SSD provides far greater I/O capacity and far lower I/O latencies than traditional magnetic disk.
The default configuration for Exadata Flash SSD is as in the Exadata Smart Flash Cache. The primary purpose of the Exadata Smart Flash Cache is to accelerate read I/O for database files by configuring Flash as a cache over the grid disks that service data file read I/O. By default the Exadata Smart Flash Cache does not accelerate Smart or full table scans, but you can configure CELL_FLASH_CACHE KEEP to alter this behavior.
Exadata Smart Flash Logging allows the Flash Cache to participate in redo log write operations. This helps alleviate the occasional very high redo write “outliers.”
The Exadata Smart Flash Cache can also operate as a write-back cache, allowing it to satisfy write requests as well as read requests; this can improve performance on systems that are experiencing data file write I/O bottlenecks—as -evidenced by free buffer waits.