[postgres@ndb2 database]$ ps -ef|grep sender
postgres 21257 21247 0 20:57 "htmlcode">

2013-08-12 20:53:42.132 CST,,,8859,,5208dad6.229b,1,,2013-08-12 20:53:42 CST,,0,FATAL,XX000,"could not connect to the primary server: FATAL: number of requested standby connections exceeds max_wal_senders (currently 0)
",,,,,,,,,""
2013-08-12 20:53:47.137 CST,,,8861,,5208dadb.229d,1,,2013-08-12 20:53:47 CST,,0,FATAL,XX000,"could not connect to the primary server: FATAL: number of requested standby connections exceeds max_wal_senders (currently 0)
",,,,,,,,,""
2013-08-12 20:53:52.142 CST,,,8862,,5208dae0.229e,1,,2013-08-12 20:53:52 CST,,0,FATAL,XX000,"could not connect to the primary server: FATAL: number of requested standby connections exceeds max_wal_senders (currently 0)
",,,,,,,,,""
2013-08-12 20:53:57.148 CST,,,8864,,5208dae5.22a0,1,,2013-08-12 20:53:57 CST,,0,FATAL,XX000,"could not connect to the primary server: FATAL: number of requested standby connections exceeds max_wal_senders (currently 0)
",,,,,,,,,"


主机上显示的日志异常:


receiver"
2013-08-12 20:43:26.937 CST,,,21064,"",5208d86e.5248,1,"",2013-08-12 20:43:26 CST,,0,LOG,00000,"connection received: host=192.25.10.71 port=46085",,,,,,,,,""
2013-08-12 20:43:26.938 CST,"repuser","",21064,"192.25.10.71:46085",5208d86e.5248,2,"authentication",2013-08-12 20:43:26 CST,2/1195,0,LOG,00000,"replication connection authorized: user=repuser",,,,,,,,,""
2013-08-12 20:43:26.938 CST,"repuser","",21064,"192.25.10.71:46085",5208d86e.5248,3,"startup",2013-08-12 20:43:26 CST,2/0,0,FATAL,53300,"number of requested standby connections exceeds max_wal_senders (currently 0)",,,,,,,,,"walreceiver"
2013-08-12 20:43:26.939 CST,"repuser","",21064,"192.25.10.71:46085",5208d86e.5248,4,"startup",2013-08-12 20:43:26 CST,,0,LOG,00000,"disconnection: session time: 0:00:00.002 user=repuser database= host=192.25.10.71 port=46085",,,,,,,,,"wa
lreceiver"
2013-08-12 20:43:41.513 CST,,,21066,"",5208d87d.524a,1,"",2013-08-12 20:43:41 CST,,0,LOG,00000,"connection received: host=192.25.10.71 port=46086",,,,,,,,,""
2013-08-12 20:43:41.514 CST,"repuser","",21066,"192.25.10.71:46086",5208d87d.524a,2,"authentication",2013-08-12 20:43:41 CST,2/1198,0,LOG,00000,"replication connection authorized: user=repuser",,,,,,,,,""
2013-08-12 20:43:41.514 CST,"repuser","",21066,"192.25.10.71:46086",5208d87d.524a,3,"startup",2013-08-12 20:43:41 CST,2/0,0,FATAL,53300,"number of requested standby connections exceeds max_wal_senders (currently 0)",,,,,,,,,"walreceiver"
2013-08-12 20:43:41.515 CST,"repuser","",21066,"192.25.10.71:46086",5208d87d.524a,4,"startup",2013-08-12 20:43:41 CST,,0,LOG,00000,"disconnection: session time: 0:00:00.002 user=repuser database= host=192.25.10.71 port=46086",,,,,,,,,"wa
lreceiver"
...


遇到如此问题，则需要检查postgresql.conf文件的max _wal_senders参数了。
3.参数生效
文档上说明需要重启DB使之生效，如果动态使之生效会报错如下：


[postgres@proxy1 ]$ psql
psql (9.2.4)
Type "help" for help.
postgres=# show max_wal_senders ;
 max_wal_senders 
-----------------
 0
(1 row)
postgres=# set max_wal_senders=1;
ERROR: parameter "max_wal_senders" cannot be changed without restarting the server
postgres=#



补充： postgresql基于流复制 (streaming replication)的warm-standby
实例一枚：
Primary:
l 归档设置：


Wal_level=archive
Archive_mode=on
archive_command = 'cp -i %p /data/pgsql/archived_wal/%f'



l 流复制相关设置：


max_wal_senders = '10'  #启动复制进程数量限制，必须大于0
max_replication_slots = '10' #为使用replication slot，必须大于0；replication slot作用是保证wal没有同步到standby之前不能从pg_xlog移走；
wal_keep_segments = '50'    #指定pg_xlog中最少保留的wal数量
 
select pg_create_physical_replication_slot(‘gp1_a_slot'); #创建replication slot
select * from pg_replication_slots;       #查询创建的replication slot



l 编辑pg_hba.conf


# Allow replication connections from localhost, by a user with the replication privilege.
#host replication  postgres  127.0.0.1/32   trust
#host replication  postgres  ::1/128     trust
local replication  postgres        trust
host replication  postgres  192.168.12.0/24   trust



l 联机备份过程（基础备份）


#touch /var/lib/pgsql/backup_in_progress
$psql –c "select pg_start_backup('hot_backup');"
$tar -cf /var/lib/pgsql/backup.tar /var/lib/pgsql/data/
$psql -c "select pg_stop_backup();"
#rm /var/lib/pgsql/backup_in_progress
tar -rf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/ #打包归档



实例：


psql -c "select pg_start_backup('pgbk10');"
tar -zcf pgbk10.tgz data/
psql -c "select pg_stop_backup();"



Standby：
l 编辑recovery.conf


standby_mode = 'on'
primary_conninfo = 'host=192.168.12.38 port=5666 user=postgres'
primary_slot_name='gp1_a_slot'
#restore_command = 'cp /data/pgsql/archived_wal/%f %p'
#archive_cleanup_command = 'pg_archivecleanup /data/pgsql/archived_wal %r'



l 将primary上的基础备份传输到standby上


$scp primary: /var/lib/pgsql/backup.tar .



解压备份到standby上的$PGDATA
l 启动standby


$pg_ctl start –D $PGDATA




启动standby后，postgres开始从primary上接收wal日志进行恢复，并且一直保持恢复状态，psql不能登录；
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。如有错误或未考虑完全的地方，望不吝赐教。