Csci 2111: Data and File Structures Week2, Lecture 1

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Csci 2111 Data and File StructuresWeek2,
Lecture 1 2 (Contd)

Secondary Storage and System Software Magnetic
Disks Tapes
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Part II Tape Outline

• Description of Tape Systems
• Organization of Data on Nine-Track Tapes
• Estimating Tape Length Requirements
• Estimating Data Transmission Times
• Disk versus Tape

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Description of Tape Systems

• No direct accessing facility, but very rapid
  sequential access.
• Compactness, resistance to rough environmental
  conditions, easy to store and transport, cheaper
  than disk
• Used to be used for application data
• Currently, tapes are primarily used as archival
  storage.

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Organization of Data on Nine-Track Tapes I

• On a tape, the logical position of a byte within
  a file corresponds directly to its physical
  position relative to the start of the file.
• The surface of a typical tape can be seen as a
  set of parallel tracks each of which is a
  sequence of bits. These bits correspond to 1 byte
  a parity bit.
• One Byte a one-bit-wide slice of tape called a
  frame.

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Organization of Data on Nine-Track Tapes II

• In odd parity, the bit is set to make the number
  of bits in the frame odd. This is done to check
  the validity of the data.
• Frames are organized into data blocks of variable
  size separated by interblock gaps (long enough to
  permit stopping and starting)

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Estimating Tape Length Requirements I

• Let b the physical length of a data block
• Let g the length of an interblock gap, and
• Let n the number of data blocks.
• The space requirement, s, for storing the file is
  s n (bg)
• b blocksize (i.e., bytes per block)/ tape
  density (i.e., bytes per inch)

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Estimating Tape Length Requirements II

• The number of records stored in a physical block
  is called the blocking factor.
• Effective Record Density a general measure of
  the effect of choosing different block sizes
  (number of bytes per block)/ (number of inches
  required to store a block)
• gt Space utilization is sensitive to the
  relative sizes of data blocks and interblock gaps.

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Estimating Data Transmission Times

• Normal Data Transmission Rate (Tape Density
  (bpi)) (Tape Speed (ips))
• Interblock gaps, however, must be taken into
  consideration gt Effective Transmission Rate/
  ((Effective Recording Density) (Tape Speed))

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Disk versus Tape

• In the past Both Disks and Tapes were used for
  secondary storage. Disks were preferred for
  random access and tape was better for sequential
  access.
• Now (1) Disks have taken over much of secondary
  storage gt Because of the decreased cost of disk
  memory storage
• Now (2) Tapes are used as Tertiary storage
  (Cheap, fast easy to stream large files or sets
  of files between tape and disk)