基於streaming replication搭建的PostgreSQL HA環境,Old Standby節點升級為New Master節點後,時間線會切換為新的時間線,比如從n變為n + 1.而Old Master節點的時間線仍然為原來的時間線,比如仍為n,透過使用pg_rewind工具,可使原來其實"完好"的Old Master成為New Standby節點.
**圖解**
如下圖所示:
![pg_rewind圖解](http://img.blog.itpub.net/blog/2019/03/28/c59ba291e37f49ba.png?x-oss-process=style/bb)
在執行主備切換後,New Master節點的時間線切換為n + 1,透過pg_rewind可使Old Master在分叉點開始與New Master同步,成為New Standby節點.

**實測**
New Master
切換後為New Master後,執行以下SQL:
```
testdb=# create table t_new(id int,flag varchar(40));
CREATE TABLE
testdb=# insert into t_new select c,'flag'||c from generate_series(1,1000000) as c;
INSERT 0 1000000
testdb=# 
```

*Old Master*
執行pg_rewind前,重啟主庫,執行以下SQL
```
testdb=# create table t_fork(id int,flag varchar(40));
CREATE TABLE
testdb=# insert into t_fork select c,'flag'||c from generate_series(1,1000000) as c;
INSERT 0 1000000
testdb=# 
```
執行pg_rewind
```
[xdb@localhost testdb]$ cp /data/archivelog/* ./pg_wal
[xdb@localhost testdb]$ pg_rewind --target-pgdata=$PGDATA --source-server="host=192.168.26.25 port=5432 dbname=testdb" --progress
connected to server
servers diverged at WAL location 0/B41F12B8 on timeline 23
rewinding from last common checkpoint at 0/AFCF99E0 on timeline 23
reading source file list
reading target file list
reading WAL in target
need to copy 360 MB (total source directory size is 501 MB)
369312/369312 kB (100%) copied
creating backup label and updating control file
syncing target data directory
Done!
```
配置recovery.conf檔案
```
[xdb@localhost testdb]$ mv recovery.done recovery.conf
[xdb@localhost testdb]$ vim recovery.conf 
[xdb@localhost testdb]$ cat recovery.conf 
standby_mode = 'on'
primary_conninfo = 'user=replicator password=replicator host=192.168.26.26 port=5432 sslmode=prefer sslcompression=0 krbsrvname=postgres target_session_attrs=any'
restore_command = 'cp /data/archivelog/%f %p'

```
重啟資料庫
```
[xdb@localhost testdb]$ pg_ctl start
waiting for server to start....2019-03-28 12:39:31.918 CST [1961] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2019-03-28 12:39:31.918 CST [1961] LOG:  listening on IPv6 address "::", port 5432
2019-03-28 12:39:31.920 CST [1961] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2019-03-28 12:39:31.970 CST [1961] LOG:  redirecting log output to logging collector process
2019-03-28 12:39:31.970 CST [1961] HINT:  Future log output will appear in directory "pg_log".
... done
server started
```
與New Master資料同步了,而在原TL上的t_fork資料表消失了.
```
testdb=# select count(*) from t_new;
  count  
---------
 1000000
(1 row)

testdb=# select count(*) from t_old;
  count  
---------
 1000000
(1 row)

testdb=# select count(*) from t_fork;
ERROR:  relation "t_fork" does not exist
LINE 1: select count(*) from t_fork;
                             ^
testdb=# 
```

PostgreSQL DBA(33) - HA#2(pg_rewind切換圖解)

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