disk structure.45
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Disk Structure
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Recap
In the last class, you have learnt:
• Working set model
• Page fault frequency
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Objective
On Completion of this period, you would be able
to know :
• Requirements of secondary memory
• Types of secondary memory
• Disk structure
• Head types
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Secondary Storage Management
Ideal requirement – Programs and data must reside in main memory permanently for process execution but,
Constraints:
• Main memory is a volatile storage
• Too small
• Expensive
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Requirements of Secondary Memory
• Should hold large amounts of data
• Should be nonvolatile
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Types of Secondary Memory
• Magnetic tapes
• Magnetic disks
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Types of secondary memory
• Magnetic Tapes
– Early secondary storage device – Slow, cannot provide random access– Currently used for backup of infrequently used
information
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Types of Secondary Memory
• Magnetic Disks
– Fast and provides random access
– Provides bulk of secondary storage
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Disk Structure
Fig 1 moving head disk mechanism
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Physical Structure
• Flat circular Shape
• Two surfaces are covered with magnetic material
• Information is recorded on both surfaces
• The disk surface is logically divided into tracks
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Operating System View of Disk
• One dimensional array of disk blocks
• Each block is a sector
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Mapping of Disk address
Disk address
Cylinder i
Surface j
Sector k
Block number = k + s ( j + i * t )
Operating System view of Disk
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Working Principle
• The drive motor spins, disk at high speed
• The read write head is positioned just above the
surface of the disk
• Information is stored magnetically on the track
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Head Types
• Fixed-head disk
• Moving-head disk
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Fixed-head Disk
– Separate head for each track
– High speed access
– Large number of Heads
– Very expensive
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Moving-head disk
– Only one head
– Moves in and out to access different tracks
– Less expensive
– Additional hardware required to move the head
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Increasing Storage Capacity
• Improving recording density
– Increasing number of tracks per inch
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Increasing Storage Capacity
• Using two sides of the platter for storing information
– Additional read write head required
– Storage capacity is doubled
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Increasing storage capacity
• Stacking several disks
– Additional read/write heads are required for each disk
– Only one motor drive is sufficient
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Disk Characteristics
• Read, modify and write it back in the same place
• Access any block directly
• Sequential or random access is allowed
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Summary
In this class, you have learnt
•Types of secondary memory
•Disk structure
•Head types
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Frequently Asked Questions
1. Explain Disk structure
2. Write a note on the Secondary storage devices
3. Explain different types of R/W heads
4. Explain the head disk characteristics
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Quiz
• Moving Head is more expensive than fixed head mechanism [T/F]
• Storage capacity can be increased by
•False
Increasing density Using two surfaces of the platter Stacking several disks All the above
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Quiz
• Magnetic tapes provide direct access [T/F]
• The disk has a Flat circular shape [T/F]
False
True
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