Lec 6-Storage Devices (1)

Hard Disk Drive

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Geometry

• Hard drives expose a large number of sectors(blocks)
  • Typically 512 or 4096 bytes(=page size -> easy to swap)
  • Individual sector writes are atomic
  • Multiple sectors writes may be interrupted
• Drive geometry
  • Sectors arranged into tracks
  • Cylinder is a particular track on multiple platters
  • Tracks arranged in concentric circles on platters
  • A disk may have double-sided platters
    

LBA: Logical Block Addresses

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Types of Delay with Disks

Three types of delay

1. Rotational delay
   • Time to rotate the desired sector to the read head
   • Related to RPM
2. Seek delay
   • Time to move the read head to a different track
3. Transfer time
   • Time to read or write bytes

• Track skew
  • offset sectors so that sequential reads across tracks incorporate seek delay
    = Consider movement of read head to a different track

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Caculating Transfer time

• $T_{I/O}=T_{seek} + T_{rotation} + T_{transfer}$

• Assume transffering 4096 bytes
• Cheetah
  • $T_{rotation} = {{60s/M}\over{15000RPM}}\times 1000ms/s\times {{1}\over{2}}=2ms$
  • $T_{transfer}={{4096B}\over{125MB/s}}\times 1000ms/s \approx0.03ms$
  • $4ms+2ms+0.03ms \approx 6ms$
• Barracuda
  • $T_{rotation} = {{60s/M}\over{7200RPM}}\times 1000ms/s\times {{1}\over{2}}\approx4.17ms$
  • $T_{transfer}={{4096B}\over{105MB/s}}\times 1000ms/s \approx0.03ms$
  • $9ms+4.17ms+0.03ms \approx 13.2ms$

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Sequential vs Random Access

• Sequential Access
  • Caculate rotational and seek time only once

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Caching

• Many disks incorporate caches
• Read caching
  • Reduces read delays
• Write caching
  • Write back cache
    • Write on only cache
    • Possibly dangerous (Volatile -> power off -> cache doesn't exist)
  • Write through cache
    • Write on disk too
• Some disks include flash memory for persistent caching

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Disk Scheduling

• Caching can't make up for poor random access time
• A queue of requests to the disk, they can be reordered to improve performance

FCFS: First come, first serve

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SSTF: Shortest seek time first

• Minimize seek time by always selecting the block with the shortest seek time
• Optimal
• Prone to starvation

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SCAN (Elevator)

• Sweeps across the disk servicing requests in order
• No starvation
• Average access times are less for requests at high and low addresses

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C-SCAN

• Service requests in one direction
• Fairer than SCAN
• Worse performance than SCAN

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C-LOOK

• Only goes as far as the last request (not until the end)

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Implementing Disk Scheduling

• Where should be implemented?
  • OS Scheduling
    • OS cannot account for rotation delay
      Do not know where the head is (mechanically)
  • On-disk Scheduling
    • Disk knows the exact position of the head and platters
    • But requires specialized hardware and drivers

Command Queuing

• Feature where a disk stores of queue of pending read / write requests
  • Called NCQ(Native Command Queuing) in SATA
• Disk may reorder items in the queue
• Tagged command queuing
  • Allow the host to place constraints on command re-ordering