Storage test: Study how the memory was stored either in the residentmemory area or on the disk.
存储测试:研究物理内存和虚拟内存的存储情况。
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From the description, it seems like you should be able to monitor the residentmemory used by the mongo db process to get an alert as shard memory ran low. Is that viable?
Accessing a memory address that has been swapped is a lot slower than reading a resident (in physical memory) address because it must be loaded from the hard drive.