ZFS uses the ZFS intent log (also referred to as a ZIL) to store synchronous writes. This has the advantage that a full transaction group doesn’t need to be written when a synchronous write occurs, and maximizes the use of I/O bandwidth. For some applications (databases come to mind), placing the ZIL on a dedicated device can be extremely helpful. But how can you tell if placing the intent log on a separate device would be useful? To answer this question, you can run Richard Elling’s super handy zilstat DTrace script. As SSD becomes cheaper and cheaper, this script will be a useful tool for folks who want to applications and database performance.