Storage Devices Summer 2016 Cornell University Today Devices that - - PowerPoint PPT Presentation

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Storage Devices Summer 2016 Cornell University Today Devices that - - PowerPoint PPT Presentation

Feb 03, 2023 959 likes •1.16k views

CS 4410 Operating Systems Storage Devices Summer 2016 Cornell University Today Devices that can persistently store data. Magnetic disk Flash storage 2 Magnetic Disks: Then 3 Magnetic Disks: Now 4 Magnetic Disk: Internal 5

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SLIDE 1

CS 4410 Operating Systems

Storage Devices

Summer 2016 Cornell University

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Today

  • Devices that can persistently store data.
  • Magnetic disk
  • Flash storage

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SLIDE 3

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Magnetic Disks: Then

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Magnetic Disks: Now

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Magnetic Disk: Internal

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Disk Speed

  • To read from disk, we must specify:

– cylinder number, track number, sector number.

  • disk access time = seek time + rotation time + transfer time

– seek time: the time to move the disk arm to the desired track – rotation time: the time for the desired sector to rotate to the disk head. – transfer time: the time for the desired data to flow between the drive and the computer.

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Disk Scheduling

  • Disk access involves moving mechanical parts,

such as arm and platter.

  • So, disk access is expensive (milliseconds),

– comparing to memory access (nanoseconds).

  • The disk should be used efficiently.

– as few movements as possible when serving a sequence of requests.

  • Goal : small access time and large bandwidth.
  • The OS may employ a disk scheduling algorithm

to achieve the above target.

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SLIDE 8

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FCFS

  • Consider a disk queue with requests for I/O to blocks on cylinders:

–

98, 183, 37, 122, 14, 124, 65, 67

  • The disk head is initially at cylinder 53.
  • Total head movement of 640 cylinders
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SLIDE 9

SSTF

  • Selects request with minimum seek time from current head

position.

  • Total head movement of 236 cylinders.

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SCAN

  • The disk arm starts at one end of the disk.
  • Moves toward the other end, servicing requests.
  • Head movement is reversed when it gets to the other end of disk.

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SLIDE 11

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C-SCAN

  • The head moves from one end of the disk to the other.
  • Servicing requests as it goes.
  • When it reaches the other end it immediately returns to the beginning of the disk.
  • Provides a more uniform wait time than SCAN.
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SLIDE 12

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C-LOOK

  • Arm only goes as far as last request in each direction.
  • Then reverses direction immediately.
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SLIDE 13

Flash storage (memory)

  • Solid state storage.
  • No moving parts.
  • Better random IO performance than disks.
  • Less power consumption.
  • Less vulnerable to physical damage.
  • Used in:

– thumb drives (USBs), – smart phones, – machine room servers, – laptop computers …

  • But, more expensive (per byte) than disks.

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SLIDE 14

Flash storage

  • NOR flash storage allows individual words

(e.g., 4 bytes) to be written and read.

– Used for storing device firmware, since it can be executed in place.

  • NAND flash storage allows reads and writes of

a page (2-4 KB).

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Accessing a NAND flash storage

  • Erase blocks

– Before writing, flash memory should be erased in blocks. – Slow process

  • Write page
  • Read page

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SLIDE 16

Today

  • Devices that can persistently store data.
  • Disk

– Scheduling requests

  • Flash storage

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SLIDE 17

Coming up…

  • Next lecture: RAID
  • HW4:

– ex 1, 2, 3, 4 – Due on Tuesday

  • Next exam on Thursday

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