26.25 port=5432 user=replicator password=replicator application_name=standby_26'
restore_command = 'cp /data/archivelog/%f %p'
archive_cleanup_command = 'pg_archivecleanup /data/archivelog %r'
trigger_file = '/tmp/trigger_file'
recovery_target_timeline = 'latest'
#生成trigger文件
[xdb@localhost testdb]$ touch /tmp/trigger_file

重启Standby节点

[xdb@localhost testdb]$ pg_ctl restart
pg_ctl: PID file "/data/pgsql/testdb/postmaster.pid" does not exist
Is server running?
trying to start server anyway
waiting for server to start....2019-03-27 15:33:28.218 CST [1901] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2019-03-27 15:33:28.219 CST [1901] LOG:  listening on IPv6 address "::", port 5432
2019-03-27 15:33:28.223 CST [1901] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2019-03-27 15:33:28.253 CST [1901] LOG:  redirecting log output to logging collector process
2019-03-27 15:33:28.253 CST [1901] HINT:  Future log output will appear in directory "pg_log".
. done
server started
[xdb@localhost testdb]$

日志输出

2019-03-27 15:33:28.259 CST,,,1903,,5c9b2748.76f,2,,2019-03-27 15:33:28 CST,,0,LOG,00000,"entering standby mode",,,,,,,,,""
2019-03-27 15:33:28.264 CST,,,1903,,5c9b2748.76f,3,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"redo starts at 0/6B0164C0",,,,,,,,,""
2019-03-27 15:33:29.416 CST,,,1903,,5c9b2748.76f,4,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"consistent recovery state reached at 0/6ED4FDA8",,,,,,,,,""
2019-03-27 15:33:29.416 CST,,,1903,,5c9b2748.76f,5,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"invalid record length at 0/6ED4FDA8: wanted 24, got 0",,,,,,,,,""
2019-03-27 15:33:29.416 CST,,,1903,,5c9b2748.76f,6,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"trigger file found: /tmp/trigger_file",,,,,,,,,""
2019-03-27 15:33:29.416 CST,,,1903,,5c9b2748.76f,7,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"redo done at 0/6ED4FD70",,,,,,,,,""
2019-03-27 15:33:29.416 CST,,,1903,,5c9b2748.76f,8,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"last completed transaction was at log time 2019-03-27 15:21:57.519456+08",,,,,,,,,""
2019-03-27 15:33:29.416 CST,,,1901,,5c9b2748.76d,2,,2019-03-27 15:33:28 CST,,0,LOG,00000,"database system is ready to accept read only connections",,,,,,,,,""
2019-03-27 15:33:29.421 CST,,,1903,,5c9b2748.76f,9,,2019-03-27 15:33:28 CST,1/0,0,LOG,00000,"selected new timeline ID: 17",,,,,,,,,""

Standby节点已升级为新Master节点
recovery.conf 文件重命名为 recovery.done
/tmp/trigger_file文件被删除
pg_is_in_recovery返回false

[xdb@localhost testdb]$ cat recovery.done 
standby_mode = 'on'
primary_conninfo = 'host=192.168.26.25 port=5432 user=replicator password=replicator application_name=standby_26'
restore_command = 'cp /data/archivelog/%f %p'
archive_cleanup_command = 'pg_archivecleanup /data/archivelog %r'
trigger_file = '/tmp/trigger_file'
recovery_target_timeline = 'latest'
[root@localhost testdb]# ls -l /tmp/trigger_file
ls: cannot access /tmp/trigger_file: No such file or directory
[xdb@localhost testdb]$ psql -d testdb
psql (11.2)
Type "help" for help.
testdb=# select pg_is_in_recovery();
 pg_is_in_recovery 
-------------------
 f
(1 row)

在pg_wal目录下生成了时间线history文件

[xdb@localhost testdb]$ ls ./pg_wal/
00000010000000000000006E.partial  00000011000000000000006F  000000110000000000000072
00000010.history                  000000110000000000000070  00000011.history
00000011000000000000006E          000000110000000000000071  archive_status
[xdb@localhost testdb]$ cat ./pg_wal/00000011.history 
7    0/27000000    no recovery target specified
8    0/2A0012E8    no recovery target specified
12    0/32000000    no recovery target specified
14    0/39A63BD0    no recovery target specified
15    0/40A63B08    no recovery target specified
16    0/6ED4FDA8    no recovery target specified
[xdb@localhost testdb]$ cat ./pg_wal/00000010.history 
7    0/27000000    no recovery target specified
8    0/2A0012E8    no recovery target specified
12    0/32000000    no recovery target specified
14    0/39A63BD0    no recovery target specified
15    0/40A63B08    no recovery target specified

生成测试数据

testdb=# create table t_new_master(id int);
CREATE TABLE
testdb=# insert into t_new_master select generate_series(1,1000000);
INSERT 0 1000000
testdb=#

旧Master节点降级为Standby节点
重新启动旧master节点,插入测试数据,用于模拟时间线分叉后的数据变动

[xdb@localhost testdb]$ pg_ctl start
pg_ctl: another server might be running; trying to start server anyway
waiting for server to start....2019-03-27 15:39:07.989 CST [1561] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2019-03-27 15:39:07.989 CST [1561] LOG:  listening on IPv6 address "::", port 5432
2019-03-27 15:39:07.991 CST [1561] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2019-03-27 15:39:08.015 CST [1561] LOG:  redirecting log output to logging collector process
2019-03-27 15:39:08.015 CST [1561] HINT:  Future log output will appear in directory "pg_log".
. done
server started
[xdb@localhost testdb]$ psql -d testdb
psql (11.2)
Type "help" for help.
testdb=# 
testdb=# create table t_fork(id int);
CREATE TABLE
testdb=# insert into t_fork select generate_series(1,100000);
INSERT 0 100000
testdb=# 
testdb=# \q
[xdb@localhost testdb]$

关闭旧主库,执行pg_rewind

[xdb@localhost testdb]$ pg_ctl stop
waiting for server to shut down.... done
server stopped
[xdb@localhost testdb]$ pg_rewind --target-pgdata=$PGDATA --source-server="host=192.168.26.26 port=5432 dbname=testdb" --progress --debug -n
connected to server
fetched file "global/pg_control", length 8192
target server needs to use either data checksums or "wal_log_hints = on"
Failure, exiting

主从设置postgres.conf中的wal_log_hints参数为on,重启数据库

[xdb@localhost testdb]$ pg_ctl start
waiting for server to start....2019-03-27 15:54:56.038 CST [1603] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2019-03-27 15:54:56.038 CST [1603] LOG:  listening on IPv6 address "::", port 5432
2019-03-27 15:54:56.042 CST [1603] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2019-03-27 15:54:56.079 CST [1603] LOG:  redirecting log output to logging collector process
2019-03-27 15:54:56.079 CST [1603] HINT:  Future log output will appear in directory "pg_log".
 done
server started
[xdb@localhost testdb]$ pg_ctl stop
waiting for server to shut down.... done
server stopped

再次执行pg_rewind

[xdb@localhost testdb]$ pg_rewind --target-pgdata=$PGDATA --source-server="host=192.168.26.26 port=5432 dbname=testdb" --progress --debug -n
connected to server
fetched file "global/pg_control", length 8192
fetched file "pg_wal/00000011.history", length 261
Source timeline history:
Target timeline history:
7: 0/0 - 0/27000000
8: 0/27000000 - 0/2A0012E8
12: 0/2A0012E8 - 0/32000000
14: 0/32000000 - 0/39A63BD0
15: 0/39A63BD0 - 0/40A63B08
16: 0/40A63B08 - 0/0
servers diverged at WAL location 0/6ED4FDA8 on timeline 16
could not open file "/data/pgsql/testdb/pg_wal/00000010000000000000006E": No such file or directory
could not find previous WAL record at 0/6ED4FDA8
Failure, exiting
[xdb@localhost testdb]$

拷贝本机和新Master节点的归档wal文件

[xdb@localhost testdb]$ scp -P 22 root@192.168.26.26:/data/archivelog/* $PGDATA/pg_wal/
The authenticity of host '192.168.26.26 (192.168.26.26)' can't be established.
ECDSA key fingerprint is SHA256:wT+B26a2wJvmsOz8QH2UK6QUqodwBYvDu/WMrmqZ5fU.
ECDSA key fingerprint is MD5:79:63:4d:55:83:d9:d6:78:21:d1:b6:a1:e8:74:0f:b0.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.26.26' (ECDSA) to the list of known hosts.
root@192.168.26.26's password: 
00000010000000000000006E.partial                                                           100%   16MB  44.1MB/s   00:00    
00000011000000000000006E                                                                   100%   16MB  71.5MB/s   00:00    
00000011000000000000006F                                                                   100%   16MB  47.0MB/s   00:00    
000000110000000000000070                                                                   100%   16MB  26.7MB/s   00:00    
000000110000000000000071                                                                   100%   16MB  26.7MB/s   00:00    
000000110000000000000072                                                                   100%   16MB  31.6MB/s   00:00    
000000110000000000000073                                                                   100%   16MB  37.1MB/s   00:00    
00000011.history                                                                           100%  261   323.1KB/s   00:00    
[xdb@localhost testdb]$ ls ./pg_wal/
00000010000000000000006A.00000028.backup  000000100000000000000074  00000010.history          000000110000000000000073
00000010000000000000006E.partial          000000100000000000000075  00000011000000000000006E  00000011.history
000000100000000000000070                  000000100000000000000076  00000011000000000000006F  archive_status
000000100000000000000071                  000000100000000000000077  000000110000000000000070  RECOVERYHISTORY
000000100000000000000072                  000000100000000000000078  000000110000000000000071
000000100000000000000073                  000000100000000000000079  000000110000000000000072
[xdb@localhost testdb]$ 
[xdb@localhost testdb]$ cp /data/archivelog/20190327/00000010000000000000006* ./pg_wal

重新执行pg_rewind

[xdb@localhost testdb]$ pg_rewind --target-pgdata=$PGDATA --source-server="host=192.168.26.26 port=5432 dbname=testdb" --progress -n
connected to server
servers diverged at WAL location 0/6ED4FDA8 on timeline 16
rewinding from last common checkpoint at 0/6B0164F8 on timeline 16
reading source file list
reading target file list
reading WAL in target
need to copy 296 MB (total source directory size is 437 MB)
303659/303659 kB (100%) copied
creating backup label and updating control file
syncing target data directory
Done!

修改配置文件recovery.conf

[xdb@localhost testdb]$ scp -P 22 root@192.168.26.26:$PGDATA/recovery.done $PGDATA
root@192.168.26.26's password: 
recovery.done                                                                              100%  317   312.7KB/s   00:00    
[xdb@localhost testdb]$ mv recovery.done recovery.conf
[xdb@localhost testdb]$ vim recovery.conf 
[xdb@localhost testdb]$ cat recovery.conf 
standby_mode = 'on'
primary_conninfo = 'host=192.168.26.26 port=5432 user=replicator password=replicator application_name=standby_25'
restore_command = 'cp /data/archivelog/%f %p'
archive_cleanup_command = 'pg_archivecleanup /data/archivelog %r'
[xdb@localhost testdb]$

重启原主库

新Master节点
日志提示有误,搭建出现问题

2019-03-27 16:22:19.800 CST,"replicator","",2360,"192.168.26.25:32974",5c9b32bb.938,1,"idle",2019-03-27 16:22:19 CST,4/0,0,ERROR,XX000,"requested starting point 0/71000000 on timeline 16 is not in this server's history","This server's history forked from timeline 16 at 0/6ED4FDA8.",,,,,,,,"standby_25"
2019-03-27 16:22:24.813 CST,"replicator","",2361,"192.168.26.25:32976",5c9b32c0.939,1,"idle",2019-03-27 16:22:24 CST,4/0,0,ERROR,XX000,"requested starting point 0/71000000 on timeline 16 is not in this server's history","This server's history forked from timeline 16 at 0/6ED4FDA8.",,,,,,,,"standby_25"

作为失败的案例,供参考.

参考资料
神奇的pg_rewind