Magnetic Disks (Hard Drives)
Optional readings for this topic from Operating Systems: Principles and Practice: Section 12.1.
- 1-5 platters, magnetically coated on each surface
- Platters spin continuously at 5000-15000 RPM
- Actuator arm positions heads, which can read and write data on the magnetic surfaces.
- Overall size of disk package: 1-8 inches.
Organization of disk data:
- Circular tracks corresponding to a particular position of the actuator arm.
- Typical density today: 200,000 tracks per radial inch.
- Tracks divided into 4096-byte sectors. Typical tracks contain a few thousand sectors.
- Typical total drive capacities: 100GB-2TB
- 100GB ~ 50M double-spaced pages of text.
- Disk technology has been one of the most rapidly advancing technologies: until recently, capacities have increased faster than Moore's Law.
Reading and writing:
- Seek: move actuator arm to position heads over desired track. Typical seek time: 2-10ms.
- Select a particular head.
- Rotational latency: wait for desired sector to pass under the head. One-half disk rotation on average (4ms @ 7500RPM)
- Transfer: read or write one or more sectors as they pass under the head. Typical transfer rates: 100-150 MBytes/sec.
- Latency refers to the sum of seek time plus rotational latency; typically 5-10ms.
API for disks:
read(startSector, sectorCount, physAddr) write(startSector, sectorCount, physAddr)
In the old days the track and surface structure of the disk was visible to software:
read(track, surface, sector, sectorCount, physAddr)
Nowadays the track structure is hidden inside the disk:
- Inner tracks have fewer sectors than outer tracks
- If some sectors are bad, disk software automatically remaps them to spare sectors.
Communicating with I/O Devices
- Each device appears in the physical address space of the machine as a few words, called device registers.
- The operating system reads and writes device registers to control the device.
- Bits in device registers serve 3 purposes:
- Parameters provided by CPU to device (e.g. number of first sector to read)
- Status bits provided by device to CPU (e.g. "operation complete" or "error occurred").
- Control bits set by CPU (e.g. "start disk read") to initiate operations.
- Device registers don't behave like ordinary memory
- "Start operation" bit may always read as 0.
- Bits may change without being written by CPU (e.g. "operation complete" bit).
Typical I/O operation:
- CPU writes device registers to start operation (e.g., read)
- Once operation has started,
readybit reads as zero
- When operation is finished, device sets
- One approach to I/O is to poll the
readybit (but then CPU wastes time until the I/O finishes)
- Most devices support DMA (Direct Memory Access):
- Device can transfer data to and from physical memory, without help from the CPU.
- CPU loads buffer address into a device register before starting operation (e.g., where to copy data read from disk).
- Device moves data directly to/from physical memory.
Interrupts: allow CPU to do other work while devices are operating.
- "Interrupt Enable" bit in device register
- CPU starts I/O operation, sets IE bit, then works on other things.
- When device needs attention (operation completes) it
interrupts the CPU:
- A forced procedure call to a particular address in the kernel.
- Operating system figures out which device interrupted,
services that device.
- Either start a new operation or turn off the IE bit.
- Operating system returns from the interrupt back to whatever it was working on.
- Interrupts make the operating system much more efficient; for example, can keep many devices busy at the same time, while also running user code.
- Multi-core machines spread the interrupts around to balance loads.