1. A method of using a LUN (logical unit number) data type for storage allocation, the method comprising: specifying, by a file system, a LUN data type for a LUN; receiving, by the file system, write data to be written into the LUN, wherein the write data has a LUN data type attribute; comparing, by the file system, the LUN data type attribute of the write data with the LUN data type of the LUN; if the LUN data type attribute of the write data matches the LUN data type of the LUN, then allocating, by the file system, at least some data blocks in the LUN for the write data; and writing, by the file system, the write data into the LUN.