页面故障发生在尝试访问内存映射文件的部分时，该部分不在内存中。若内存有空闲空间，操作系统会在磁盘上找到页面并直接加载到内存；若无空闲内存，则必须替换现有页面并从磁盘读取请求的页面。这一过程，在活跃系统中可能非常耗时。
What are page faults?
Page faults will occur if you’re attempting to access part of a memory-mapped file that isn’t in memory.
If there is free memory, then the operating system can find the page on disk and load it to memory directly. However, if there is no free memory, the operating system must:
- find a page in memory that is stale or no longer needed, and write the page to disk.
- read the requested page from disk and load it into memory.
This process, particularly on an active system can take a long time, particularly in comparison to reading a page that is already in memory.
参考：http://docs.mongodb.org/manual/faq/storage/#what-are-page-faults
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