Hard Drive Hardware

Addressing and 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(torn write)
  

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 multiple, double-sided platters

각속도: 각이 같으면 같은 속도
선속도: 각속도가 같으면 바깥쪽으로 갈수록 선속도가 빨라짐
seek -> arm으로 원하는 track을 찾는다.

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Common Disk Interfaces

• ST-506 -> ATA -> IDE -> SATA
• Recent versions support Logical Block Addresses (LBA)

Small(=server) Computer System Interface

• Packet based, like TCP/IP
• Device translates LBA to internal format
• Transport independent

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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

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How To Calculate Transfer Time

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Sequential vs. Random Access

Caching

• Many disks incorporate caches (track buffer)
  • Small amount of RAM (8,16, 32 MB)
• Read caching
  • Reduces read delays due to seeking and rotation
• Write caching
  • Write back cache: drive reports that writes are complete after they have been cached
• Write through cache: drive reports that writes are complete after they have been written to disk
• Today, some disks include flash memory for persistent caching (hybrid drives)

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Disk Scheduling

First come, first serve (FCFS)

• Most basic scheduler, serve requests in order
  

Shortest seek time first (SSTF)

• minimize seek time by always selecting the block with the shortest seek time
• pros
  • SSTF is optimal, and it can be easily implemented
• cons
  • SSTF is prone to starvation

SCAN, otherwise known as the elevator algorithm

• Head sweeps across the disk servicing requests
  in order
  

C-SCAN

• Like SCAN, but only service requests in one direction
  

C-LOOK

• Peek at the upcoming addresses in the queue
• Head only goes as far as the last request
  

Implementing Disk Scheduling

Where

• OS scheduling
  • OS can implement SSTF or LOOK by ordering the queue by LBA
  • However, the OS cannot account for rotation delay
• On-disk scheduling
  • Disk knows the exact position of the head and platters
  • Can implement more advanced schedulers (SPTF)
  • But, requires specialized hardware and drivers

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Command Queuing

• Feature where a disk stores of queue of pending read/write requests
• Disk may reorder items in the queue to improve performance
  • E.g. batch operations to close sectors/tracks
• Supported by SCSI and modern SATA drives
• Tagged command queuing: allows the host to place constraints on command re-ordering