Chapter 8

Chapter 8: Types of Storage

Storage holds data, instructions and information for future use. A storage medium is the physical material on which a computer keeps data, instructions and information. Every computer user must use a storage as it is used to store various informations. And every computer has preinstalled hard disk too. And storage has the limit to store data and it is called capacity. Usual capacity we can found is kilobyte (kb), megabyte (mb) and gigabyte (gb). We have mentioned hard disk just now, and hard disk is one of the storage device, which is defined as computer hardware that records and/or retrieves items to and from storage media. Two types of process found in storage device is reading (process of transferring items from storage medium to memory) and writing (process of transferring items from memory to storage medium). The speed or the time required for storage device to locate item on storage medium and deliver item from memory to processor is called access time. RAM has the fastest access time and hard disk, memory cards and USB flash drive, optical discs and tape has slower access time according to the sequence.

Types of storage device:
A) Hard disk- contains one or more inflexible, circular platters that use magnetic particles to store data, instructions and informations. It can store data using longitudinal recording or perpendicular recording.
Characteristics of hard disk:
1) Capacity- amount of data in bytes that can be stored.
2) Platters- circular disk on which magnetic data is stored.
3) Read/Write heads- mechanical head which move above platters and perform reading (transform platter's magnetic field into electric current) and writing (transform electric current into magnetic field).
4) Cylinders- two corresponding tracks on a fixed disk.
5) Sectors and tracks- sector is subdivision of a track  (a data storage ring).
6) Revolutions per minute- rotational speed.
7) Transfer rate- rate at which data is transferred from memory to platter or from platter to memory.
8) Access time- refer above.
One of the hard disk function is formatting (process of dividing disk into tracks and sectors so that operating system can store and locate data and information on the disk.
How a hard disk work:
1) Circuit board controls the movement of head actuator and small motor.
2) Small motor will spin the platter.
3) Head actuator will position read/write head over the correct location on the platter to read/write data.
One of the failure of hard disk is caused by head crash where a read/write head touches the platter.
How disk cache work:
1) A special purpose chip on the hard disk called controller receive request for data from processor.
2) It will checks disk cache for requested item.
3) If the item is not found in disk cache, it will locate the requested item on platters.
4) Lastly, it will transfer the item to processor.
Types of hard disk:
1) Redundant array of independent disk (RAID)- a group of two of more integrated hard disk.
2) Network attached storage (NAS)- a server connected to a network with the sole of providing storage.
3) External hard disk- separate free-standing hard disk that connects to a computer with cable or wirelessly.
4) Removable hard disk- hard disk that can be inserted or removed from a drive.
5) Miniature hard disk- hard disk in miniature size which is available in both form of internal and external hard disk. For internal miniature hard disk, it is often used in laptop.
Disk controller in hard disk contain special purpose chip and electronic circuits that control the transfer of data, instructions and informations from a disk to and from the system bus and other components of the computer.
Types of chip in disk controller:
1) Serial Advanced Technology Attachment (SATA)- a computer bus interface that connect host bus adapter (HBA) to storage device.
2) Enhance Integrated Drive Electronics (EIDE)- standard electronic interface between computer and storage device.
3) Small Computer System Interface (SCSI)- a set of parallel interface standards for attaching printers, disk drives, scanners and other peripherals to computers.
4) Serial Attached SCSI (SAS)- replaces SCSI.

B) Flash memory storage- a type of solid state media and contain no moving parts.
Solid state drive (SSD) have several advantages over magnetic hard disk such as faster access time, faster transfer rates, generate less heat and consume less power and last longer.
Types of flash memory storage:
1) Memory card- removable flash memory device that can be inserted and removed from a slot in computer, mobile devices and card reader/writer.
For example:
i) Compact Flash (CF)
ii) Secure Digital (SD)
iii) Secure Digital High Capacity (SDHC)
iv) Micro SD
v) Micro SDHC
vi) xD Picture Card
vii) Memory Stick
viii) Memory Stick Micro (M2)
2) USB flash drive- plug into USB port on a computer or mobile device.
3) Express card module-removable device that fits in express card slot.
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C) Cloud storage- internet service that provides storage to computer users.
Cloud storage providers:
1) Windows Live Sky Drive- Backup or additonal storage for any type of files.
2) Flickr- Digital photos.
3) YouTube- Digital videos.
4) Facebook- Digital photos, digital videos, messages and personal information.
5) Google Docs- Documents, spreadsheets and presentations.
6) Gmail, Outlook- E-mail messages.
7) Amazon EC2, Amazon S3- Enterprise-level storage.
Benefits of cloud storage:
1) Access files from any computer.
2) Store large files instantaneously.
3) Allow others to access their files.
4) View time critical data and images immediately.
5) Store off site backups.
6) Provide data center functions.

D) Optical Discs- consists of a flat, round, portable disc made of metal, plastic and lacquer that is written and read by a laser.
How laser reads data on an optical disc:
1) Laser diode shines a light beam toward the disc.
2) If light strikes a pit, it scatters. If light strikes a land, it is reflected back toward the laser diode.
3) Reflected light is deflected to a light sensing diode, which sends a digital signal of 1 to the computer. Absence of reflected light is read as digital signal of 0.
Types of optical disc:
1) CD-ROM- can be read but not written to.
2) CD-R- multisession optical disc on which users can write but not erase.
3) CD-RW- erasable multisession optical disc.
4) DVD-ROM- high capacity optical disc, can be read but not written to.
5) Blu-ray Disc-ROM (BD-ROM)- has storage capacity of 100GB.
6) DVD-RW, DVD+RW, DVD+RAM- high capacity rewritable DVD formats.

E) Tape- magnetically coated ribbon of plastic capable of storing large amount of data and informations. Require tape drive to read and write data and information.

F) Magnetic stripe card- contains magnetic stripe that stores information.

G) Smart card- stores data on thin microprocessor embedded in the card.

H) Microfilm and microfiche- store microscopic images of documents on a roll or sheet film.

I) Enterprise storage- uses special hardware to store huge volume of data and information for large businesses.