Postgresl選擇正確的關閉模式

停止數據庫的命令：

pg_ctl stop -D $PGDATA [-m shutdown-mode]

shutdown-mode有如下幾種模式：

1. smart: 等所有的連接中止后，關閉數據庫。如果客戶端連接不終止， 則無法關閉數據庫。

開啟一個空會話：

[root@localhost ~]# su - postgres
[postgres@localhost ~]$ psql
psql (9.4.4)
Type "help" for help.
 
postgres=#

用smart關閉數據庫：

[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m smart
waiting for server to shut down............................................................... failed
pg_ctl: server does not shut down
HINT: The "-m fast" option immediately disconnects sessions rather than
waiting for session-initiated disconnection

2. fast: 快速關閉數據庫， 斷開客戶端的連接，讓已有的事務回滾，然后正常關閉數據庫。

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

查看關閉日志：

LOG: received fast shutdown request
LOG: aborting any active transactions
FATAL: terminating connection due to administrator command
LOG: shutting down
LOG: database system is shut down

會話被強制中斷，然后關閉數據庫。

起一個事務，然后測試關閉：

postgres=# create table t(id int primary key, name varchar(9));
CREATE TABLE
postgres=# begin;
BEGIN
postgres=# insert into t values(1,'a')
postgres-# ;
INSERT 0 1

不提交， 然后用FAST MODE去關閉數據庫：

[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m fast
waiting for server to shut down.... done
server stopped
查看日志：
LOG: received fast shutdown request
LOG: aborting any active transactions
LOG: autovacuum launcher shutting down
FATAL: terminating connection due to administrator command
LOG: shutting down
LOG: database system is shut down

同樣是直接中斷會話， 而不去管事務有沒有提交。

postgres=# select * from t;
id | name
----+------
(0 rows)

沒有提交的數據， 在重啟之后並不能查到。

 

3. immediate: 立即關閉數據庫，立即停止數據庫進程，直接退出，下次啟動時會進行實例恢復。

postgres=# insert into t values(1,'a')
;
INSERT 0 1
postgres=# select * from t;
id | name
----+------
1 | a
(1 row)

關閉數據庫：

[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m immediate
waiting for server to shut down.... done
server stopped
查看日志：
LOG: received immediate shutdown request
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and repeat your command.
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and repeat your command.

啟動數據庫：

[postgres@localhost ~]$ pg_ctl -D /apps/pgsql/pgdata -l 1.log start
server starting
 
查看日志：
LOG: database system was interrupted; last known up at 2017-04-27 18:56:47 PDT
LOG: database system was not properly shut down; automatic recovery in progress #提示非正常關機，自動開啟恢復。
LOG: redo starts at 0/181F910
LOG: record with zero length at 0/181FA90
LOG: redo done at 0/181FA60
LOG: last completed transaction was at log time 2017-04-27 18:59:13.727213-07
LOG: MultiXact member wraparound protections are now enabled
LOG: autovacuum launcher started
LOG: database system is ready to accept connections

查看數據：

[postgres@localhost ~]$ psql
psql (9.4.4)
Type "help" for help.
 
postgres=# select * from t;
id | name
----+------
1 | a
(1 row)

提交的數據已通過實例恢復。

 

小結：

對比以上三種關庫模式：

smart最為安全，但最慢， 需要將所有連接都斷開后，才會關庫，默認關庫模式。

fast強制中斷會話，而不管有操作有沒有提交，在做系統維護（系統維護時一般應用都正常關閉了，或者不再會有事務操作。）時，需要這種模式來關閉數據庫。

immediate最暴力的方式，不管數據有沒有落盤（POSGRE是遵循WAL機制），就直接關掉， 待啟動時進行實例恢復， 如果在關庫前有大量的事務沒有寫入磁盤， 那這個恢復過程可能會非常的漫長。