Secondary Storage
Memory
Rashedul Hasan
Secondary Storage
Necessity of Secondary Storage
• Primary storage has the following limitations,
Limited Capacity: the storage capacity of primary
storage of today’s computer is not sufficient to
store large volume of data.
Volatile: the data stored in it is lost when the
power is turned off or interrupted.
In order to overcome the above mentioned
limitations we need auxiliary memory.
Secondary Storage
Secondary storage devices permanently
store hold data and Information as well as
programs.
It is non-volatile.
Generally has an operating speed far slower
than that of Primary storage.
Direct access devices
Sequential access devices
[Magnetic Tape]
Magnetic Disks
Floppy
Disk
Hard Disk
Optical Disk
CDROM
CD-R
Flash drive or Pen
Drive
Memory Storage
Device
CD-RW
DVD
Memory
Card
(SD/MMC)
Storage
Magnetic Tape
• Magnetic tape is a one-half inch or one-quarter
inch ribbon of plastic material on which data is
recorded. The tape drive is an input/output device
that reads, writes and erases data on tapes.
Magnetic tapes are erasable, reusable and durable.
They are made to store large quantities of data
inexpensively and therefore are often used for
backup. Magnetic tape is not suitable for data files
that are revised or updated often because it stores
data sequentially.
Magnetic Disk.
• Magnetic disks are the most widely used
storage medium for computers. A magnetic
disk offers high storage capacity, reliability,
and the capacity to directly access stored
data. Magnetic disks hold more data in a
small place and attain faster data access
speeds. Types of magnetic disks mainly
include, floppy disk and hard disks.
Floppy Disk
• The Floppy was introduced in the early
1970s by IBM as a new type of secondary
storage. Originally they were eight inches in
diameter and were thin and flexible which
gave them the name floppy disks, or
floppies. It was used as the principle
medium of secondary storage for personal
computers. They are available in two
different sizes: 3 1/2 inch and 5 1/4 inch.
Hard Disks
• Hard disks provide larger and faster
secondary storage capabilities than
diskettes. Usually hard disks are
permanently mounted inside the computer.
Most personal computers have at least one
hard disk. The input/output device that
transfers data to and from the hard disk is
the hard disk drive.
A Western Digital 3.5 inch 250 GB
SATA HDD.
A hard disk drive with the metal
cover removed
Hard Disk
Hard Disk
Sector, Track & R/W Head
• Tracks: Data are recorded in concentric
circles called Track.
• Sectors: Sections used for storage reference
purpose.
• Read/Write Head: is used to transfer
between the computer and the Disk.
Close-up of a hard disk head
An IBM HDD head resting on a disk
platter.
Characteristics
• Data transfer rate: As of 2008, a typical 7200 rpm
desktop hard drive has a data transfer rate of about 70
megabytes per second. This rate depends on the track
location, so it will be highest for data on the outer tracks
(where there are more data sectors) and lower toward the
inner tracks (where there are fewer data sectors); and is
generally somewhat higher for 10,000rpm drives. A current
widely-used standard for the data transfer rate is 3.0 Gbit/s
SATA, which can send about 300 megabyte/s. from the
buffer to the computer, and it is rapidly increasing
everyday.
• RPM = Rotation per Minute.
Characteristics
• Seek time currently ranges from just under
2 ms for high-end server drives, to 15 ms
for miniature drives, with the most common
desktop type typically being around 9 ms.
There has not been any significant
improvement in this speed for some years.
• Ms = Millisecond.
Characteristics
• Shock resistance is especially important for
mobile devices. Some laptops now include a
motion sensor that parks the disk heads if
the machine is dropped,
Characteristics
• Latency : also known as rotational delay
time. The time it takes to access a particular
location in storage from the initial position.
The lower the latency, the faster the hard
disk drive.
Optical Disk
• Optical storage is referring to the storage
of data on an optically readable medium.
Data is recorded by making marks in a
pattern that can be read back with the aid of
light.
• A common modern technique used by
computers involves a tiny beam of laser
light precisely focused on a spinning disc.
Optical Disk
• Unlike magnetic disks having several
concentric tracks, an optical disk has one
long track starting at the outer edge and
spiraling inwards to the centre.
CD-ROM, CD-R, CD-RW
• CD-ROM- Compact Disk Read Only
Memory.
• CD-R- Compact Disk Recordable. [Write
once read more.]
• CD-RW- Compact Disk Read Write.[ For
Re-Writing many times]
• CD, CD-ROM: Read only storage, used for mass
distribution of digital information (music, video,
computer programs)
• CD-R: Write once storage, used for tertiary and
off-line storage
• CD-RW: Slow write, fast read storage, used for
tertiary and off-line storage
• Ultra Density Optical or UDO is similar in
capacity to BD-R or BD-RE and is slow write, fast
read storage used for tertiary and off-line storage.
Blu-ray
• Blu-ray is a newer optical disc storage media format. Its
main uses are high-definition video and data storage. The
disc has the same dimensions as a CD or DVD. The term
"Blu-ray" comes from the blue laser used to read and write
to the disc. The Blu-ray discs can store much more data
then CDs or DVDs. A dual layer Blu-ray disc can store up
to 50GB, almost six times the capacity of a dual layer
DVD (WOW!). Blu-ray discs have similar devices used to
read them and write to them as CDs have. A BD-ROM
drive can only read a Blu-ray disc and a BD writer can
read and write a Blu-ray disc.
An old 4x CD-ROM Drive.
Characteristics
• Capacity
• A standard 120 mm, "700 MB" CD-ROM
can hold about 847 MB of data.
• In comparison, a single-layer DVD-ROM
can hold 4.7 GB of error-protected data,
more than 6 CD-ROMs.
• Transfer rates :
As of 2004, the fastest transfer rate
commonly available is about 52x or 10,350
rpm and 7.62 megabytes per second.
[1x or 1-speed which gives a data transfer
rate of 150 kilobytes per second in the most
common data format.]
• CD-Recordable drives are often sold with three
different speed ratings, one speed for write-once
operations, one for re-write operations, and one
for read-only operations. The speeds are typically
listed in that order; ie a 12x/10x/32x CD drive can,
CPU and media permitting, write to CD-R discs at
12x speed (1.80 MB/s), write to CD-RW discs at
10x speed (1.50 MB/s), and read from CD discs at
32x speed (4.80 MB/s).
• The 1x speed rating for CD-ROM (150
kB/s) is different than 1x speed rating for
audio CD (172.3 kB/s) and is not to be
confused with the 1x speed rating for DVDs
(1.32 MB/s).
DVD
• DVD (also known as "Digital Versatile
Disc" or "Digital Video Disc") is a popular
optical disc storage media format. Its main
uses are video and data storage. Most DVDs
are of the same dimensions as compact
discs (CDs) but store more than six times as
much data.
• DVD-ROM has data that can only be read
and not written,
• DVD-R and DVD+R can record data only
once and then function as a DVD-ROM.
• DVD-RW, DVD+RW and DVD-RAM can
both record and erase data multiple times.
Internal mechanism of a DVD-ROM
Drive.
Memory Storage
Device
• Pen Drive/USB Drive/Flash Drive:
It is a plug and play (PnP) device that simply
connects into a USB port of a computer.
One can read, write, copy, delete and move data
from it to computer’s HDD or from HDD to pen
drive.
One can even run application like MP3 files or
music video directly from it.
Pen Drive
Memory Storage
Device
• Memory Card (SD/MMC)
Flash memory based cards are removable
storage.
in addition to computer, These cards are
used in digital camera, cell phone etc.
SD = Secure Digital
MMC = Multimedia Card.
Online Storage
• Internet Can also be a Source of Storage.
www.atbackup.com
www.connected.com
www.mcafee.com
www.shii.com