magnetic tapes

8/3/2019 Magnetic Tapes

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Magnetic Tapes



• Magnetic tape used for secondary storage on early computer systems; now used for routine archiving &storing back-up data.

• Records on magnetic tapes are stored serially, one after

other.• Each record can be of any length.

– Length is usually determined by the application program.

• Each record can be identified by its position on the

tape.• To access a single record, tape is mounted & “fast-

forwarded” from its beginning until locate desired pos



• Ninth track holds parity bit used for routine

error checking.

• Number of characters that can be recorded

per inch is determined by density of tape (e.g.,

1600 or 6250 bpi).

• Backup of PC/network servers



Characteristics

• 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 applicationdata; nowadays, it’s mostly used for backups

or archives.



• 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.



CENG 351 6

Reel 1

Reel 2

tape

Read/write head



CENG 351 7

Tape Organization

…

2400’

logical record

BOT

marker

Header block

(describes data blocks)

Data blocks Interblock gap(for acceleration &

deceleration of tape)

EOT

marker



Recording method

• Linear

linear serpentine

•

Scanningtransverse scan

arcuate scan

Helical scan



Linear

Linear serpentine



Helical



Storing Records on Magnetic Tapes

• Can store records individually or grouped into blocks. – If individually, each record is separated by a space to indicate its

starting and ending places.

– If blocks, then entire block is preceded by a space and followedby a space, but individual records are stored sequentially within

block.

• Interrecord gap (IRG) is gap between records about 1/2 inchlong regardless of the sizes of the records it separates.

• Interblock gap (IBG) the gap between blocks of records;still 1/2 inch long.



Data Blocks and Records

• Each data block is a sequence of contiguousrecords.

• A record is the unit of data that a user’s

program deals with.• The tape drive reads an entire block of records

at once.

•Unlike a disk, a tape starts and stops.

• When stopped, the read/write head is over aninterblock gap



Pros & Cons of Blocking

• Fewer I/O operations are needed because asingle READ command can move an entire blockinto main memory.

• Less tape is wasted because size of physicalrecord exceeds size of gap.

• Overhead and software routines are needed forblocking, deblocking, and record keeping.

• Buffer space may be wasted if you need only onelogical record but must read an entire block to getit.



Transfer Rates & Speeds

• Block size set to take advantage of transfer

rate.

• Transfer rate -- density of the tape, multiplied

by the tape transport speed (speed of the

tape)

transfer rate = density * transport speed

.



Magnetic Tape Access Times Vary

Widely

Benchmarks Access time

Maximum access 2.5 minutes

Average access 1.25 minutesSequential access 3 milliseconds

• Variability makes magnetic tape a poormedium for routine secondary storage except

for files with very high sequential activity.



10.5 inch reel of 9 tracktape

Quarter-Inch

cartridges.

magnetic tapes

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