说明
- 迁移过程参考：http://www.jianshu.com/p/2d07339774c0
遇到问题：当配置完从库，然后启动时发生如下错误：
“FATAL: requested WAL segment 0000000800002A0000000000 has already been removed”
原因
- PostgreSQL的流复制的原理是通过传递主机(master)上的wal日志信息到备机(slave)然后恢复，这中间就有一个潜在的问题，如果主机端比较忙，wal日志被覆盖了，而从机可能因为网络或者其他原因没有接收到该日志，就会造成主从不一致，这时主从就断了，并且在主机端留下很多的流复制错误信息；通常这种情况不影响主库的使用，但如果有在备机上做一些查询，就需要注意了。出现了这个问题，需要到归档的日志文件里把文件拷贝到备机的pg_xlog下面。 如果主机的wal日志已经循环覆盖了，而且没有做wal的归档，那出现这种情况只能重新做流复制了；
- 在重新做流复制的时候，我也调大了postgresql.conf中wal_keep_segments的值（wal_keep_segments=16）
官方对wal_keep_segments的解释：
Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is normally 16 megabytes. If a standby server connected to the sending server falls behind by more than wal_keep_segments segments, the sending server might remove a WAL segment still needed by the standby, in which case the replication connection will be terminated. Downstream connections will also eventually fail as a result. (However, the standby server can recover by fetching the segment from archive, if WAL archiving is in use.)
This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint. If wal_keep_segments is zero (the default), the system doesn’t keep any extra segments for standby purposes, so the number of old WAL segments available to standby servers is a function of the location of the previous checkpoint and status of WAL archiving. This parameter can only be set in the postgresql.conf file or on the server command line.