The document discusses the components of accessing a sector in a disk, which include seek time, rotational delay, and transfer time. It details how seek time varies based on disk size and specifications, while rotational delay depends on the disk's rotation speed, and transfer time depends on the number of sectors. Additionally, it covers different types of storage media such as optical disks and magnetic tape, highlighting their characteristics and usage.

1. Lecture 8
External Memory
1

2.  The time to access a sector in a track on a surface is divided
into 3 components:
Time Component Action
Seek Time Time to move the read/write arm to
the correct cylinder
Rotational delay (or
latency)
Time it takes for the disk to rotate so
that the desired sector is under the
read/write head
Transfer time Once the read/write head is
positioned over the data, this is the
time it takes for transferring data

4.  Seek time is the time required to move the arm to the
correct cylinder.
 Largest in cost.
 Difficult quantity to pin down
 Startup time
 Time taken to traverse the tracks
 Settling time (positioning head over track until track
identification)
 The smaller the disk the lesser the seek time
 31/2 inch (8.9 cm) in diameter common size

5.  It is usually impossible to know exactly how
many tracks will be traversed in every seek
 Manufacturer’s specifications for disk drives
often list this figure as the average seek time for
the drives.
 Most hard disks today have s under 10 ms, and
high-performance disks have s as low as 7.5 ms.

6.  Seek time depends on the speed with which the head
rack moves, and the number of tracks that the head
must move across to reach its target.
 Given the following (which are constant for a
particular disk):
 Hs = the time for the I/ O head to start moving
 Ht = the time for the I/ O head to move from one track to
the next
 Then the time for the head to move n tracks is:
 Seek(n)= Hs+ Ht*n

7.  The rotational delay is the time required for the addressed area of the
disk to rotate into a position where it is accessible by the read/write
head.
 Maximum rotational delay is the time it takes to do a full rotation (as the
relevant part of the disk may have just passed the head when the request
arrived).
 Most rotating storage devices rotate at a constant angular rate (constant
number of revolutions per second).
 The maximum rotational delay is simply the reciprocal of the rotational
speed

8.  Latency is the time needed for the disk to rotate so the sector
wanted is under the read/write head.
 Hard disks usually rotate at about 5000-7000 rpm,
 12-9 msec per revolution.
 Note:
 Min latency = 0
 Max latency = Time for one disk revolution
 Average latency (r) = (min + max) / 2
= max / 2
= time for ½ disk revolution
 Typically 6 – 4.5 ms, at average

9.  Given the following:
 R = the rotational speed of the spindle (in rotations
per second)
  = the number of radians through which the track
must rotate
 then the rotational latency  radians is:
 Latency= (/2)*(1000/R), in ms

10.  The transfer time is given by the formula:
number of sectors
Transfer time = --------------------------------------- x rotation time
track capacity in number of sectors
 e.g. if there are St sectors per track, the time to
transfer one sector would be 1/ St of a revolution.

11.  The transfer time depends only on the speed at which
the spindle rotates, and the number of sectors that
must be read.
 Given:
 St = the total number of sectors per track
 the transfer time for n contiguous sectors on the same track
is:
 Transfer Time =(n/St)*(1000/R), in ms

13.  Optical storage
 CD-ROM
 CD-Recordable (CD-R)
 CD-R/W
 DVD

14.  Compact disk originally for audio, but now used as computer storage
device.
 Disk formed from polycarbonate
 Digital information is imprinted as a series of microscopic pits on the
surface.
 Done by high-intensity laser to create a master disk.
 Master used to create die to stamp out copies.
 Then coated with highly reflective coat, usually aluminium or gold.
 Clear acrylic – top coat to protect against dust and scratches.
 Finally label on acrylic.

15.  Read by low –powered laser reflected through
clear polycarbonate.
 Intensity is different for pits and lands
 detected and converted into digital signal.
 Pits rough – low intensity
 Lands smooth – high intensity

17.  Contains single spiral track, from near centre to
outer edge of the disk.
 Same length sectors
 Constant packing density

18.  for
 Easy to mass produce inexpensively
 Removable
 against
 Expensive for small runs
 Slower than magnetic disk
 Read only

19.  CD-Recordable (CD-R)
 Write Once Read Many (WORM)
 Medium includes dye layer.
 Dye used to change reflectivity and is activated by a
high-intensity laser.
 Compatible with CD-ROM drives or CD drives
 CD-RW
 Erasable
 Mostly CD-ROM drive compatible
 Phase change
 Disk uses material with two different reflectivities in
different phase states
 A beam of laser light can change the material from one phase to another

20.  Digital Versatile Disk
 Will read computer disks and play video disks
 Multi-layer
 Very high capacity

22.  Magnetic Tape

23.  No direct access, but very fast sequential access.
 Resistant to different environmental conditions.
 Easy to transport, store, cheaper than disk.
 Before it was widely used to store application
data; nowadays, it’s mostly used for backups or
archives.

24.  A sequence of bits are stored on magnetic tape.
 For storage, the tape is wound on a reel.
 To access the data, the tape is unwound from one
reel to another.
 As the tape passes the head, bits of data are read
from or written onto the tape.

25. Reel 1 Reel 2
tape
Read/write head

26.  Typically data on tape is stored in 9 separate bit
streams, or tracks.
 Each track is a sequence of bits.
 Recording density = # of bits per inch (bpi).
Typically 800 or 1600 bpi.
30000 bpi on some recent devices.

27. ½”
0
1
1
0
1
1
0
1
0
0
1
1
0
1
1
0
1
0
0
1
1
0
1
1
0
1
0
0
1
1
0
1
1
0
1
0
…
… …
… …
…
parity bit
8 bits = 1 byte

28. …
2400’
logical record
BOT
marker
Header block
(describes data blocks)
Data blocks Interblock gap
(for acceleration &
deceleration of tape)
EOT
marker