Disk Controller Overhead
Disk controller contains a microprocessor + buffer memory + possibly a cache (for disk sectors)
Overhead to perform an access (of the order of 1 ms)
- Receiving orders from CPU and interpreting them
- Managing the transfer between disk and memory (e.g., managing the DMA)
Transfer rate between disk and controller is smaller than between disk and memory, hence:
- Need for a buffer in controller
- This buffer might take the form of a cache (mostly for read-ahead)