mongodb datafiles size and ｏｃｃｕｐｙ memory up to 99% without stopping

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when I using　MongoDB, I use df -h shell command and it shows the memory is occupied by 99%, I know it is because MongoDB's datafiles. But what I wonder is that why MongoDB server is still running other than stop when occupy memory up to 99%. Thanks in advance for good help.

mongodb datafiles size and ｏｃｃｕｐｙ memory up to 99% without stopping