Data Storage TechnologyData Storage Technology

By MaikaBy Maika

IntroductionIntroduction

A A storage devicestorage device consists of a consists of a read/write mechanism and a storage read/write mechanism and a storage medium.medium.

The The storage mediumstorage medium is the device or is the device or substance that actually holds data.substance that actually holds data.

The The read/write mechanismread/write mechanism is the is the device used to read or write data to and device used to read or write data to and from the storage medium.from the storage medium.

A typical computer system has many storage devicesA typical computer system has many storage devices Primary storage:Primary storage: Static RAM (SRAM) Static RAM (SRAM)

Dynamic RAM (DRAM)Dynamic RAM (DRAM)

Flash RAMFlash RAM

ROMROM Secondary storage:Secondary storage: Magnetic tape Magnetic tape

Magnetic diskMagnetic disk

Optical diskOptical disk

Multiple devices are required because no one device Multiple devices are required because no one device or technology can cost-effectively meet all storage or technology can cost-effectively meet all storage needs for a single computer system or user.needs for a single computer system or user.

Primary CharacteristicsPrimary Characteristics

SpeedSpeed VolatilityVolatility Access methodAccess method PortabilityPortability Cost and capacityCost and capacity

Speed

Is the most important characteristic that differentiate primary and secondary storage

With current technology, primary storage speed is typically greater than secondary storage speed by a factor of 105 or more.

Storage device speed is called access time.

Is the time required to execute on read or write operation completely.

Access time of primary storage devices generally are expressed in nanoseconds.

Access time of secondary storage devices typically are expressed in milliseconds.

A complete measure of data access speed consists of access time and the unit of data transfer to/from the storage device.

Data transfer units vary from one storage device to another:

The data transfer unit for primary storage devices is usually a word.

The data transfer unit for secondary storage devices is block.

The date transfer unit for magnetic and optical disk drives is sector.

A storage device’s data transfer rate is computed by dividing 1 by the access time and multiplying the result by the unit of data transfer.

Eg. The data transfer rate for a primary storage device with 15 nanosecond access time and a 32-bit word data transfer unit can be computed as:

(1second / 15 nanoseconds) * 32 bits

= 266,666,667 (bytes/second)

VolatilityVolatility

A storage device or medium is A storage device or medium is non-volatilenon-volatile if it holds if it holds data without loss over long periods of time.data without loss over long periods of time.

A storage device or medium is A storage device or medium is volatilevolatile if it cannot if it cannot reliably hold data for long periods.reliably hold data for long periods.

Primary storage devices are generally volatile.Primary storage devices are generally volatile.

Secondary storage devices are generally non-volatile.Secondary storage devices are generally non-volatile.

Access MethodAccess Method

The physical structure of a storage device’s The physical structure of a storage device’s read/write mechanism and storage medium read/write mechanism and storage medium determines the way(s) in which data can be accessed.determines the way(s) in which data can be accessed.

Three types of access methods:Three types of access methods: Serial accessSerial access Radom accessRadom access Parallel accessParallel access

Serial AccessSerial Access A serial access storage device stores and retrieves A serial access storage device stores and retrieves

data items in a linear or sequential order.data items in a linear or sequential order.

Once written, data can be read back only in that same Once written, data can be read back only in that same order.order.

Serial access time depends on the current position of Serial access time depends on the current position of the read/write mechanism and on the position of the the read/write mechanism and on the position of the desired date item within the storage medium.desired date item within the storage medium.

Serial access devices are not used for frequently Serial access devices are not used for frequently accessed data but for holding backup copies of data accessed data but for holding backup copies of data stored on other storage devices.stored on other storage devices.

Random AccessRandom Access

A random access device is not restricted to any A random access device is not restricted to any specific order when accessing data but directly access specific order when accessing data but directly access any desired data item stored on the storage medium.any desired data item stored on the storage medium.

All primary storage devices and disk storage devices All primary storage devices and disk storage devices are random access devices.are random access devices.

Access time may or may not be a constant.Access time may or may not be a constant. It is a constant for most primary storage devices.It is a constant for most primary storage devices. It is not a constant for disk storage.It is not a constant for disk storage.

Parallel AccessParallel Access

A parallel access device is capable of simultaneously A parallel access device is capable of simultaneously accessing multiple storage locations.accessing multiple storage locations.

If one considers the unit of data access to be a bit, If one considers the unit of data access to be a bit, then access is parallel.then access is parallel.

Parallel access also can be implemented by Parallel access also can be implemented by subdividing data items and storing the component subdividing data items and storing the component pieces on multiple storage devices.pieces on multiple storage devices.

PortabilityPortability

Data can be made portable by storing it on a Data can be made portable by storing it on a removable storage medium or device.removable storage medium or device.

External disk drives are portable if they can External disk drives are portable if they can be added or removed easily from a computer be added or removed easily from a computer system and transported without damage to system and transported without damage to the device or its data content.the device or its data content.

Portable devices typically sacrifice access Portable devices typically sacrifice access speed to compensate for the lack of speed to compensate for the lack of environmental control.environmental control.

Cost and CapacityCost and Capacity

Each of Each of the the storage storage device device attributes attributes is directly is directly related to related to device device costcost

CharacteristicsCharacteristics CostCost

SpeedSpeed

VolatilityVolatility

Access methodAccess method

PortabilityPortability

CapacityCapacity

Cost increases as speed increasesCost increases as speed increases

For devices of similar type, cost For devices of similar type, cost decreases as volatility increasesdecreases as volatility increases

Serial is the least expensive, Serial is the least expensive, Random is more expensive than serial, Random is more expensive than serial, Parallel is more expensive than non-Parallel is more expensive than non-parallelparallel

For devices of similar type, portability For devices of similar type, portability increases costincreases cost

Cost usually increases in direct Cost usually increases in direct proportion to capacityproportion to capacity

Primary Storage DevicesPrimary Storage DevicesStoring Electrical SignalsStoring Electrical Signals

• Data are represented within the CPU as digital electrical signals – the basis of data transmission among all devices attached to the system bus

• Electrical power can be stored directly by various devices including batteries and capacitors

• An electrical signal can be stored indirectly by using its energy to alter the state of a device such as a mechanical switch

Random Access Memory (RAM)Random Access Memory (RAM)

RAMRAM is a generic term describing primary is a generic term describing primary storage devices with the following storage devices with the following characteristics:characteristics:

Microchip implementation using Microchip implementation using semiconductorssemiconductors

Ability to read and write with equal speedAbility to read and write with equal speed Random access to stored bytes, words, or Random access to stored bytes, words, or

larger data unitslarger data units

There are two basic RAM typesThere are two basic RAM types Static RAMStatic RAM (SRAM) (SRAM) Dynamic RAMDynamic RAM (DRAM) (DRAM)

Static RAMStatic RAM

Is implemented entirely with Is implemented entirely with transistorstransistors

The basic unit of storage is a flip-flop The basic unit of storage is a flip-flop circuitcircuit

A A flip-flop circuitflip-flop circuit which contains two which contains two transistors for bit storage is an transistors for bit storage is an electrical switch that remembers its electrical switch that remembers its last positionlast position

SRAM is SRAM is volatile volatile unless a continuous unless a continuous supply of power can be guaranteed.supply of power can be guaranteed.

Dynamic RAMDynamic RAM

Uses transistors and capacitors.Uses transistors and capacitors.

The capacitors are the The capacitors are the dynamicdynamic elementelement

DRAM chips include circuitry that DRAM chips include circuitry that automatically performs refresh automatically performs refresh operationsoperations

Each refresh operation is called a Each refresh operation is called a refresh cyclerefresh cycle

SRAM is more expensive to fabricate SRAM is more expensive to fabricate than DRAM due to its more complex than DRAM due to its more complex circuitry.circuitry.

DRAM is slower than SRAM due to its DRAM is slower than SRAM due to its required refresh cycles.required refresh cycles.

Neither RAM type can match current Neither RAM type can match current microprocessor clock rates which microprocessor clock rates which range from 500MHz to 1.5GHz.range from 500MHz to 1.5GHz.

The fastest DRAM is at least 25 The fastest DRAM is at least 25 times slower than modern times slower than modern microprocessors. SRAM is at least microprocessors. SRAM is at least 2.5 times slower.2.5 times slower.

Read-Only Memory (ROM)Read-Only Memory (ROM)

Is a random access memory device that can Is a random access memory device that can store data permanently or semipermanently.store data permanently or semipermanently.

Is used to stored programs such as Is used to stored programs such as computer system boot subroutines like the computer system boot subroutines like the system BIOS.system BIOS.

Two ROM technologies are currently in use Two ROM technologies are currently in use are: Electrically Erasable Programmable are: Electrically Erasable Programmable Read-Only Memory (EEPROM) and Flash Read-Only Memory (EEPROM) and Flash Memory.Memory.

EEPROM can be programmed, erased, and EEPROM can be programmed, erased, and reprogrammed by signals sent from an external reprogrammed by signals sent from an external control source, such as a CPU.control source, such as a CPU.

The primary drawback of EEPROM is that The primary drawback of EEPROM is that read and erase operations require relatively read and erase operations require relatively high voltages and repeated cycles to cause high voltages and repeated cycles to cause permanent change.permanent change.

Flash Memory can be erased and rewritten much Flash Memory can be erased and rewritten much more quickly and is used for programs and data more quickly and is used for programs and data that aren’t frequently updated.that aren’t frequently updated.

It tends to wear out after 100,000 or more It tends to wear out after 100,000 or more write operations, making it unsuitable for write operations, making it unsuitable for primary storage.primary storage.

Memory PackagingMemory Packaging

Memory circuits are embedded within microchips and Memory circuits are embedded within microchips and groups of chips are packed on a small circuit board that can groups of chips are packed on a small circuit board that can be installed or removed easily from a computer system.be installed or removed easily from a computer system.

Early RAM and ROM circuits were packaged in Early RAM and ROM circuits were packaged in dual in-line dual in-line packagedpackaged (DIPs) (DIPs)

Later, Later, single in-line memory modulesingle in-line memory module (SIMM) is adopted (SIMM) is adopted which incorporated multiple DIPs on a tiny printed circuit which incorporated multiple DIPs on a tiny printed circuit board.board.

The The double in-line memory moduledouble in-line memory module (DIMM) is a newer (DIMM) is a newer packaging standard. It is a double-sided SIMM with memory packaging standard. It is a double-sided SIMM with memory DIPs and electrical contacts on both sides of the module.DIPs and electrical contacts on both sides of the module.

CPU Memory AccessCPU Memory Access

The physical organization of memory, The physical organization of memory,

The organization of programs and data within memory, The organization of programs and data within memory,

And the method(s) of referencing specific memory locations And the method(s) of referencing specific memory locations

are critical design issues for both primary and secondary are critical design issues for both primary and secondary storage devices and processors.storage devices and processors.

Physical Memory Organization

The main memory can be regarded as a sequence of contiguous or adjacent memory cells.

Cell 0 Cell 1 Cell 2 Cell 3 Cell 4

In a byte sequence, the leftmost byte is called the most significant byte and the rightmost byte is called the least significant byte.

Big endian describes architectures that store the most significant byte at the lowest memory address.

Little endian describes architecture that store the least significant byte at the lowest memory address.

The addressable memory of a CPU is the highest numbered storage byte that can be represented.

Addressable memory is determined by the number of bits used to represent an address.

Physical memory is the actual number of memory bytes that physically are installed in the machine.

Physical memory is usually less than addressable memory.

Memory Allocation and Addressing

Memory allocation describes the assignment of specific memory address to system software, application programs, and data.

Unused

Program data

Program code

Operating system

High memory

Low memoryProgram

offset 0000H

3000H

FFFFH

Absolute addressing describes memory address operands that refer to actual physical memory locations.

Usually, the operating system calculates and stores the program offset in a register when the program first is loaded into memory.

During program execution, the CPU automatically adds the program offset to all memory address operands before accessing memory.

The method of automatically computing physical memory addresses is called indirect addressing.

The register that holds the offset value is called offset register.

Magnetic StorageMagnetic Storage

► A magnetic storage device A magnetic storage device converts electrical signals into magnetic charges, converts electrical signals into magnetic charges, captures the magnetic charge on a storage medium, captures the magnetic charge on a storage medium, and later uses the stored magnetic charge to and later uses the stored magnetic charge to

generate an electrical current.generate an electrical current.

► Magnetic storage devices must control or Magnetic storage devices must control or compensate for some undesirable compensate for some undesirable characteristics of magnetism and magnetic characteristics of magnetism and magnetic storage mediastorage media

Magnetic Decay and LeakageMagnetic Decay and Leakage

Magnetic decay:Magnetic decay:

► Is the tendency of magnetically charged particles to Is the tendency of magnetically charged particles to lose their charge over time.lose their charge over time.

► Is constant over time and proportional to the power of Is constant over time and proportional to the power of the charge.the charge.

► Will cause the stored charge power to fall below the Will cause the stored charge power to fall below the threshold required for a successful read operation threshold required for a successful read operation

the data content of the storage medium effectively is the data content of the storage medium effectively is lost.lost.

Magnetic leakage:Magnetic leakage:

► The strength of individual bit charges also can The strength of individual bit charges also can decrease due to magnetic leakage from adjacent bits.decrease due to magnetic leakage from adjacent bits.

StorageStorage DensityDensity

► CoercivityCoercivity is the ability of a substance or magnetic is the ability of a substance or magnetic storage medium to accept and hold magnetic charge. storage medium to accept and hold magnetic charge. It varies widely among elements and compounds.It varies widely among elements and compounds.

► The amount of surface area allocated to a bit is The amount of surface area allocated to a bit is referred to as the referred to as the recording densityrecording density, which is , which is expressed in bits, bytes, or tracks per inch.expressed in bits, bytes, or tracks per inch.

TIP:TIP: Designers and purchasers of magnetic media and Designers and purchasers of magnetic media and devices must find a suitable balance between high devices must find a suitable balance between high recording density and the reliability of the media, recording density and the reliability of the media, especially over relatively long periods of time.especially over relatively long periods of time.

Media IntegrityMedia Integrity

► The integrity of magnetic storage medium The integrity of magnetic storage medium depends on the nature of its construction and depends on the nature of its construction and the environmental factors to which it is the environmental factors to which it is subjected.subjected.

► Loss of coercible coating represent a loss of Loss of coercible coating represent a loss of strength in stored magnetic charges strength in stored magnetic charges data data becomes unreadable when the remaining becomes unreadable when the remaining charge falls below the threshold of readability.charge falls below the threshold of readability.

TIP:TIP: To extent the life of magnetic media, they To extent the life of magnetic media, they must be protected from physical abuse and must be protected from physical abuse and temperature and humidity extremestemperature and humidity extremes

Magnetic Tape

► Is a ribbon of plastic with a coercible surface Is a ribbon of plastic with a coercible surface coating.coating.

► Tapes are mounted in a tape drive for Tapes are mounted in a tape drive for reading and writing.reading and writing.

► Tape drive contains motors that wind and Tape drive contains motors that wind and unwind the tape.unwind the tape.

► Tapes primarily are used to make back up Tapes primarily are used to make back up copies of data stored on faster secondary copies of data stored on faster secondary storage devices and to physically transport storage devices and to physically transport large data sets.large data sets.

► There are two basic approaches to There are two basic approaches to record data onto a tape surface:record data onto a tape surface:

Linear recordingLinear recording places bits along places bits along parallel tracks that run along the entire parallel tracks that run along the entire length of the tape.length of the tape.

Helical scanningHelical scanning reads and writes data to reads and writes data to or from a tape by rotating the read/write or from a tape by rotating the read/write head at an angle to the tape and moving head at an angle to the tape and moving from tape edge to tape edge.from tape edge to tape edge.

Magnetic disk

Magnetic disk media are flat, circular platters with metallic coating that are rotated beneath read/write heads.

Data normally are recorded on both sides of a platter. A track is one concentric circle of a platter. A sector is a fractional portion of a track. A read/write head is placed for each side of each platter. Read/write heads are mounted on the end of an access

arm. Access arms are attached to a positioning servo. The servo moves the access arms so that the read/write

heads can be positioned anywhere between the outermost track and the innermost track.

Hard disk:

Is a magnetic disk media with a rigid metal base.

Typical platter size is between three and five inches in diameter.

High speeds is up to 10,000 revolutions per minute.

Drive capacity depends on the number of platters, platter size, and recording density.

Multiple hard drives often are enclosed in a single storage cabinet and referred to as a drive array.

Floppy disk (diskette):

Uses a base of flexible or rigid plastic material.

Rang from 2.5 to 3.5 inches in diameter.

Contains a single flexible plastic platter coated with an iron or other metallic compound on both sides.

The platter is mounted permanently in a plastic case that can be removed from a diskette drive.

The case has an access door that is closed when the diskette is removed from the drive and opened automatically when the diskette is inserted into the derive.

Disk access time depends on several factors including:

Time required to switch among read/write heads Time required to position the read/write heads Rotational delay – the time that the disk controller

must wait for the proper sector to rotate beneath the heads

Optical Mass Storage Optical Mass Storage DevicesDevices

Came of age in the 1990sCame of age in the 1990s

Primary advantages: higher recording density Primary advantages: higher recording density and longer data lifeand longer data life

Can retain data for decadesCan retain data for decades

Not subject to problems of magnetic decay and Not subject to problems of magnetic decay and leakageleakage

Popular due to standardized and relatively Popular due to standardized and relatively inexpensive storage media.inexpensive storage media.

Store bit values as variations in light reflectionStore bit values as variations in light reflection

Optical storage devices are available Optical storage devices are available currently in a wide variety of storage currently in a wide variety of storage formats and write technologies including:formats and write technologies including:

CD-ROMCD-ROM WORM (CD-R)WORM (CD-R) Magneto-opticalMagneto-optical CD-RWCD-RW DVDDVD