rs是一个全局的变量，其中那个包含与复制相关的辅助函数，这些函数大多只是数据库命令的包装器，例如
>db.adminCommand({“replSetInitiate”:config})就和
>rs.initiate(config)命令是等价的，但是明显后者更容易操作；多了解一下辅助函数还是比较利于操作的。
执行rs.help()可查看可用的辅助函数，如下：
spock:PRIMARY> rs.help()
rs.status() { replSetGetStatus : 1 } checks repl set status
rs.initiate() { replSetInitiate : null } initiates set with default settings
rs.initiate(cfg) { replSetInitiate : cfg } initiates set with configuration cfg
rs.conf() get the current configuration object from local.system.replset
rs.reconfig(cfg) updates the configuration of a running replica set with cfg (disconnects)
rs.add(hostportstr) add a new member to the set with default attributes (disconnects)
rs.add(membercfgobj) add a new member to the set with extra attributes (disconnects)
rs.addArb(hostportstr) add a new member which is arbiterOnly:true (disconnects)
rs.stepDown([secs]) step down as primary (momentarily) (disconnects)
rs.syncFrom(hostportstr) make a secondary to sync from the given member
rs.freeze(secs) make a node ineligible to become primary for the time specified
rs.remove(hostportstr) remove a host from the replica set (disconnects)
rs.slaveOk() shorthand for db.getMongo().setSlaveOk()
rs.printReplicationInfo() check oplog size and time range
rs.printSlaveReplicationInfo() check replica set members and replication lag
db.isMaster() check who is primary
reconfiguration helpers disconnect from the database so the shell will display an error, even if the command succeeds.
see also http://<mongod_host>:28017/_replSet for additional diagnostic info
