单实例，当MongoDB单表数据文件太大导致写入速度变慢
解决办法（待测试）：
文件拆分成小文件。主要参数有
Storage options:
--storageEngine arg what storage engine to use - defaults
to wiredTiger if no data files present
--directoryperdb each database will be stored in a
separate directory
--quota limits each database to a certain
number of files (8 default)
--quotaFiles arg number of files allowed per db, implies
--quota