Question about GNOME Disk Utility in Ubuntu 12.04

[open.source]

Does the included Read-Only benchmark write stuff to the disk.

I know that it says Read-Only, but how can it test read speeds and access times without writing stuff, does it use files already on the disk or does it read empty blocks?

I'm just very curious about this.

[2F4U]

Why would a read-only benchmark need to write anything to the disk? As the name says, it is testing read performance is therefore reading existing bits and bytes from the hdd.