If you know the exact difference between the ways in which data files and log files of a database are accessed, you can understand why you should place data files and log files on separate physical disks for better performance.
The data file of a database, which is normally a file with a
.ndf extension, is used to store the actual data in the database. The data is stored in pages that are 8 KB in size. When particular data is queried by the user, SQL Server reads the required data pages from the disk into memory containing the requested data from the data file. In case SQL Server needs to make any modification in the existing data, it reads the required data pages into the buffer ...