可以参考之前写的这篇博客：
Changed in version 3.0.0.
Only for MMAPv1 Storage Engine. The total amount of data mapped in megabytes. This is the total data size at the time of the last mongostat call.
The amount of virtual memory in megabytes used by the process at the time of the last mongostatcall.
原文：http://www.cnblogs.com/xiaoit/p/4522718.html