Emery Berger

1
Operating SystemsCMPSCI 377Lecture 18 Storage
Systems

• Emery Berger
• University of Massachusetts, Amherst

2
Last TimeImproving I/O Performance

• Approaches
• Caching reduces data transfer
• Large data transfers
• DMA

3
Today

• Improving performance of storage systems (i.e.,
  the disk)
• Scheduling
• Interleaving
• Read-ahead
• Tertiary storage

4
Disk Operations

• Disks are SLOW!
• 1 disk seek 40,000,000 cycles
• Latency
• Seek position head over track/cylinder
• Rotational delay time for sector to rotate
  underneath head
• Bandwidth
• Transfer time move bytes from disk to memory

5
Calculating Disk Operations Time

• Read/write n bytes
• Cant reduce transfer time
• Can we reduce latency?

I/O time seek rotational delay transfer(n)
6
Reducing Latency

• Minimize seek time rotational latency
• Smaller disks
• Faster disks
• Sector size tradeoff
• Scheduling
• Layout

7
Disk Head Scheduling

• Change order of disk accesses
• Reduce length number of seeks
• Algorithms
• FCFS
• SSTF
• SCAN, C-SCAN
• LOOK, C-LOOK

8
FCFS
I/O time seek rotational delay transfer(n)

• First-come, first-serve
• Example disk tracks (65, 40, 18, 78)
• assume head at track 50
• Total seek time?
• 15252260 122

65
18
78
40
disk
9
SSTF
I/O time seek rotational delay transfer(n)

• Shortest-seek-time first (like SJF)
• Example disk tracks (65, 40, 18, 78)
• assume head at track 50
• Total seek time?
• 10224713 92
• Is this optimal?

65
18
78
40
disk
10
SCAN
I/O time seek rotational delay transfer(n)

• Always move back and forth across diska.k.a.
  elevator
• Example disk tracks (65, 40, 18, 78)
• assume head at track 50, moving forwards
• Total seek time?
• 1513226022132
• When is this good?

65
18
78
40
disk
11
C-SCAN
I/O time seek rotational delay transfer(n)

• Circular SCANGo back to start of disk after
  reaching end
• Example disk tracks (65, 40, 18, 78)
• assume head at track 50, moving forwards
• Total seek time?
• 1513221001840208
• Can be expensive, but more fair

65
18
78
40
disk
12
LOOK variants

• Instead of going to end of disk,go to last
  request in each direction
• SCAN, C-SCAN ) LOOK, C-LOOK

13
Exercises

• 5000 cylinders, currently at 143, moving
  forwards(86, 1470, 913, 1774, 948, 1509, 1022,
  1750, 130 )
• FCFS
• SSTF
• SCAN
• LOOK
• C-SCAN
• C-LOOK
• Find sequence total seek time
• First 0, last 4999

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Solutions

• FCFS
• 143, 86, 1470, 913, 1774, 948, 1509, 1022, 1750,
  130 distance 7,081  
• SSTF
• 143, 130, 86, 913, 948, 1022, 1470, 1509, 1750,
  1774 distance 1,745 
• SCAN
• 143, 913, 948, 1022, 1470, 1509, 1750, 1774,
  4999, 130, 86 distance 9,769 
• LOOK
• 143, 913, 948, 1022, 1470, 1509, 1750, 1774, 130,
  86 distance 3,319 
• C-SCAN
• 143, 913, 948, 1022, 1470, 1509, 1750, 1774,
  4999, 0, 86, 130 distance 9,813
• C-LOOK
• 143, 913, 948, 1022, 1470, 1509, 1750, 1774, 86,
  130 distance 3,363

15
Today

• Improving performance of storage systems (i.e.,
  the disk)
• Scheduling
• Interleaving
• Read-ahead
• Tertiary storage

16
Disk Interleaving

• Contiguous allocation
• Requires OS response before disk spins past next
  block
• Instead of physically contiguous
  allocation,interleave blocks
• Relative to OS speed, rotational speed

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Read-Ahead

• Reduce seeks by reading blocks from disk ahead of
  users request
• Place in buffer on disk controller
• Similar to pre-paging
• Easier to predict future accesses on disk?

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Tertiary Storage

• a.k.a. long-term storage
• disks secondary storage
• used for archival data or backups
• Examples
• tape drives
• CD-R, DVD-R/W
• Performance still somewhat important
• not much OS can do

19
Summary

• OS can improve storage system performance
• Scheduling
• Interleaving
• Read-ahead