背景
1、PG物理流复制的从库，当激活后，可以开启读写，使用pg_rewind可以将从库回退为只读从库的角色。而不需要重建整个从库。
2、当异步主从发生角色切换后，主库的wal目录中可能还有没完全同步到从库的内容，因此老的主库无法直接切换为新主库的从库。使用pg_rewind可以修复老的主库，使之成为新主库的只读从库。而不需要重建整个从库。
3、如果没有pg_rewind，遇到以上情况，需要完全重建从库。或者你可以使用存储层快照，回退回脑裂以前的状态。又或者可以使用文件系统快照，回退回脑裂以前的状态。
原理与修复步骤
1、使用pg_rewind功能的前提条件：必须开启full page write，必须开启wal hint或者data block checksum。
2、需要被修复的库：从激活点开始，所有的WAL必须存在pg_wal目录中。如果WAL已经被覆盖，只要有归档，拷贝到pg_wal目录即可。
3、新的主库，从激活点开始，产生的所有WAL必须存在pg_wal目录中，或者已归档，并且被修复的库可以使用restore_command访问到这部分WAL。
4、修改(source db)新主库或老主库配置，允许连接。
5、修复时，连接新主库，得到切换点。或连接老主库，同时比对当前要修复的新主库的TL与老主库进行比对，得到切换点。
6、解析需要被修复的库的从切换点到现在所有的WAL。同时连接source db(新主库（或老主库）)，进行回退操作（被修改或删除的BLOCK从source db获取并覆盖，新增的BLOCK，直接抹除。）回退到切换点的状态。
7、修改被修复库(target db)的recovery.conf, postgresql.conf配置。
8、启动target db，连接source db接收WAL，或restore_command配置接收WAL，从切换点开始所有WAL，进行apply。
9、target db现在是source db的从库。
以EDB PG 11为例讲解
export PS1="$USER@`/bin/hostname -s`-> "
export PGPORT=4000
export PGDATA=/data04/ppas11/pg_root4000
export LANG=en_US.utf8
export PGHOME=/usr/edb/as11
export LD_LIBRARY_PATH=$PGHOME/lib:/lib64:/usr/lib64:/usr/local/lib64:/lib:/usr/lib:/usr/local/lib:$LD_LIBRARY_PATH
export DATE=`date +"%Y%m%d%H%M"`
export PATH=$PGHOME/bin:$PATH:.
export MANPATH=$PGHOME/share/man:$MANPATH
export PGHOST=127.0.0.1
export PGUSER=postgres
export PGDATABASE=postgres
alias rm='rm -i'
alias ll='ls -lh'
unalias vi
1、初始化数据库集群
initdb -D /data04/ppas11/pg_root4000 -E UTF8 --lc-collate=C --lc-ctype=en_US.UTF8 -U postgres -k --redwood-like
2、配置recovery.done
cd $PGDATA
cp $PGHOME/share/recovery.conf.sample ./
mv recovery.conf.sample recovery.done
vi recovery.done
restore_command = 'cp /data04/ppas11/wal/%f %p'
recovery_target_timeline = 'latest'
standby_mode = on
primary_conninfo = 'host=localhost port=4000 user=postgres'
3、配置postgresql.conf
要使用rewind功能：
必须开启full_page_writes
必须开启data_checksums或wal_log_hints
postgresql.conf
listen_addresses = '0.0.0.0'
port = 4000
max_connections = 8000
superuser_reserved_connections = 13
unix_socket_directories = '.,/tmp'
unix_socket_permissions = 0700
tcp_keepalives_idle = 60
tcp_keepalives_interval = 10
tcp_keepalives_count = 10
shared_buffers = 16GB
max_prepared_transactions = 8000
maintenance_work_mem = 1GB
autovacuum_work_mem = 1GB
dynamic_shared_memory_type = posix
vacuum_cost_delay = 0
bgwriter_delay = 10ms
bgwriter_lru_maxpages = 1000
bgwriter_lru_multiplier = 10.0
effective_io_concurrency = 0
max_worker_processes = 128
max_parallel_maintenance_workers = 8
max_parallel_workers_per_gather = 8
max_parallel_workers = 24
wal_level = replica
synchronous_commit = off
full_page_writes = on
wal_compression = on
wal_buffers = 32MB
wal_writer_delay = 10ms
checkpoint_timeout = 25min
max_wal_size = 32GB
min_wal_size = 8GB
checkpoint_completion_target = 0.2
archive_mode = on
archive_command = 'cp -n %p /data04/ppas11/wal/%f'
max_wal_senders = 16
wal_keep_segments = 4096
max_replication_slots = 16
hot_standby = on
max_standby_archive_delay = 300s
max_standby_streaming_delay = 300s
wal_receiver_status_interval = 1s
wal_receiver_timeout = 10s
random_page_cost = 1.1
effective_cache_size = 400GB
log_destination = 'csvlog'
logging_collector = on
log_directory = 'log'
log_filename = 'edb-%a.log'
log_truncate_on_rotation = on
log_rotation_age = 1d
log_rotation_size = 0
log_min_duration_statement = 1s
log_checkpoints = on
log_error_verbosity = verbose
log_line_prefix = '%t '
log_lock_waits = on
log_statement = 'ddl'
log_timezone = 'PRC'
autovacuum = on
log_autovacuum_min_duration = 0
autovacuum_max_workers = 6
autovacuum_freeze_max_age = 1200000000
autovacuum_multixact_freeze_max_age = 1400000000
autovacuum_vacuum_cost_delay = 0
statement_timeout = 0
lock_timeout = 0
idle_in_transaction_session_timeout = 0
vacuum_freeze_table_age = 1150000000
vacuum_multixact_freeze_table_age = 1150000000
datestyle = 'redwood,show_time'
timezone = 'PRC'
lc_messages = 'en_US.utf8'
lc_monetary = 'en_US.utf8'
lc_numeric = 'en_US.utf8'
lc_time = 'en_US.utf8'
default_text_search_config = 'pg_catalog.english'
shared_preload_libraries = 'auto_explain,pg_stat_statements,$libdir/dbms_pipe,$libdir/edb_gen,$libdir/dbms_aq'
edb_redwood_date = on
edb_redwood_greatest_least = on
edb_redwood_strings = on
db_dialect = 'redwood'
edb_dynatune = 66
edb_dynatune_profile = oltp
timed_statistics = off
4、配置pg_hba.conf，允许流复制
local all all trust
host all all 127.0.0.1/32 trust
host all all ::1/128 trust
local replication all trust
host replication all 127.0.0.1/32 trust
host replication all ::1/128 trust
host all all 0.0.0.0/0 md5
5、配置归档目录
mkdir /data04/ppas11/wal
chown enterprisedb:enterprisedb /data04/ppas11/wal
6、创建从库
pg_basebackup -h 127.0.0.1 -p 4000 -D /data04/ppas11/pg_root4001 -F p -c fast
7、配置从库
cd /data04/ppas11/pg_root4001
mv recovery.done recovery.conf
vi postgresql.conf
port = 4001
8、启动从库
pg_ctl start -D /data04/ppas11/pg_root4001
9、压测主库
pgbench -i -s 1000
pgbench -M prepared -v -r -P 1 -c 24 -j 24 -T 300
10、检查归档
postgres=# select * from pg_stat_archiver ;
archived_count | last_archived_wal | last_archived_time | failed_count | last_failed_wal | last_failed_time | stats_reset
----------------+--------------------------+----------------------------------+--------------+-----------------+------------------+----------------------------------
240 | 0000000100000000000000F0 | 28-JAN-19 15:08:43.276965 +08:00 | 0 | | | 28-JAN-19 15:01:17.883338 +08:00
(1 row)
postgres=# select * from pg_stat_archiver ;
archived_count | last_archived_wal | last_archived_time | failed_count | last_failed_wal | last_failed_time | stats_reset
----------------+--------------------------+----------------------------------+--------------+-----------------+------------------+----------------------------------
248 | 0000000100000000000000F8 | 28-JAN-19 15:08:45.120134 +08:00 | 0 | | | 28-JAN-19 15:01:17.883338 +08:00
(1 row)
11、检查从库延迟
postgres=# select * from pg_stat_replication ;
-[ RECORD 1 ]----+---------------------------------
pid | 8124
usesysid | 10
usename | postgres
application_name | walreceiver
client_addr | 127.0.0.1
client_hostname |
client_port | 62988
backend_start | 28-JAN-19 15:07:34.084542 +08:00
backend_xmin |
state | streaming
sent_lsn | 1/88BC2000
write_lsn | 1/88BC2000
flush_lsn | 1/88BC2000
replay_lsn | 1/88077D48
write_lag | 00:00:00.001417
flush_lag | 00:00:00.002221
replay_lag | 00:00:00.097657
sync_priority | 0
sync_state | async
例子1，从库激活后产生读写，使用pg_rewind修复从库，回退到只读从库
1、激活从库
pg_ctl promote -D /data04/ppas11/pg_root4001
2、写从库
pgbench -M prepared -v -r -P 1 -c 4 -j 4 -T 120 -p 4001
此时从库已经和主库不在一个时间线，无法直接变成当前主库的从库
enterprisedb@pg11-test-> pg_controldata -D /data04/ppas11/pg_root4001|grep -i time
Latest checkpoint's TimeLineID: 1
Latest checkpoint's PrevTimeLineID: 1
Time of latest checkpoint: Mon 28 Jan 2019 03:56:38 PM CST
Min recovery ending loc's timeline: 2
track_commit_timestamp setting: off
Date/time type storage: 64-bit integers
enterprisedb@pg11-test-> pg_controldata -D /data04/ppas11/pg_root4000|grep -i time
Latest checkpoint's TimeLineID: 1
Latest checkpoint's PrevTimeLineID: 1
Time of latest checkpoint: Mon 28 Jan 2019 05:11:38 PM CST
Min recovery ending loc's timeline: 0
track_commit_timestamp setting: off
Date/time type storage: 64-bit integers
3、修复从库，使之继续成为当前主库的从库
4、查看切换点
cd /data04/ppas11/pg_root4001
ll pg_wal/*.history
-rw------- 1 enterprisedb enterprisedb 42 Jan 28 17:15 pg_wal/00000002.history
cat pg_wal/00000002.history
1 6/48C62000 no recovery target specified
5、从库激活时间开始产生的WAL必须全部在pg_wal目录中。
-rw------- 1 enterprisedb enterprisedb 42 Jan 28 17:15 00000002.history
-rw------- 1 enterprisedb enterprisedb 16M Jan 28 17:16 000000020000000600000048
............
000000020000000600000048开始，所有的wal必须存在从库pg_wal目录中。如果已经覆盖了，必须从归档目录拷贝到从库pg_wal目录中。
6、从库激活时，主库从这个时间点开始所有的WAL还在pg_wal目录，或者从库可以使用restore_command获得(recovery.conf)。
recovery.conf
restore_command = 'cp /data04/ppas11/wal/%f %p'
7、pg_rewind命令帮助
https://www.postgresql.org/docs/11/app-pgrewind.html
pg_rewind --help
pg_rewind resynchronizes a PostgreSQL cluster with another copy of the cluster.
Usage:
pg_rewind [OPTION]...
Options:
-D, --target-pgdata=DIRECTORY existing data directory to modify
--source-pgdata=DIRECTORY source data directory to synchronize with
--source-server=CONNSTR source server to synchronize with
-n, --dry-run stop before modifying anything
-P, --progress write progress messages
--debug write a lot of debug messages
-V, --version output version information, then exit
-?, --help show this help, then exit
Report bugs to <support@enterprisedb.com>.
8、停库（被修复的库，停库）
pg_ctl stop -m fast -D /data04/ppas11/pg_root4001
9、尝试修复
pg_rewind -n -D /data04/ppas11/pg_root4001 --source-server="hostaddr=127.0.0.1 user=postgres port=4000"
servers diverged at WAL location 6/48C62000 on timeline 1
rewinding from last common checkpoint at 5/5A8CD30 on timeline 1
Done!
10、尝试正常，说明可以修复，实施修复
pg_rewind -D /data04/ppas11/pg_root4001 --source-server="hostaddr=127.0.0.1 user=postgres port=4000"
servers diverged at WAL location 6/48C62000 on timeline 1
rewinding from last common checkpoint at 5/5A8CD30 on timeline 1
Done!
11、已修复，改配置
cd /data04/ppas11/pg_root4001
vi postgresql.conf
port = 4001
mv recovery.done recovery.conf
vi recovery.conf
restore_command = 'cp /data04/ppas11/wal/%f %p'
recovery_target_timeline = 'latest'
standby_mode = on
primary_conninfo = 'host=localhost port=4000 user=postgres'
12、删除归档中错误时间线上产生的文件否则会在启动修复后的从库后，走到00000002时间线上，这是不想看到的。
mkdir /data04/ppas11/wal/error_tl_2
mv /data04/ppas11/wal/00000002* /data04/ppas11/wal/error_tl_2
13、启动从库
pg_ctl start -D /data04/ppas11/pg_root4001
14、建议对主库做一个检查点，从库收到检查点后，重启后不需要应用太多WAL，而是从新检查点开始恢复
psql
checkpoint;
15、压测主库
pgbench -M prepared -v -r -P 1 -c 16 -j 16 -T 200 -p 4000
16、查看归档状态
postgres=# select * from pg_stat_archiver ;
archived_count | last_archived_wal | last_archived_time | failed_count | last_failed_wal | last_failed_time | stats_reset
----------------+--------------------------+----------------------------------+--------------+-----------------+------------------+----------------------------------
1756 | 0000000100000006000000DC | 28-JAN-19 17:41:57.562425 +08:00 | 0 | | | 28-JAN-19 15:01:17.883338 +08:00
(1 row)
17、查看从库健康、延迟，观察修复后的情况
postgres=# select * from pg_stat_replication ;
-[ RECORD 1 ]----+--------------------------------
pid | 13179
usesysid | 10
usename | postgres
application_name | walreceiver
client_addr | 127.0.0.1
client_hostname |
client_port | 63198
backend_start | 28-JAN-19 17:47:29.85308 +08:00
backend_xmin |
state | catchup
sent_lsn | 7/DDE80000
write_lsn | 7/DC000000
flush_lsn | 7/DC000000
replay_lsn | 7/26A8DCB0
write_lag | 00:00:18.373263
flush_lag | 00:00:18.373263
replay_lag | 00:00:18.373263
sync_priority | 0
sync_state | async
例子2，从库激活成为新主库后，老主库依旧有读写，使用pg_rewind修复老主库，将老主库降级为新主库的从库
1、激活从库
pg_ctl promote -D /data04/ppas11/pg_root4001
2、写从库
pgbench -M prepared -v -r -P 1 -c 16 -j 16 -T 200 -p 4001
3、写主库
pgbench -M prepared -v -r -P 1 -c 16 -j 16 -T 200 -p 4000
此时老主库已经和新的主库不在一个时间线
enterprisedb@pg11-test-> pg_controldata -D /data04/ppas11/pg_root4000|grep -i timeline
Latest checkpoint's TimeLineID: 1
Latest checkpoint's PrevTimeLineID: 1
Min recovery ending loc's timeline: 0
enterprisedb@pg11-test-> pg_controldata -D /data04/ppas11/pg_root4001|grep -i timeline
Latest checkpoint's TimeLineID: 1
Latest checkpoint's PrevTimeLineID: 1
Min recovery ending loc's timeline: 2
enterprisedb@pg11-test-> cd /data04/ppas11/pg_root4001/pg_wal
enterprisedb@pg11-test-> cat 00000002.history
1 8/48DE2318 no recovery target specified
enterprisedb@pg11-test-> ll *.partial
-rw------- 1 enterprisedb enterprisedb 16M Jan 28 17:48 000000010000000800000048.partial
4、修复老主库，变成从库
4.1、从库激活时，老主库从这个时间点开始所有的WAL，必须全部在pg_wal目录中。
000000010000000800000048 开始的所有WAL必须存在pg_wal，如果已经覆盖了，必须从WAL归档拷贝到pg_wal目录
4.2、从库激活时间开始产生的所有WAL，老主库必须可以使用restore_command获得(recovery.conf)。
recovery.conf
restore_command = 'cp /data04/ppas11/wal/%f %p'
5、关闭老主库
pg_ctl stop -m fast -D /data04/ppas11/pg_root4000
6、尝试修复老主库
pg_rewind -n -D /data04/ppas11/pg_root4000 --source-server="hostaddr=127.0.0.1 user=postgres port=4001"
servers diverged at WAL location 8/48DE2318 on timeline 1
rewinding from last common checkpoint at 6/CCCEF770 on timeline 1
Done!
7、尝试成功，可以修复，实施修复
pg_rewind -D /data04/ppas11/pg_root4000 --source-server="hostaddr=127.0.0.1 user=postgres port=4001"
8、修复完成后，改配置
cd /data04/ppas11/pg_root4000
vi postgresql.conf
port = 4000
mv recovery.done recovery.conf
vi recovery.conf
restore_command = 'cp /data04/ppas11/wal/%f %p'
recovery_target_timeline = 'latest'
standby_mode = on
primary_conninfo = 'host=localhost port=4001 user=postgres'
9、启动老主库
pg_ctl start -D /data04/ppas11/pg_root4000
10、建议对新主库做一个检查点，从库收到检查点后，重启后不需要应用太多WAL，而是从新检查点开始恢复
checkpoint;
11、压测新主库
pgbench -M prepared -v -r -P 1 -c 16 -j 16 -T 200 -p 4001
12、查看归档状态
psql -p 4001
postgres=# select * from pg_stat_archiver ;
archived_count | last_archived_wal | last_archived_time | failed_count | last_failed_wal | last_failed_time | stats_reset
----------------+--------------------------+----------------------------------+--------------+-----------------+------------------+----------------------------------
406 | 0000000200000009000000DB | 28-JAN-19 21:18:22.976118 +08:00 | 0 | | | 28-JAN-19 17:47:29.847488 +08:00
(1 row)
13、查看从库健康、延迟
psql -p 4001
postgres=# select * from pg_stat_replication ;
-[ RECORD 1 ]----+---------------------------------
pid | 17675
usesysid | 10
usename | postgres
application_name | walreceiver
client_addr | 127.0.0.1
client_hostname |
client_port | 60530
backend_start | 28-JAN-19 21:18:36.472197 +08:00
backend_xmin |
state | streaming
sent_lsn | 9/E8361C18
write_lsn | 9/E8361C18
flush_lsn | 9/E8361C18
replay_lsn | 9/D235B520
write_lag | 00:00:00.000101
flush_lag | 00:00:00.000184
replay_lag | 00:00:03.028098
sync_priority | 0
sync_state | async
小结
1 适合场景
1、PG物理流复制的从库，当激活后，可以开启读写，使用pg_rewind可以将从库回退为只读从库的角色。而不需要重建整个从库。
2、当异步主从发生角色切换后，主库的wal目录中可能还有没完全同步到从库的内容，因此老的主库无法直接切换为新主库的从库。使用pg_rewind可以修复老的主库，使之成为新主库的只读从库。而不需要重建整个从库。
如果没有pg_rewind，遇到以上情况，需要完全重建从库，如果库占用空间很大，重建非常耗时，也非常耗费上游数据库的资源（读）。
2 前提
要使用rewind功能：
1、必须开启full_page_writes
2、必须开启data_checksums或wal_log_hints
initdb -k 开启data_checksums
3 原理与修复流程
1、使用pg_rewind功能的前提条件：必须开启full page write，必须开启wal hint或者data block checksum。
2、需要被修复的库：从激活点开始，所有的WAL必须存在pg_wal目录中。如果WAL已经被覆盖，只要有归档，拷贝到pg_wal目录即可。
3、新的主库，从激活点开始，产生的所有WAL必须存在pg_wal目录中，或者已归档，并且被修复的库可以使用restore_command访问到这部分WAL。
4、修改(source db)新主库或老主库配置，允许连接。
5、修复时，连接新主库，得到切换点。或连接老主库，同时比对当前要修复的新主库的TL与老主库进行比对，得到切换点。
6、解析需要被修复的库的从切换点到现在所有的WAL。同时连接source db(新主库（或老主库）)，进行回退操作（被修改或删除的BLOCK从source db获取并覆盖，新增的BLOCK，直接抹除。）回退到切换点的状态。
7、修改被修复库(target db)的recovery.conf, postgresql.conf配置。
8、启动target db，连接source db接收WAL，或restore_command配置接收WAL，从切换点开始所有WAL，进行apply。
9、target db现在是source db的从库。
参考
https://www.postgresql.org/docs/11/app-pgrewind.html
《PostgreSQL primary-standby failback tools : pg_rewind》
《PostgreSQL 9.5 new feature - pg_rewind fast sync Split Brain Primary & Standby》
《PostgreSQL 9.5 add pg_rewind for Fast align for PostgreSQL unaligned primary & standby》
《MTK使用 - PG,PPAS,oracle,mysql,ms sql,sybase 迁移到 PG, PPAS (支持跨版本升级)》
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