在Linux平台下，无论是64位或者32位的MongoDB默认最大连接数都是819，WIN平台不知道，估计也没有人在 WIN平台下使用MongoDB做生产环境

[root@DELL113 mongodb-linux-i686-2.4.1]# ./bin/mongo 192.168.6.42
MongoDB shell version: 2.4.1
connecting to: 192.168.6.42/test
> db.serverStatus().connections
{ "current" : 1, "available" : 818, "totalCreated" : NumberLong(1) }
途中available显示818少了一个，表示空闲的。current表示已经占用了的连接数，两数一加就等于819，如果我现在在连接一个，那么available就是817，current就是2

[root@DELL113 mongodb-linux-i686-2.4.1]# ./bin/mongo 192.168.6.42
MongoDB shell version: 2.4.1
connecting to: 192.168.6.42/test
> db.serverStatus().connections
{ "current" : 1, "available" : 818, "totalCreated" : NumberLong(1) }
> db.serverStatus().connections
{ "current" : 2, "available" : 817, "totalCreated" : NumberLong(2) }
819个连接数对于一般的站点我认为已经够用，并且都是现连现取现断。但这个连接数也可以修改，只要在启动的时候加入--maxConns即可

服务器启动

[root@lee mongodb-linux-x86_64-2.4.1]# ./bin/mongod --dbpath=/root/db --maxConns=2000
Wed Apr 3 11:06:21.905 [initandlisten] MongoDB starting : pid=2812 port=27017 dbpath=/root/db 64-bit host=lee
Wed Apr 3 11:06:21.957 [initandlisten] db version v2.4.1
Wed Apr 3 11:06:21.957 [initandlisten] git version: 1560959e9ce11a693be8b4d0d160d633eee75110
Wed Apr 3 11:06:21.957 [initandlisten] build info: Linux ip-10-2-29-40 2.6.21.7-2.ec2.v1.2.fc8xen #1 SMP Fri Nov 20 17:48:28 EST 2009 x86_64 BOOST_LIB_VERSION=1_49
Wed Apr 3 11:06:21.957 [initandlisten] allocator: tcmalloc
Wed Apr 3 11:06:21.957 [initandlisten] options: { dbpath: "/root/db", maxConns: 2000 }
Wed Apr 3 11:06:21.982 [initandlisten] journal dir=/root/db/journal
Wed Apr 3 11:06:21.982 [initandlisten] recover : no journal files present, no recovery needed
Wed Apr 3 11:06:22.297 [initandlisten] preallocateIsFaster=true 2.62
Wed Apr 3 11:06:22.717 [initandlisten] --maxConns too high, can only handle 819
Wed Apr 3 11:06:22.724 [initandlisten] waiting for connections on port 27017
Wed Apr 3 11:06:22.725 [websvr] admin web console waiting for connections on port 28017
Wed Apr 3 11:06:25.126 [initandlisten] connection accepted from 192.168.4.86:53917 #1 (1 connection now open)
查询最大连接数

[root@DELL113 mongodb-linux-i686-2.4.1]# ./bin/mongo 192.168.6.42
MongoDB shell version: 2.4.1
connecting to: 192.168.6.42/test
> db.serverStatus().connections
{ "current" : 1, "available" : 818, "totalCreated" : NumberLong(1) }
> 
发现还是819？其实是Linux默认进程能打开最大文件数有关，可以通过ulimit 解决

[root@lee mongodb-linux-x86_64-2.4.1]# ulimit -n 2500
[root@lee mongodb-linux-x86_64-2.4.1]# ./bin/mongod --dbpath=/root/db --maxConns=2000
Wed Apr 3 11:11:07.013 [initandlisten] MongoDB starting : pid=2930 port=27017 dbpath=/root/db 64-bit host=lee
Wed Apr 3 11:11:07.013 [initandlisten] db version v2.4.1
Wed Apr 3 11:11:07.013 [initandlisten] git version: 1560959e9ce11a693be8b4d0d160d633eee75110
Wed Apr 3 11:11:07.013 [initandlisten] build info: Linux ip-10-2-29-40 2.6.21.7-2.ec2.v1.2.fc8xen #1 SMP Fri Nov 20 17:48:28 EST 2009 x86_64 BOOST_LIB_VERSION=1_49
Wed Apr 3 11:11:07.013 [initandlisten] allocator: tcmalloc
Wed Apr 3 11:11:07.013 [initandlisten] options: { dbpath: "/root/db", maxConns: 2000 }
Wed Apr 3 11:11:07.031 [initandlisten] journal dir=/root/db/journal
Wed Apr 3 11:11:07.031 [initandlisten] recover : no journal files present, no recovery needed
Wed Apr 3 11:11:07.170 [initandlisten] waiting for connections on port 27017
Wed Apr 3 11:11:07.171 [websvr] admin web console waiting for connections on port 28017
Wed Apr 3 11:11:10.076 [initandlisten] connection accepted from 192.168.4.86:53161 #1 (1 connection now open)
再查看最大连接数，搞定

[root@DELL113 mongodb-linux-i686-2.4.1]# ./bin/mongo 192.168.6.42
MongoDB shell version: 2.4.1
connecting to: 192.168.6.42/test
> db.serverStatus().connections
{ "current" : 1, "available" : 1999, "totalCreated" : NumberLong(1) }
> 
关于ulimit的更多知识大家可以去网上检索检索