Module 3: Explains How Data is Stored on Different Storage Devices

storage medium ( secondary storage)

physical device that data is stored for long term.

Storage devices

can be internal, external, or remote.

Internal devices

 are located inside the system unit;

• external devices are attached to the computer via the ports; 

• remote (online) storage systems are accessed via the Internet by any computer with an Internet connection. 

• Anything stored on a storage medium —----->  file and is given a filename. 

• Users often create logical folders (for example Pictures, Programs, Homework) to organize and store related files. 

• The physical storage of files on the storage media is different from where the files are stored logically (within folders).

A critical aspect of any storage device is

its storage capacity (measured in bytes)

Storage vs. memory:

•  stored in secondary storage (normally hard disk) until the user opens them. 

  • At this point the operating system copies the program/data to RAM (memory) for faster access. 

  • When the user closes the program/data the operating system deletes the program/data from RAM.

Access time

• measures the time it takes for a device to locate the data on the storage medium and transfer it to the CPU.

Hard drives

• most common secondary storage device.

• can be internal or external, hard disks or solid-state drives.

Hard drives can be internal or external, hard disks or solid-state drives.

• contain one or more circular, metallic platters. 

• The top and bottom surface of each platter has hundreds of circular tracks which contain very small magnetic particles. 

• The read/write head of the hard drive aligns the particles one way to represent a 0 bit and another way to represent a 1 bit.

• normally built into the system unit although external hard disks are available.

Magnetic hard drives

• have a read/write head for each recording surface. The read/write head never touches the surface of the disk, but is extremely close to it. 

• The read/write heads re-align the magnetic particles on the platters when saving data and read the orientation of the particles when retrieving data.

A head crash (or hard drive crash)

• can occur when a contaminant (smoke, dust or hair particle) lands on the platter or the read/write head touches the surface of the platter.

Formatting

•  process of dividing the disk into tracks and sectors (fig 8-5).

• There are thousands of tracks and sectors on a typical platter.

• The operating system keeps track of where data is stored by its track number and sector number.

Partitioning a hard drive

• enables you to divide the physical capacity of a drive into separate areas called partitions. 

  • Each partition can be treated as a separate drive. Partitioning a hard drive allows you to put two operating systems on your computer—one on each partition.

Disk defragmentation

• reorganizes the magnetic particles on the platter so that all the files related to a program are stored in the same area of the disk and all unused particles are grouped in the same areas on the disk.

Solid-State hard drives use flash memory. 

• Because solid-state drives do not contain moving parts they are more reliable and more resistant to vibrations than magnetic hard drives. 

• They also consume less power, are quieter, lighter, last longer, generate less heat and are faster than magnetic hard drives. However, they are also more expensive than magnetic hard drives.

A memory card

• removable flash memory storage device that you insert into a slot on a computer, mobile device, or card reader. There are many types of memory cards (SDHC, SDXC, miniSD, etc.). Devices that use memory cards include smartphones, digital cameras and printers.

A USB flash drive (also called thumb drive)

•  flash memory storage device that plugs in a USB port on a computer or mobile device. 

  • Due to their small size they are ideal for transferring data easily between devices with USB ports. 

  • Flash drive storage capacities range from 64 GB to 2 TB or more.

  • have less capacity per price than external hard disks.

Cloud storage

• when a person or company stores their data on a remote storage device that is accessed via the Internet. Many web sites offer limited amounts of storage space for free. Individuals or businesses that require large amounts of storage capacity are charged according to the amount of storage capacity they need and how often the data is transferred. Online storage systems require the user to log onto the system with a username and password.

Advantages of cloud storage include:

• Access to files from any device that has internet access

• Sharing files over the internet

•  Having a data backup location

• Saving storage space on your physical device by storing large files on the cloud

Optical drives (CDs and DVDs)

• use laser beams to write and read data from metal or plastic discs. 

  • Data can be stored on both sides of an optical disc.

Optical read-only memory (ROM) discs

• represent 0s and 1s by using lasers to burn tiny depressions (pits) onto some areas of the disc and leaving other areas (called lands) unchanged.

  • A low power laser light reads data by reflecting light through the bottom of the disc. A land area causes light to reflect, which represents a 1 bit. The pit area absorbs the light. The absence of light represents a 0 bit.

CDs and DVDs, like hard discs, use

• tracks and sectors to identify storage locations.

CD-ROMs and DVD ROMs

•  cannot be erased or used to save data

CD-R, DVD-R (recordable)

• can be written to just once

CD-RW, DVD-RW (rewritable)

•  can be written to many times

Large organizations often use specialized storage devices that provide

• large amounts of storage capacity and extremely high performance in a heavy use environment. 

RAID (redundant array of independent disks)

• uses two or more small hard drives working together to do the job of a larger drive.

  • more expensive than a traditional hard drive, but it provides better performance and (with some levels) reliability because it automatically creates backup copies of data. 

  • be implemented on a single device or on a storage server.

A NAS  (network attached storage)

• device is a server connected to a network through a wired or wireless connection. Because it is made for the sole purpose of saving and retrieving data it is faster than a traditional server. 

  • typically has between two to eight hard drives configured in a RAID to provide a large amount of storage. NAS devices have their own IP address because they have their own connection to the network. They are used by companies that want many users to simultaneously access the stored data directly via their network connection. 

A SAN (storage area network)

•  separate network of storage devices (hard drives, RAID, optical discs and/or magnetic tape) that provide a vast amount of storage to large companies. 

  • usually connect to the rest of the network through fiber optic cable to improve performance. Fig 8-20 illustrates a SAN.

Magnetic tape

• magnetically coated ribbon of plastic that is capable of storing large amounts of data at a comparatively low cost. 

  • Due to its low expense, magnetic tape used to be the most prominent data-storage alternative.

  •  However, because magnetic tape is slow and uses sequential access, it is used today primarily for archiving data that is no longer actively being used.

A magnetic stripe card

• plastic card about the size of a credit card that contains information identifying you and the card. 

  • Examples of stripe cards are credit cards, bank cards and entertainment cards.

Smart cards

• look similar to a magnetic stripe card except it stores data on an integrated circuit embedded in the card. 

  • Areas where smart cards are used are tracking medical information, prepaid money cards, tracking customer purchases or providing access to a building.

RFID

•  technology that uses radio signals to communicate with a tag placed in or attached to an object, animal or person. 

  • An RFID reader wirelessly reads the information stored in an RFID tag