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Mass Storage Structure
Notice: The slides for this lecture have been largely based on those accompanying the textbook
Operating Systems Concepts with Java, by Silberschatz, Galvin, and Gagne (2003). Many, if not all, of
the illustrations contained in this presentation come from this source.
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CSCI 315 Operating Systems Design
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Last Time: Performance
I/O a major factor in system performance:
– Demands CPU to execute device driver,
kernel I/O code.
– Context switches due to interrupts.
– Data copying.
– Network traffic especially stressful.
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CSCI 315 Operating Systems Design
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Intercomputer Communications
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Improving Performance
• Reduce number of context switches.
• Reduce data copying.
• Reduce interrupts by using large transfers,
smart controllers, polling.
• Use DMA.
• Balance CPU, memory, bus, and I/O
performance for highest throughput.
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CSCI 315 Operating Systems Design
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Device-Functionality
Progression
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Disk Structure
Points to consider:
Sector sizes (number of bits
per sector) should be fixed.
The density of the magnetic
material is constant on the
surface of the disk.
sector
direction of rotation
read/write head
direction of movement
Size of the sector gets
smaller as the radius of the
track gets smaller.
The disk rotates at a
constant speed. To find a
block, the head is moved to
the appropriate track, and
then the correct sector is
found as the disk rotates.
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arm
track
Organization of a disk surface
CSCI 315 Operating Systems Design
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Disk Structure
The disk rotation is given in
rotations per minute (RPM).
The time to find a track is
proportional to the distance
the head must travel.
sector
read/write head
The average time to find a
sector within a track is
roughly half the time for a
full rotation.
direction of movement
Question: If the time to
move from track i to track
(i+1) is given by d,
assuming that the disk head
is at track 0 (all the way
out), could you calculate the
time to get to sector 4 in
track 5?
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direction of rotation
arm
track
Organization of a disk surface
CSCI 315 Operating Systems Design
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Disk Structure
Multi-surface disk
A cylinder is the collection of all
the same tracks across all the
multiple disk surfaces.
There is a time associated with
turning heads on and off so that a
different surface can be accessed.
We call this overhead the headswitching time.
direction of rotation
read/write heads
cylinder
arm
direction of movement
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The time to move the arm to read
another cylinder is due to the
mechanics of the arm. It is
certainly much large than the
head-switching time, which is due
to electronics only.
Question: How should one
organize data across multiple
surfaces to minimize access
overhead?
CSCI 315 Operating Systems Design
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Disk Scheduling
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