pitr 原理_PostgreSQL基于时间点恢复(PITR)
OS 環境：CentOS 6.2
數據庫 ：PostgreSQL 9.1.3
pg_home=/home/postgres/
pg_data=/database/pgdata/
一、前期工作既要恢復，肯定是需要一個備份基礎的，否則再怎么的巧婦也難為無米之炊。
1.修改數據庫參數，修改postgresql.conf：
archive_mode = on
archive_timeout = 300 --單位是秒，此處以5分鐘為限強制歸檔，僅作測試
archive_command = 'cp -i %p /home/postgres/archive/%f'
wal_level = archive
修改完重啟下reload,DB
2.基于文件級別的持續備份，
a.基礎備份
postgres=# select pg_start_backup('backup_2012_05_20_14:22:10');
b.打包備份pg_data
# cd /database
# tar -cvzf pgdata.tar ./pgdata
c.結束基礎備份并切換歸檔
postgres=# select pg_stop_backup();
postgres=# select pg_switch_xlog();
pg_switch_xlog
----------------
0/C000020
(1 row)
postgres=# select pg_current_xlog_location();
pg_current_xlog_location
--------------------------
0/C000020
(1 row)
postgres=# create table test_1(id int,name varchar(50));
postgres=# insert into test_1 values (1,'kenyon');
INSERT 0 1
此時在pg_data路徑下會產生一個label,可以查看內容有checkpoint時間,基礎備份的開始和結束時間，以及標簽名稱等。因為之前已經設置了archive的三個參數，可以在archive的備份路徑pg_home/archive下看到歸檔的文件會定時傳過來。
二、恢復過程
停數據庫
# pg_stop
假定數據庫的崩潰場景，將pgdata數據刪除
# rm -rf /database/pgdata
恢復之前備份的tar文件
# tar xvf pgdata.tar
刪除pg_xlog文件夾并重建
# rm -rf pg_xlog # mkdir -p pg_xlog/archive_status
拷貝recovery.conf文件并修改
# cp $PG_HOME/recovery.conf.sample /database/pgdata/
# vi /database/pgdata/recovery.conf
--新增內容，指定恢復文件和路徑，%f,%p見上面說明
restore_command = 'cp /home/postgres/archive/%f "%p"'
啟動數據庫
# pg_start
[postgres@localhost archive]$ psql
spsql (9.1.3)
Type "help" for help.
postgres=# select * from test_1;
id | name
----+--------
1 | kenyon
(1 rows) --恢復成功，會恢復到之前接收到的最后一個歸檔文件。另外recovery.conf會改名變成recovery.done
日志內容:
LOG: shutting down
LOG: database system is shut down
LOG: database system was interrupted; last known up at 2012-05-20 22:23:15 CST
LOG: starting archive recovery
LOG: restored log file "000000010000000000000002" from archive
LOG: redo starts at 0/8000078
LOG: consistent recovery state reached at 0/C000000
LOG: restored log file "000000010000000000000003" from archive
LOG: restored log file "000000010000000000000004" from archive
LOG: restored log file "000000010000000000000005" from archive
LOG: restored log file "000000010000000000000006" from archive
LOG: restored log file "000000010000000000000007" from archive
cp: cannot stat `/home/postgres/archive/000000010000000000000008': No such file or directory
LOG: could not open file "pg_xlog/000000010000000000000008" (log file 0, segment 8): No such file or directory
LOG: redo done at 0/1C000078
LOG: last completed transaction was at log time 2012-05-20 23:01:22.960591+08
LOG: restored log file "000000010000000000000007" from archive
cp: cannot stat `/home/postgres/archive/00000002.history': No such file or directory
LOG: selected new timeline ID: 2
cp: cannot stat `/home/postgres/archive/00000001.history': No such file or directory
LOG: archive recovery complete
LOG: database system is ready to accept connections
LOG: autovacuum launcher started
PS:若要恢復到指定時間，還需要再recovery.conf中設置recovrey_target_time,recovery_target_timeline等參數
總結：pitr技術對于7*24小時支撐是至關重要的，但是如果數據庫非常小，增大pg_dump備份的頻率可能更方便，但對于大數據庫就需要了。
總結
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