Secondary StorageMemory

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 StorageSecondary 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

Magnetic Disks Optical Disk Memory StorageDevice

Sequential access devices[Magnetic Tape]

FloppyDisk

Hard DiskCD-ROM

CD-R CD-RW DVD

Flash drive or Pen Drive

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 StorageDevice

• 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 StorageDevice

• 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