Operating Systems: Three Easy Pieces Ch. 37

Hard Disk Drives

• the main form of persistent data storage
• The interface
  • Consist of a large number of sectors (512 - bytes block)
  • Multi-sector operation
    • Updating the disk is atomic
    • it will either complete in its entirety or it won’t complete at all
    • torn write
• Basic Geometry
  • Platter
    • Circular hard surface
    • Each Platter has 2 surfaces
  • Spindle
    • Connected to motor
    • RPM (rotations per minute)
      • 7200 ~ 15000 RPM
    • Data is encoded on each surface in concrete circle
      • Track
  • Reading and writing is accomplished by the disk head
    • Disk head is attached to disk arm
• Latency
  • Single-track Latency
    • Rotational Delay
      • it must just wait for the desired sector to rotate under the disk head
  • Multiple Tracks
    • Seek Time
      • Seek: Move disk arm to correct track
      • Acceleration → disk arm gets moving
      • Coasting → arm moving as full speed
      • Deceleration → arm moves slow down
      • Settling → head is carefully positioned
        • settling time is significant
    • Transfer
      • where data is either read from or written to the surface
• Track Skew
  • make sure that sequential reads can be properly serviced even when crossing track boundaries
  • When the disk needs time to reposition head
    • If there is now skew, the head need to wait for next block which requries whole rotation
  • Outer track has more sectors than inner tracks
    • More room geometrically
    • Multi-zoned disk drives
    • Each zone has same number of sectors
• Track buffer
  • Cache
    • Just small amount of the memory
  • Drive has a choice
    • acknowledge
      • After put the data in its memory
        • Write back caching
      • write has actually been written to disk
        • Write through caching
  • Dimensional analysis
    • RPM
• I/O time
  • Workload
    • Random
    • Sequential
  • Capcity and performance
    • High performance for searching
    • Low capacity
    • Low performance on searching
    • High capacity
• Disk Scheduling
  • Due to the high cost of I/O
  • SSTF: Shortest Seek Time First
    • order the queue of I/O request by track
      • drive geometry is not available to the host OS
        • Nearest-block-first
    • Starvation
  • SCAN
    • simply moves back and forth across the disk servicing requests in order across the track
      • Sweep
    • F-SCAN
      • freezes the queue to be serviced when it is doing a sweep
    • C-SCAN
      • Circular SCAN
      • sweeps from outer-to-inner, and then resets at the outer track to begin again
    • Elevator algorithm → going up and down
  • Rotation Cost
  • shortest positioning time first, SPTF, SATF is the solution
• I/O merging
  • Merge the request, reordering is performed upon the merged request
• Work-conversing
  • How long should the system wait before issuing an I/O to disk?
    • immediately issue the request to the drive
  • anticipatory disk scheduling
    • non-work-conserving
    • waiting for a new and better request results better performance in average