If you wonder how this witchcraft is achieved:
#dvdram disks are "hard sectored" more or less like older style hard disks and floppies were. This essentially turns the DVD-RAM disk into thousands of individually addressable "mini rewritable dvds"
The DVD drive can easily find the correct little bit of the disk to erase and overwrite.
This is how the disk can appear like any normal block device!
A really weird fact: DVD-RAM is predates DVD-RW or DVD+RW!