You can improve system performance by placing logs and database objects on separate devices.
Place a table on one hard disk and nonclustered indexes on another to ensure that physical reads and writes are faster, since the work is split between two disk drives.
Split large tables across two disks to improve performance, particularly for multiuser applications.
When log and data share devices, disable user log cache buffering of transaction log records.
Use partitioning to provide multiple insertion points for a heap table, add a degree of parallelism to systems configured to perform parallel query processing, and make it possible to distribute a table’s I/O across multiple database devices.
See Performance and Tuning Series: Physical Database Tuning > Controlling Physical Data Placement for a detailed discussion of how object placement affects performance.