pg_xlog 的重要性不言而喻，不可以随意直接rm，官方文档中也有明确说明。
当随着时间的推移，xlog目录会越来越大，网上有些方法是先停机然后再删除，但我不想停机删除，那么如何优雅清理xlog日志呢？
方法：
1.登录到postgres中查看当前的wal_keep_segments
postgres=# show wal_keep_segments ;
wal_keep_segments
-------------------
1024
(1 row)
2.在 postgres.conf中修改 降低wal_keep_segments的值，本案例中降低到512
3.重载 postgres配置文件
-bash-4.1$ pg_ctl reload -D $PGDATA
- 再次查看 wal_keep_segments的值
postgres=# show wal_keep_segments ;
wal_keep_segments
-------------------
512
(1 row)
--> 查看到值已经降低，但是pg_xlog的日志文件数量仍然是那么多，
原因是因为每隔 checkpoints时间wal 会自动检查一次,checkpoint_timeout默认是5分钟。
postgres=# show checkpoint_timeout ;
checkpoint_timeout
--------------------
5min
(1 row)
5.想要立刻见效，可以手动执行checkpoint
postgres=# checkpoint ;
Note:
“ If a standby server connected to the sending server falls behind by more than wal_keep_segments segments, the sending server might remove a WAL segment still needed by the standby, in which case the replication connection will be terminated” 官方 文档中明确说了如果wal_keep_segments不一致，可能会导致流复制不同步，因此在各个节点一定保持wal_keep_segments数量一致。
关于wal_keep_segments的说明
pg归档的删除方法
-bash-4.1$ pg_archivecleanup --help
pg_archivecleanup removes older WAL files from PostgreSQL archives.
Usage:
pg_archivecleanup [OPTION]... ARCHIVELOCATION OLDESTKEPTWALFILE
Options:
-d generate debug output (verbose mode)
-n dry run, show the names of the files that would be removed
-V, --version output version information, then exit
-x EXT clean up files if they have this extension
-?, --help show this help, then exit
For use as archive_cleanup_command in recovery.conf when standby_mode = on:
archive_cleanup_command = 'pg_archivecleanup [OPTION]... ARCHIVELOCATION %r'
e.g.
archive_cleanup_command = 'pg_archivecleanup /mnt/server/archiverdir %r'
Or for use as a standalone archive cleaner:
e.g.
pg_archivecleanup /mnt/server/archiverdir 000000010000000000000010.00000020.backup
Report bugs to <pgsql-bugs@postgresql.org>.
删除0000001D0000000000000054 之前的归档
-bash-4.1$ pg_archivecleanup . 0000001D0000000000000054
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