So, what is a raw device? Well, a raw device (or raw partition) is a disk partition that isn't mounted or written to via the UNIX filesystem, instead it is accessed via a "character-special" device driver.

This means that it is up to any application using the raw device to determine how the data is written since there is no filesystem to do this on the application's behalf.

The performance benefit from using raw devices exists because a write to a raw device bypasses the UNIX buffer cache. Additionally there is no inode allocation and maintenance or free block allocation and maintenance to serve as an overhead to the I/O, instead the data is transferred directly from the Oracle buffer cache to the disk.

The trade-off is that raw devices are very useful for write operations, but, having no block definition are not so useful for read operations. Outside of any write dominant datafiles, Redo logs are especially suitable as they are write-intensive and they are written to sequentially.