随着3.0.2的发布，将对业务线的数据库版本进行升级
Upgrade Requirements
To upgrade an existing MongoDB deployment to 3.0, you must be running 2.6. If you’re running a version of MongoDB before 2.6, you must upgrade to 2.6 before upgrading to 3.0. See Upgrade MongoDB to 2.6 for the procedure to upgrade from 2.4 to 2.6. Once upgraded to MongoDB 2.6, you cannot downgrade to any version earlier than MongoDB 2.4.
If your existing MongoDB deployment is already running with authentication and authorization, your user data model authSchema must be at least version 3. To verify the version of your existing authSchema, see MongoDB 2.4 User Model Removed. To upgrade your authSchema version, see Upgrade User Authorization Data to 2.6 Format for details.
以上所有的就是要向升级到3.0.必须先升级到2.6大版本，然后再从2.6升级到3.0，一旦你升级到2.6，那么就不能降级到比2.4更低的版本。还需要注意的是，一单升级到3.0版本，就不能降级到低于2.6.5版本以下。
可以参阅http://docs.mongodb.org/manual/release-notes/3.0-compatibility/ 3.0兼容性的变化
单节点的升级：
1.下载3.0版本
2.关闭mongodb，替换2.6版本，用3.0的mongod启动即可
replica sets升级：
1.首先确认集群的所有节点已经是2.6版本。
2.首先对secondary进行升级，用3.0的mongod替代2.6的mongod启动，直接recover到secondary状态算成功。
3.强制failover，用 rs.stepDown()完成，强制将primary切换为secondary，再重复第二步进行升级即可。
参考http://docs.mongodb.org/manual/release-notes/3.0-upgrade/