Data Storage TechnologyData Storage Technology

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 is the device or substance that actually holds data.or substance that actually holds data.

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

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

Primary CharacteristicsPrimary Characteristics

SpeedSpeed

VolatilityVolatility

Access methodAccess method

PortabilityPortability

Cost and capacityCost and capacity

SpeedSpeed

Is the most important characteristic that Is the most important characteristic that differentiate primary and secondary storagedifferentiate primary and secondary storage

With current technology, primary storage With current technology, primary storage speed is typically greater than secondary speed is typically greater than secondary storage speed by a factor of 10storage speed by a factor of 1055 or more. or more.

Storage device speed is called Storage device speed is called access timeaccess time..

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

Access time of Access time of primary storage devicesprimary storage devices generally generally are expressed in are expressed in nanosecondsnanoseconds..

Access time of Access time of secondary storage devicessecondary storage devices typically are expressed in typically are expressed in millisecondsmilliseconds..

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

Data transfer units vary from one storage device to Data transfer units vary from one storage device to another:another: The data transfer unit for primary storage The data transfer unit for primary storage

devices is usually a devices is usually a wordword.. The data transfer unit for secondary storage The data transfer unit for secondary storage

devices is devices is blockblock.. The date transfer unit for magnetic and optical The date transfer unit for magnetic and optical

disk drives is disk drives is sectorsector. .

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

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

(1second / 15 nanoseconds) * 32 (1second / 15 nanoseconds) * 32 bitsbits

= 266,666,667 (bytes/second)= 266,666,667 (bytes/second)

VolatilityVolatility

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

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

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

Secondary storage devices are generally non-Secondary storage devices are generally non-volatile.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 determines the way(s) in which data can be accessed.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 Once written, data can be read back only in that same order.same 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 specific order when accessing data but directly access any desired data item stored on the access any desired data item stored on the storage medium.storage medium.

All primary storage devices and disk storage All primary storage devices and disk storage devices are random access devices.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 A parallel access device is capable of simultaneously accessing multiple storage simultaneously accessing multiple storage locations.locations.

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

Parallel access also can be implemented by Parallel access also can be implemented by subdividing data items and storing the subdividing data items and storing the component pieces on multiple storage devices.component 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 Data are represented within the CPU as digital digital electrical signalselectrical signals – the basis of data – the basis of data transmission among all devices attached to the transmission among all devices attached to the system bussystem bus

Electrical power can be stored Electrical power can be stored directlydirectly by by various devices including batteries and capacitorsvarious devices including batteries and capacitors

An electrical signal can be stored An electrical signal can be stored indirectlyindirectly by by using its energy to alter the state of a device such using its energy to alter the state of a device such as a mechanical switchas 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 semiconductorsMicrochip implementation using semiconductors Ability to read and write with equal speedAbility to read and write with equal speed Random access to stored bytes, words, or larger Random access to stored bytes, words, or larger

data unitsdata 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 transistorsIs implemented entirely with transistors

The basic unit of storage is a flip-flop circuitThe basic unit of storage is a flip-flop circuit

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

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

Dynamic RAMDynamic RAM

Uses transistors and capacitors.Uses transistors and capacitors.

The capacitors are the The capacitors are the dynamicdynamic element element

DRAM chips include circuitry that automatically DRAM chips include circuitry that automatically performs refresh operationsperforms refresh operations

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

A number of technologies are used A number of technologies are used to bridge the performance gap to bridge the performance gap between memory and between memory and microprocessors:microprocessors:– Read-ahead memory accessRead-ahead memory access– Synchronous read operationsSynchronous read operations– On-chip cachingOn-chip caching

Synchronous DRAM (SDRAM)Synchronous DRAM (SDRAM)Enhanced DRAM (EDRAM)Enhanced DRAM (EDRAM)Ferroelectric RAMFerroelectric RAM

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

Is a random access memory device that Is a random access memory device that can store data can store data permanentlypermanently or or semipermanentlysemipermanently..

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.

EEPROMEEPROM can be programmed, erased, and can be programmed, erased, and reprogrammed by signals sent from an external control reprogrammed by signals sent from an external control source, such as a CPU.source, such as a CPU.The primary drawback of EEPROM is that read and The primary drawback of EEPROM is that read and

erase operations require relatively high voltages erase operations require relatively high voltages and repeated cycles to cause permanent change.and repeated cycles to cause permanent change.

Flash MemoryFlash Memory can be erased and rewritten much more can be erased and rewritten much more quickly and is used for programs and data that aren’t quickly and is used for programs and data that aren’t frequently updated.frequently updated. It tends to wear out after 100,000 or more write It tends to wear out after 100,000 or more write

operations, making it unsuitable for primary operations, making it unsuitable for primary storage.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)(SIMM) is adopted 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)(DIMM) is a newer 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.

Dual in-line package (DIP)

Single in-line memory module (SIMM)

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 And the method(s) of referencing specific memory locations 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 OrganizationPhysical Memory Organization

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

Cell 0Cell 0 Cell 1Cell 1 Cell 2Cell 2 Cell 3Cell 3 Cell 4Cell 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 endianBig endian describes architectures that store the describes architectures that store the most significant byte at the lowest memory most significant byte at the lowest memory address.address.

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

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

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

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

– Physical memory is usually less than addressable Physical memory is usually less than addressable memory.memory.

Memory Allocation and Memory Allocation and AddressingAddressing

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

UnusedUnused

Program dataProgram data

Program codeProgram code

Operating Operating systemsystem

High memory

Low memoryProgram

offset 0000H

3000H

FFFFH

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

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

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

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

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

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

Is constant over time and proportional to the power of the Is constant over time and proportional to the power of the charge.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 lost.the data content of the storage medium effectively is lost.

Magnetic leakage:Magnetic leakage:

The strength of individual bit charges also can decrease The strength of individual bit charges also can decrease due to magnetic leakage from adjacent bits.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. It storage medium to accept and hold magnetic charge. It varies widely among elements and compounds.varies widely among elements and compounds.

The amount of surface area allocated to a bit is referred The amount of surface area allocated to a bit is referred to as the to as the recording densityrecording density, which is expressed in bits, , which is expressed in bits, bytes, or tracks per inch.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 depends on The integrity of magnetic storage medium depends on the nature of its construction and the environmental the nature of its construction and the environmental factors to which it is subjected.factors to which it is subjected.

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

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

Magnetic Tape

Is a ribbon of plastic with a Is a ribbon of plastic with a coercible surface coating.coercible surface coating.Tapes are mounted in a Tapes are mounted in a tape drive for reading and tape drive for reading and writing.writing.Tape drive contains motors Tape drive contains motors that wind and unwind the that wind and unwind the tape.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 record There are two basic approaches to record data onto a tape surface:data onto a tape surface:

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

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

Magnetic diskMagnetic disk

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

Data normally are recorded on both sides of a platter.Data normally are recorded on both sides of a platter.

A track is one concentric circle of a platter.A track is one concentric circle of a platter.

A sector is a fractional portion of a track.A sector is a fractional portion of a track.

A read/write head is placed for each side of each platter.A read/write head is placed for each side of each platter.

Read/write heads are mounted on the end of an access Read/write heads are mounted on the end of an access arm.arm.

Access arms are attached to a positioning servo.Access arms are attached to a positioning servo.

The servo moves the access arms so that the read/write The servo moves the access arms so that the read/write heads can be positioned anywhere between the heads can be positioned anywhere between the outermost track and the innermost track.outermost track and the innermost track.

Hard disk:Hard disk:

Is a magnetic disk media with a rigid metal base.Is a magnetic disk media with a rigid metal base.

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

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

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

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

Floppy disk (diskette):Floppy disk (diskette):

Uses a base of flexible or rigid plastic material.Uses a base of flexible or rigid plastic material.

Rang from 2.5 to 3.5 inches in diameter.Rang from 2.5 to 3.5 inches in diameter.

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

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

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

Disk access time depends on several factors Disk access time depends on several factors including:including:

Time required to switch among read/write Time required to switch among read/write headsheads

Time required to position the read/write Time required to position the read/write headsheads

Rotational delayRotational delay – the time that the disk – the time that the disk controller must wait for the proper sector to controller must wait for the proper sector to rotate beneath the headsrotate 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 Not subject to problems of magnetic decay and leakageand leakage

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 formats and write technologies including:including:

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

Guideline for the proper care of Guideline for the proper care of Compact DiscsCompact Discs

Do’sDo’s– Store the disc in a jewel box when not in useStore the disc in a jewel box when not in use– Hold a disc by its edgesHold a disc by its edges

Don’tDon’t– Expose the disc to excessive heat or sunlightExpose the disc to excessive heat or sunlight– Eat, smoke, or drink near a discEat, smoke, or drink near a disc– Stack discsStack discs– Touch the underside of the discTouch the underside of the disc