Postgresql 如何选择正确的关闭模式
停止数据库的命令：
pg_ctl stop -D $PGDATA [-m shutdown-mode]
shutdown-mode有如下几种模式：
1. smart: 等所有的连接中止后，关闭数据库。如果客户端连接不终止， 则无法关闭数据库。
开启一个空会话：
[root@localhost ~]# su - postgres [postgres@localhost ~]$ psql psql (9.4.4) Type "help" for help. postgres=#
用smart关闭数据库：
[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m smart waiting for server to shut down............................................................... failed pg_ctl: server does not shut down HINT: The "-m fast" option immediately disconnects sessions rather than waiting for session-initiated disconnection
2. fast: 快速关闭数据库， 断开客户端的连接，让已有的事务回滚，然后正常关闭数据库。
[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m fast waiting for server to shut down.... done server stopped
查看关闭日志：
LOG: received fast shutdown request LOG: aborting any active transactions FATAL: terminating connection due to administrator command LOG: shutting down LOG: database system is shut down
会话被强制中断，然后关闭数据库。
起一个事务，然后测试关闭：
postgres=# create table t(id int primary key, name varchar(9)); CREATE TABLE postgres=# begin; BEGIN postgres=# insert into t values(1,'a') postgres-# ; INSERT 0 1
不提交， 然后用FAST MODE去关闭数据库：
[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m fast waiting for server to shut down.... done server stopped
查看日志：
LOG: received fast shutdown request LOG: aborting any active transactions LOG: autovacuum launcher shutting down FATAL: terminating connection due to administrator command LOG: shutting down LOG: database system is shut down
同样是直接中断会话， 而不去管事务有没有提交。
postgres=# select * from t; id | name ----+------ (0 rows)
没有提交的数据， 在重启之后并不能查到。
3. immediate: 立即关闭数据库，立即停止数据库进程，直接退出，下次启动时会进行实例恢复。
postgres=# insert into t values(1,'a') ; INSERT 0 1 postgres=# select * from t; id | name ----+------ 1 | a (1 row)
关闭数据库：
[postgres@localhost ~]$ pg_ctl stop -D $PGDATA -m immediate waiting for server to shut down.... done server stopped
查看日志：
LOG: received immediate shutdown request WARNING: terminating connection because of crash of another server process DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory. HINT: In a moment you should be able to reconnect to the database and repeat your command. WARNING: terminating connection because of crash of another server process DETAIL: The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory. HINT: In a moment you should be able to reconnect to the database and repeat your command.
启动数据库：
[postgres@localhost ~]$ 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 #提示非正常关机，自动开启恢复。 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
查看数据：
[postgres@localhost ~]$ psql psql (9.4.4) Type "help" for help. postgres=# select * from t; id | name ----+------ 1 | a (1 row)
提交的数据已通过实例恢复。
小结：
对比以上三种关库模式：
smart最为安全，但最慢， 需要将所有连接都断开后，才会关库，默认关库模式。
fast强制中断会话，而不管有操作有没有提交，在做系统维护（系统维护时一般应用都正常关闭了，或者不再会有事务操作。）时，需要这种模式来关闭数据库。
immediate最暴力的方式，不管数据有没有落盘（POSGRE是遵循WAL机制），就直接关掉， 待启动时进行实例恢复， 如果在关库前有大量的事务没有写入磁盘， 那这个恢复过程可能会非常的漫长。
补充：postgresql 异步 stream replication 环境关闭 master 的验证
os: ubuntu 16.04
db: postgresql 9.6.8
验证在异步 stream replication环境下，主动关闭master时，数据是否有丢失，能丢失多少。
版本
# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.5 LTS
Release: 16.04
Codename: xenial
# su - postgres -c "psql -c \"select version();\""
version
----------------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.6.8 on x86_64-pc-linux-gnu (Ubuntu 9.6.8-1.pgdg16.04+1), compiled by gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609, 64-bit
(1 row)
用 pgbench 模拟数据库的大量数据操作
postgres=# create database pgbenchdb; # su - postgres $ pgbench -i -s 20 pgbenchdb $ pgbench -r -j2 -c4 -T300 pgbenchdb
关闭 master
# su - postgres $ /usr/lib/postgresql/9.6/bin/pg_ctl stop -m fast -D "/data/pg9.6/main"
提升 slave
# su - postgres $ /usr/lib/postgresql/9.6/bin/pg_ctl promote -D "/data/pg9.6/main"
查看 old master 的 xlog location
$ /usr/lib/postgresql/9.6/bin/pg_xlogdump 000000010000000000000016 rmgr: XLOG len (rec/tot): 106/ 106, tx: 0, lsn: 0/16000028, prev 0/152C9A10, desc: CHECKPOINT_SHUTDOWN redo 0/16000028; tli 1; prev tli 1; fpw true; xid 0:118746; oid 16432; multi 1; offset 0; oldest xid 543 in DB 1; oldest multi 1 in DB 1; oldest/newest commit timestamp xid: 550/118745; oldest running xid 0; shutdown pg_xlogdump: FATAL: error in WAL record at 0/16000028: invalid record length at 0/16000098: wanted 24, got 0
可以看到 lsn: 0/16000028, prev 0/152C9A10, desc: CHECKPOINT_SHUTDOWN redo 0/16000028;
查看 new master 的 .history文件
$ ls -lt|more total 360456 -rw------- 1 postgres postgres 16777216 Nov 30 10:32 000000020000000000000016 drwx------ 2 postgres postgres 4096 Nov 30 10:16 archive_status -rw------- 1 postgres postgres 42 Nov 30 10:16 00000002.history -rw------- 1 postgres postgres 16777216 Nov 30 10:16 000000010000000000000016.partial -rw------- 1 postgres postgres 16777216 Nov 30 10:16 000000010000000000000015 -rw------- 1 postgres postgres 16777216 Nov 30 10:16 000000010000000000000014 -rw------- 1 postgres postgres 16777216 Nov 30 10:05 000000010000000000000013 $ cat 00000002.history 1 0/16000098 no recovery target specified $ /usr/lib/postgresql/9.6/bin/pg_xlogdump 000000010000000000000016 rmgr: XLOG len (rec/tot): 106/ 106, tx: 0, lsn: 0/16000028, prev 0/152C9A10, desc: CHECKPOINT_SHUTDOWN redo 0/16000028; tli 1; prev tli 1; fpw true; xid 0:118746; oid 16432; multi 1; offset 0; oldest xid 543 in DB 1; oldest multi 1 in DB 1; oldest/newest commit timestamp xid: 550/118745; oldest running xid 0; shutdown rmgr: XLOG len (rec/tot): 42/ 42, tx: 0, lsn: 0/16000098, prev 0/16000028, desc: END_OF_RECOVERY tli 2; prev tli 1; time 2018-11-30 10:16:57.249408 CST rmgr: Standby len (rec/tot): 50/ 50, tx: 0, lsn: 0/160000C8, prev 0/16000098, desc: RUNNING_XACTS nextXid 118746 latestCompletedXid 118745 oldestRunningXid 118746 rmgr: XLOG len (rec/tot): 51/ 312, tx: 0, lsn: 0/16000100, prev 0/160000C8, desc: FPI_FOR_HINT , blkref #0: rel 1664/0/1260 blk 0 FPW rmgr: Standby len (rec/tot): 50/ 50, tx: 0, lsn: 0/16000238, prev 0/16000100, desc: RUNNING_XACTS nextXid 118746 latestCompletedXid 118745 oldestRunningXid 118746 pg_xlogdump: FATAL: error in WAL record at 0/16000238: invalid record length at 0/16000270: wanted 24, got 0
可以看到关键记录
lsn: 0/16000028, prev 0/152C9A10, desc: CHECKPOINT_SHUTDOWN redo 0/16000028 lsn: 0/16000098, prev 0/16000028, desc: END_OF_RECOVERY
而 END_OF_RECOVERY 对应的 lsn 为 0/16000098，和 00000002.history 时间线文件的内容完全一致。
所以在异步 stream replication 环境下，主动关闭master时，会将最后一条记录(CHECKPOINT_SHUTDOWN)发送给slave，不会造成数据的丢失。
而 synchronous_commit = on 保证事务有两份持久化的落盘数据。
分析 pg_log 日志
old master 上的最后几条日志
2018-11-30 10:16:40.986 CST,"postgres","pgbenchdb",7559,"[local]",5c009d79.1d87,4,"UPDATE waiting",2018-11-30 10:16:25 CST,,0,LOG,00000,"disconnection: session time: 0:00:15.723 user=postgres database=pgbenchdb host=[local]",,,,,,,,,"pgbench" 2018-11-30 10:16:40.993 CST,"postgres","pgbenchdb",7558,"[local]",5c009d79.1d86,3,"idle",2018-11-30 10:16:25 CST,4/0,0,FATAL,57P01,"terminating connection due to administrator command",,,,,,,,,"pgbench" 2018-11-30 10:16:40.994 CST,"postgres","pgbenchdb",7560,"[local]",5c009d79.1d88,3,"idle",2018-11-30 10:16:25 CST,5/0,0,FATAL,57P01,"terminating connection due to administrator command",,,,,,,,,"pgbench" 2018-11-30 10:16:40.994 CST,"postgres","pgbenchdb",7558,"[local]",5c009d79.1d86,4,"idle",2018-11-30 10:16:25 CST,,0,LOG,00000,"disconnection: session time: 0:00:15.729 user=postgres database=pgbenchdb host=[local]",,,,,,,,,"pgbench" 2018-11-30 10:16:40.994 CST,"postgres","pgbenchdb",7560,"[local]",5c009d79.1d88,4,"idle",2018-11-30 10:16:25 CST,,0,LOG,00000,"disconnection: session time: 0:00:15.725 user=postgres database=pgbenchdb host=[local]",,,,,,,,,"pgbench" 2018-11-30 10:16:40.999 CST,"postgres","pgbenchdb",7561,"[local]",5c009d79.1d89,3,"idle",2018-11-30 10:16:25 CST,6/0,0,FATAL,57P01,"terminating connection due to administrator command",,,,,,,,,"pgbench" 2018-11-30 10:16:41.001 CST,"postgres","pgbenchdb",7561,"[local]",5c009d79.1d89,4,"idle",2018-11-30 10:16:25 CST,,0,LOG,00000,"disconnection: session time: 0:00:15.731 user=postgres database=pgbenchdb host=[local]",,,,,,,,,"pgbench" 2018-11-30 10:16:41.010 CST,,,7156,,5c009735.1bf4,7,,2018-11-30 09:49:41 CST,,0,LOG,00000,"shutting down",,,,,,,,,"" 2018-11-30 10:16:41.209 CST,,,7156,,5c009735.1bf4,8,,2018-11-30 09:49:41 CST,,0,LOG,00000,"checkpoint starting: shutdown immediate",,,,,,,,,"" 2018-11-30 10:16:47.623 CST,,,7156,,5c009735.1bf4,9,,2018-11-30 09:49:41 CST,,0,LOG,00000,"checkpoint complete: wrote 29357 buffers (89.6%); 0 transaction log file(s) added, 0 removed, 0 recycled; write=5.931 s, sync=0.399 s, total=6.418 s; sync files=53, longest=0.179 s, average=0.007 s; distance=311294 kB, estimate=311294 kB",,,,,,,,,"" 2018-11-30 10:16:47.683 CST,"repl","",7227,"192.168.56.90:52556",5c009795.1c3b,3,"streaming 0/16000098",2018-11-30 09:51:17 CST,,0,LOG,00000,"disconnection: session time: 0:25:30.149 user=repl database= host=192.168.56.90 port=52556",,,,,,,,,"walreceiver" 2018-11-30 10:16:47.730 CST,,,7153,,5c009735.1bf1,5,,2018-11-30 09:49:41 CST,,0,LOG,00000,"database system is shut down",,,,,,,,,""
注意倒数第二条信息 streaming 0/16000098 ，说明当时的master关闭时，已经和salve沟通过，确认已经接收到 END_OF_RECOVERY 之前所有的数据了。
old slave 日志
2018-11-30 10:16:47.660 CST,,,7256,,5c009795.1c58,2,,2018-11-30 09:51:17 CST,,0,LOG,00000,"replication terminated by primary server","End of WAL reached on timeline 1 at 0/16000098.",,,,,,,,"" 2018-11-30 10:16:47.660 CST,,,7256,,5c009795.1c58,3,,2018-11-30 09:51:17 CST,,0,FATAL,XX000,"could not send end-of-streaming message to primary: no COPY in progress ",,,,,,,,,"" 2018-11-30 10:16:47.660 CST,,,7255,,5c009795.1c57,5,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"invalid record length at 0/16000098: wanted 24, got 0",,,,,,,,,"" 2018-11-30 10:16:47.854 CST,,,7443,,5c009d8f.1d13,1,,2018-11-30 10:16:47 CST,,0,FATAL,XX000,"could not connect to the primary server: could not connect to server: Connection refused Is the server running on host ""192.168.56.119"" and accepting TCP/IP connections on port 5432? ",,,,,,,,,"" 2018-11-30 10:16:52.668 CST,,,7444,,5c009d94.1d14,1,,2018-11-30 10:16:52 CST,,0,FATAL,XX000,"could not connect to the primary server: could not connect to server: Connection refused Is the server running on host ""192.168.56.119"" and accepting TCP/IP connections on port 5432? ",,,,,,,,,"" 2018-11-30 10:16:56.875 CST,,,7255,,5c009795.1c57,6,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"received promote request",,,,,,,,,"" 2018-11-30 10:16:56.875 CST,,,7255,,5c009795.1c57,7,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"redo done at 0/16000028",,,,,,,,,"" 2018-11-30 10:16:56.875 CST,,,7255,,5c009795.1c57,8,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"last completed transaction was at log time 2018-11-30 10:16:40.986869+08",,,,,,,,,"" 2018-11-30 10:16:56.888 CST,,,7255,,5c009795.1c57,9,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"selected new timeline ID: 2",,,,,,,,,"" 2018-11-30 10:16:57.166 CST,,,7255,,5c009795.1c57,10,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"archive recovery complete",,,,,,,,,"" 2018-11-30 10:16:57.267 CST,,,7255,,5c009795.1c57,11,,2018-11-30 09:51:17 CST,1/0,0,LOG,00000,"MultiXact member wraparound protections are now enabled",,,,,,,,,"" 2018-11-30 10:16:57.267 CST,,,7257,,5c009795.1c59,1,,2018-11-30 09:51:17 CST,,0,LOG,00000,"checkpoint starting: force",,,,,,,,,"" 2018-11-30 10:16:57.275 CST,,,7253,,5c009795.1c55,3,,2018-11-30 09:51:17 CST,,0,LOG,00000,"database system is ready to accept connections",,,,,,,,,"" 2018-11-30 10:16:57.276 CST,,,7447,,5c009d99.1d17,1,,2018-11-30 10:16:57 CST,,0,LOG,00000,"autovacuum launcher started",,,,,,,,,""
信息也是相当的清晰。
wal_retrieve_retry_interval = 5s 控制 salve 到 master 失败时，再次重试的等待时间。
以上为个人经验，希望能给大家一个参考，也希望大家多多支持潘少俊衡。如有错误或未考虑完全的地方，望不吝赐教。
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