DBMS_Resource_Manager.Calibrate_IO Usage
It really is to be the number of physical disks, in the old fashioned style, not using a SAN. The best article about this I found here http://blog.bensmann.com/testing-oracle-io. Most others blogs/docs I found were more like copies from manuals than original contents that explains something new. When using this on a SAN, the cache is going to play a big role, as is the inner working of those beasts. For example, they write in a buffer and give an acknowledgement to the rdbms that the write is complete, even when the data did not even come close to the intended disk. When continuously other blocks are written, that cache could be choked and then we get a better view of the real io performance. Reading and updating blocks from a regular sized datafile won't be sufficient to reach that.