PostgreSQL：“ FATAL: requested WAL segment00800002A0 has already been removed”
在使用配置了热备的PostgreSQL数据库时，在执行大量事务时，尤其是一个需要插入几千万条数据的insert事务时（典型的做法是持续 insert into t select * from t;），后台log中报错如下：
csv格式日志：
2013-07-01 13:25:29.430 CST,,,27738,,51d112c8.6c5a,1,,2013-07-01 13:25:28 CST,,0,LOG,00000,"streaming replication successfully connected to primary",,,,,,,,"libpqrcv_connect, libpqwalreceiver.c:171",""
2013-07-01 13:25:29.430 CST,,,27738,,51d112c8.6c5a,2,,2013-07-01 13:25:28 CST,,0,FATAL,XX000,"could not receive data from WAL stream:FATAL: requested WAL segment 0000000800002A0000000000 has already been removed
",,,,,,,,"libpqrcv_receive, libpqwalreceiver.c:389",""
备注：根据报错信息，很容易知道是由于在主库产生了大量xlog，由于postgreSQL在执行事务时，在提交时才发送到备库。由于该事务需要执行的时间过长，超过了checkpoint的默认间隔，所以导致有的xlog还未发送到备库却被remove掉了。要解决该问题，一般可用的方案有：
一、调整wal_keep_segments的值
将GUC参数wal_keep_segments设大一些，比如设置为2000，而每个segment默认值为16MB，就相当于有32000MB，即大概30多个GB的空间作为缓存空间。
不过，该方法并不能从根本上解决该问题。毕竟，在生产环境中或TPCC等测试灌数时，如果某条事务需要插入几十亿条记录，有可能还是会出现该问题。
二、启用归档
归档，就是将未发送到备库的xlog备份到某个目录下，待重启数据库时再将其恢复到备库中去。
GUC参数设置示例如下：
主库的postgresql.conf文件中：
wal_level = hot_standby
archive_mode = on
archive_command = 'rsync -zaq %p postgres@pg-slave:/var/lib/pgsql/wal_restore/%f && test ! -f /var/lib/pgsql/backup/wal_archive/%f && cp %p /var/lib/pgsql/backup/wal_archive/'
archive_timeout = 300
max_wal_senders = 5
wal_keep_segments = 0 # not sure why I've set it to this?
备库的postgresql.conf文件中：
wal_level = hot_standby
archive_mode = on
archive_command = 'test ! -f /var/lib/pgsql/backup/wal_archive/%f && cp -i %p /var/lib/pgsql/backup/wal_archive/%f < /dev/null'
hot_standby = on
wal_keep_segments = 1
备库的recovery.conf文件中：
standby_mode = 'on'
primary_conninfo = 'host=pg-master port=5432 user=replicator'
restore_command = 'cp /var/lib/psql/wal_restore/%f %p'
archive_cleanup_command = 'pg_archivecleanup /var/lib/pgsql/wal_restore/ %r'
三、启用replication slot （pg9.4以后才有）
这个方法算是根本解决方法，不会造成xlog的丢失。也就是说，在xlog被拷贝之前，不会删除。
启用方法：
（1）在postgresql.conf中添加：
max_replication_slots = 2000
（2）在拷贝到备库之前，主库要创建一个slot：
postgres=# SELECT * FROM pg_create_physical_replication_slot('node_a_slot');
slot_name | xlog_position
-------------+---------------
node_a_slot |
postgres=# SELECT * FROM pg_replication_slots;
slot_name | slot_type | datoid | database | active | xmin | restart_lsn
-------------+-----------+--------+----------+--------+------+-------------
node_a_slot | physical | | | f | |
(1 row)
（3）在备库的recovery.conf文件中添加一行：
standby_mode = 'on'
primary_conninfo = 'host=192.168.4.225 port=19000 user=wslu password=xxxx'
primary_slot_name = 'node_a_slot'
如何配置备库，参考：
http://blog.csdn.net/prettyshuang/article/details/50898363#t10
参考：
https://www.postgresql.org/docs/9.4/static/runtime-config-replication.html
https://www.postgresql.org/docs/9.4/static/warm-standby.html#CASCADING-REPLICATION
http://blog.2ndquadrant.com/postgresql-9-4-slots/
http://grokbase.com/t/postgresql/pgsql-general/13654jchy3/trouble-with-replication
http://stackoverflow.com/questions/28201475/how-do-i-fix-a-postgresql-9-3-slave-that-cannot-keep-up-with-the-master