When a process sends requests to a rdsk type special file, it is talking directly to the driver. A read or a write goes directly to or from device. Reads and writes must be aligned on DEV_BSIZE boundaries or the results are undefined.

When a process sends requests to a dsk type special file, it is talking to high level os routines. A read or write goes to or from the buffer cache. If needed, the os will read new data into the buffer cache. There are no alignment restrictions. You can read or write any collection of bytes anywhere on the device. The os will send aligned requests to the driver, but this is hidden from the process. I/O requests that arise from accesses to a dsk type special file enter the driver via its strategy entry point, not the read and write entry points. The driver may resequence the requests to optimize overall performance.