How can I clear the mongoDB oplog?

Is it normal that MongoDB resident memory keeps growing?

• I have one MongoDB(version 2.0.2) instance in production environment whose resident memory keeps growing and, in about 3 months, consumes up 24G physical RAM. The memory usage stats is as below: Currently, the MongoDB instance runs well and there is no latency spike is observed.  I'm wondering whether this is normal for MongoDB. Does MongoDB just try to consume all RAM to be more efficient? Is there way to limit RAM usage of MongoDB instance?

• Answer:

  Does MongoDB just try to consume all RAM to be mos efficient? Most databases just try to consume all available RAM. However, in MongoDB's cases, it is using Memory-Mapped Files. So MongoDB effectively pretends that its entire data set is in RAM and it allows the operating system to "go to disk" when necessary. From a performance stand-point, MongoDB will generally start to get worse as more data lives outside of RAM, especially if that data is being accessed. Generally speaking, when the indexes exceed RAM the speed tends to drop off dramatically. So you will want to keep an eye on your index and data sizes. Is there way to limit RAM usage of MongoDB instance? Yes, this is theoretically possible. But you generally don't want to do that. You typically want MongoDB running on its own machine and basically owning all resources on that machine. If this is not the case, you'll want to consider the type of growth you are expecting, how you plan to clean up old data and how you plan to grow that system.