PostgreSQL如何刪除pg_xlog數據

以前討論過若是歸檔或者流複製出現問題的時候，pg_xlog下面會不斷地產生WAL日誌，嚴重會塞滿磁盤空間。可是pg_xlog下面的日誌又不能隨意刪除，不然有可能致使數據庫起不來。PostgreSQL自帶了清理wal日誌的工具叫pg_resetxlog，實際上是經過重置xlog的值來清理xlog文件。可是這個工具一般在到萬不得已，不要使用。下面示例以下：

環境： PostgreSQL 9.1.3
VMWARE 6.0
IP：192.168.1.103

1、重置前環境

[postgres@localhost pg_xlog]$ ll
total 1310732
-rw-------. 1 postgres postgres 67108864 Aug 19 19:46 00000004000000010000003A
-rw-------. 1 postgres postgres 67108864 Nov 10 22:13 00000004000000010000003B
-rw-------. 1 postgres postgres 67108864 Nov 10 22:24 00000004000000010000003C
-rw-------. 1 postgres postgres      290 Nov 10 22:24 00000004000000010000003C.00000020.backup
-rw-------. 1 postgres postgres 67108864 Nov 10 22:54 00000004000000010000003D
-rw-------. 1 postgres postgres 67108864 Nov 10 23:15 00000004000000010000003E
-rw-------. 1 postgres postgres 67108864 Nov 10 23:30 000000040000000200000000
-rw-------. 1 postgres postgres 67108864 Nov 10 23:39 000000040000000200000001
-rw-------. 1 postgres postgres 67108864 Nov 11 08:31 000000040000000200000002
-rw-------. 1 postgres postgres 67108864 Nov 11 08:38 000000040000000200000003
-rw-------. 1 postgres postgres 67108864 Nov 17 08:09 000000040000000200000004
-rw-------. 1 postgres postgres 67108864 Dec 31 12:20 000000040000000200000005
-rw-------. 1 postgres postgres 67108864 Jan  3 09:35 000000040000000200000006
-rw-------. 1 postgres postgres 67108864 Jan 10 07:46 000000040000000200000007
-rw-------. 1 postgres postgres 67108864 Jan 10 07:48 000000040000000200000008
-rw-------. 1 postgres postgres 67108864 Jan 10 07:52 000000040000000200000009
-rw-------. 1 postgres postgres 67108864 Jan 10 07:52 00000004000000020000000A
-rw-------. 1 postgres postgres 67108864 Jan 10 07:52 00000004000000020000000B
-rw-------. 1 postgres postgres 67108864 Jan 10 07:52 00000004000000020000000C
-rw-------. 1 postgres postgres      113 Jun 17  2012 00000004.history
drwxrwxr-x. 2 postgres postgres     4096 Jan 10 07:52 archive_status
-rw-------. 1 postgres postgres 67108864 Nov 17 08:09 xlogtemp.1461
-rw-------. 1 postgres postgres 67108864 Jan  3 09:35 xlogtemp.1518
[postgres@localhost pg_xlog]$ cd ..
[postgres@localhost pgdata]$ du -sh pg_xlog/
1.3G    pg_xlog/

2、關閉數據庫

[postgres@localhost pg_xlog]$ pg_stop
waiting for server to shut down...... done
server stopped

3、查看版本控制信息

[postgres@localhost ~]$ pg_controldata
pg_control version number:            903
Catalog version number:               201105231
Database system identifier:           5741432812849707775
Database cluster state:               shut down
pg_control last modified:             Thu 10 Jan 2013 07:56:36 AM CST
Latest checkpoint location:           2/38000020
Prior checkpoint location:            2/34009190
Latest checkpoint's REDO location:    2/38000020
Latest checkpoint's TimeLineID:       4
Latest checkpoint's NextXID:          0/1888
Latest checkpoint's NextOID:          65595
Latest checkpoint's NextMultiXactId:  1
Latest checkpoint's NextMultiOffset:  0
Latest checkpoint's oldestXID:        1792
Latest checkpoint's oldestXID's DB:   1
Latest checkpoint's oldestActiveXID:  0
Time of latest checkpoint:            Thu 10 Jan 2013 07:56:35 AM CST
.....

注意這兩個參數：
Latest checkpoint's NextXID: 0/ 1888
Latest checkpoint's NextOID: 65595

4、重置，刪除pg_xlog

[postgres@localhost ~]$ pg_resetxlog -o 65595 -x1888 -f /database/pgdata/
Transaction log reset

5、再次查看，並重啓

[postgres@localhost pgdata]$ du -sh pg_xlog/
193M    pg_xlog/

[postgres@localhost pg_xlog]$ ll
total 196620
-rw-------. 1 postgres postgres      290 Nov 10 22:24 00000004000000010000003C.00000020.backup
-rw-------. 1 postgres postgres 67108864 Jan 10 08:02 00000004000000020000000F
-rw-------. 1 postgres postgres      113 Jun 17  2012 00000004.history
drwxrwxr-x. 2 postgres postgres     4096 Jan 10 08:02 archive_status
-rw-------. 1 postgres postgres 67108864 Nov 17 08:09 xlogtemp.1461
-rw-------. 1 postgres postgres 67108864 Jan  3 09:35 xlogtemp.1518

[postgres@localhost 12780]$ pg_start
server starting
[postgres@localhost 12780]$ psql
Password: 
psql (9.1.3)
Type "help" for help.

postgres=# \q

已經被清理並正常啓動了。以上測試是單機版環境，主備狀況須要作好WAL日誌的備份。

另外使用這個操做可能會引發事務或數據的丟失,操做時應當慎重

6、其餘

[postgres@localhost ~]$ pg_resetxlog --help
 pg_resetxlog resets the PostgreSQL transaction log 

 Usage:   pg_resetxlog [OPTION]... DATADIR 

 Options:   -e XIDEPOCH     set next transaction ID epoch   -f              force update to be done   -l TLI,FILE,SEG force minimum WAL starting location for new transaction log   -m XID          set next multitransaction ID   -n              no update, just show extracted control values (for testing)   -o OID          set next OID   -O OFFSET       set next multitransaction offset   -x XID          set next transaction ID   --help          show this help, then exit   --version       output version information, then exit 

 Report bugs to <pgsql-bugs@postgresql.org>