William Stallings Computer Organization and Architecture

Chapter 5External Memory

Types of External Memory

Magnetic DiskMagnetic Disk RAID( redundant array of independent disks )RAID( redundant array of independent disks )

RAID refers to a family of techniques for using multiple disks as a parallel array of data storage devices, with redundancy built in to compensate for disk failure.

Removable

OpticalOptical CD-ROM CD-Writable (WORM) CD-R/W DVD (digital video disk)

Magnetic TapeMagnetic Tape

5.1 Magnetic Disk

Disk substrate coated with magnetizable material (iron oxide…rust)

Substrate used to be aluminiumNow glass

Improved surface uniformityIncreases reliability

Reduction in surface defectsReduced read/write errors

Better stiffness Better shock/damage resistance

Magnetic Disk

Range of packaging Floppy Winchester hard disk Removable hard disk

Read and Write Mechanisms

Recording and retrieval via conductive coil called a headhead May be single read/write head or separate ones During read/write, head is stationary, platter rotates Write

CurrentCurrent through coil produces magnetic fieldmagnetic field Pulses sent to head Magnetic pattern recorded on surface below

Read (traditional) Magnetic field moving relative to coil produces current Coil is the same for read and write

Read (contemporary) Separate read head, close to write head Partially shielded magneto resistive (MR) sensor Electrical resistance depends on direction of magnetic field High frequency operation

Higher storage density and speed

Inductive Write MR Read

Data Organization and Formatting

Concentric rings or trackstracks Gaps between tracks Reduce gap to increase capacity Same number of bits per trackSame number of bits per track (variable packing packing

densitydensity)

Constant angular velocity

Tracks divided into sectorssectorsMinimum block size is one sectorMay have more than one sector per block

Disk Data Layout

Finding Sectors

Must be able to identify start of track and start of track and sectorsector

Format disk Additional information not available to user Marks tracks and sectors

Gap1 Id Gap2 Data Gap3 Gap1 Id Gap2 Data Gap3

TrackSyncByte

Head Sector CRC SyncByte

Data CRC

ST506 format (old!)

Figure.5.2 Winchester Disk Track Format

Physical Sector 0 Physical Sector 0 600B/Sector600B/Sector Physical Sector 1Physical Sector 1 ……Sector 29Sector 29

The ID field is a unique bit identifier or address used to locate a particular sector.The ID field is a unique bit identifier or address used to locate a particular sector.

The SYNCH byte is a special bit pattern that delimits the beginning of the field.The SYNCH byte is a special bit pattern that delimits the beginning of the field.

Physical Characteristics

Fixed (rare) or movable head Removable or fixed disk Single or double (usually) sided disk Single or multiple platter Head mechanism

Contact (Floppy) Fixed gap Flying (Winchester)

The head is actually an aerodynamic foil that rests lightly on the platter’s surface when the disk is motionless. The air pressure generated by the spinning disk is enough to make the foil rise above the surface. The resulting noncontact noncontact systemsystem can be engineered to use narrower heads that operate closer to the platter’s surface than conventional rigid disk head.

Fixed/Movable Head Disk

Fixed head One read write head per track Heads mounted on fixed ridged arm

Movable head One read write head per side Mounted on a movable arm

As shown in figure 5.3 (P158)

Removable or Not

Removable disk Can be removed from drive and replaced with

another disk Provides unlimited storage capacity Easy data transfer between systems

Nonremovable disk Permanently mounted in the drive

Multiple Platter

One head per sideHeads are joined and alignedAligned tracks on each platter form

cylindersData is striped by cylinder

reduces head movement Increases speed (transfer rate)

Multiple Platters

One head per side

Heads are joined and aligned

Cylinders

Aligned tracks on each platter form cylinders

Data is striped by cylinder reduces head

movement Increases speed (transfer rate)

Floppy Disk

8”, 5.25”, 3.5”Small capacity

Up to 1.44Mbyte (2.88M never popular)

SlowUniversalCheapObsolete

Winchester Hard Disk (1)

Developed by IBM in Winchester (USA)Sealed unitOne or more platters (disks)Heads fly on boundary layer of air as disk

spinsVery small head to disk gapGetting more robust

Winchester Hard Disk (2)

UniversalCheap Fastest external storageFastest external storageGetting larger all the time

Multiple Gigabyte now usual

Removable Hard Disk

ZIP Cheap Very common Only 100M

JAZ Not cheap 1G

L-120 (a: drive) Also reads 3.5” floppy Becoming more popular

All obsoleted by CD-R and CD-R/W

Disk Performance ParametersDisk Performance Parameters

Seek timeSeek time Moving head to correct tracktrack

TTss=m=m××n+sn+s

where Ts=estimated seek time

n=number of tracks traversed m=constant that depend on the disk drive

s= startup time

RotationalRotational delay/ Rotational delay/ Rotational latency latency Waiting for data to rotate under head

Access time = Seek + LatencyAccess time = Seek + Latency

Transfer rateTransfer rate T=b/rNT=b/rNWhere T=transfer time b=number of bytes to be transferred

N=number of bytes on a track

r=rotation speed, in revolutions per second

The total average access timeThe total average access timeTa=Ts+1/2r+b/rN

Where Ts is the average seek time.

Note: Transfer speed 1X means 150KB/s

Timing of Disk I/O Transfer

Figure 5.5 Timing of a Disk I/O Transfer

5.2 RAID (Redundant Array of Independent Disks)

Advantages of multiple disks Separate I/O requests can be handled in parallel, as

long as the data required reside on separate disks. A single I/O requests can be executed in parallel if

the block of data to be accessed is distributed across multiple disks.

A standardized scheme for multiple-disk database design— (RAID) 7 levels in common use Not a hierarchy

RAID Design architecturesRAID Design architectures common common characteristicscharacteristics: Set of physical disks viewed as a single logical

drive by Operating system Data distributed across the physical drives of an

array Redundant disk capacity is used to Redundant disk capacity is used to store store

parity informationparity information, which guarantees data recoverability in case of a disk failure.

RAID 0

No redundancyNo redundancyData striped across all disksRound Robin striping Increase speedIncrease speed

Multiple data requests probably not on same disk Disks seek in parallel A set of data is likely to be striped across multiple

disks

RAID 1 Mirrored DisksMirrored Disks

Parity calculation the simple expedient of duplicating all the data ( 2 copies of each stripe on separate disks)

Data is striped across disksRead from eitherWrite to bothRecovery is simple

Swap faulty disk & re-mirror No down time

Expensive

RAID 2

Disks synchronizedDisks synchronized The spindles of the individual drives are synchronized so that each disk

head is in the same position on each disk at any given time.

Very small stripes Often single byte/word

Parallel accessParallel access Redundant via Hamming codeRedundant via Hamming code

Error correction calculated across corresponding bits on disks Multiple parity disks store Hamming error correctMultiple parity disks store Hamming error correctinging code in code in

corresponding positionscorresponding positions

Lots of redundancy Expensive Not used

RAID 3

Similar to RAID 2 Only one redundant diskOnly one redundant disk Parallel accessParallel access Bit-interleaved parityBit-interleaved parity

Simple parity bitSimple parity bit for each set of corresponding bits Instead of an error-correcting code, a simple parity

bit is computed for the set of individual bits in the set of individual bits in the same position on all of the data disksthe same position on all of the data disks.

Data on failed drive can be reconstructed from surviving data and parity info

Very high transfer rates

RAID 4

Each disk operates Each disk operates independentlyindependently Good for high I/O request rate Large stripes Block-interleaved parityBlock-interleaved parity

Bit by bit parity calculated across stripes on each disk A bit-by-bit parity strip is calculated across corresponding

strips on each data disk, and the parity bits are stored in the corresponding strip on the parity disk.

Parity stored on parity disk Write penalty

Update not only the user data but also the corresponding parity bits

each strip write involves two reads and two writes BottleneckBottleneck

every write operation must involve the parity disk

RAID 5

Like RAID 4 Block-interleaved distributed parityBlock-interleaved distributed parity

Parity striped across all disksParity striped across all disks Round robin allocation for parity stripe

Avoids RAID 4 bottleneckAvoids RAID 4 bottleneck at parity at parity diskdisk

Commonly used in network servers

RAID 6

Block-interleaved dual distributed Block-interleaved dual distributed parityparity Two parity calculationsTwo parity calculations Stored in separate blocks on different disks

User requirement of N disks needs N+2 High data availabilityHigh data availability

Three Three disks need to fail for data loss

Significant write penalty Each write affects two parity blocks

RAID 0, 1, 2

Figure 5.6 RAID Levels (page 1 of 2)

RAID 3 & 4

Figure 5.6 RAID Levels (page 2 of 2)

RAID 5 & 6

Applications requiring extremely high availability

Table 5.2 RAID LevelsTable 5.2 RAID Levels

Data Mapping For RAID 0

All of the user and system data are viewed as being stored on a logical a logical diskdisk..

The disk is divided into stripsstrips; these strips may be physical blocks, sectors, or some other unit.

5.3 OPTICAL MEMORY

Optical Storage CD-ROM

Originally for audio650Mbytes giving over 70 minutes audioPolycarbonate (聚碳酸脂 ) coated with highly

reflective coat, usually aluminiumData stored as pitsRead by reflecting laserConstant packing densityConstant linear velocity

CD Operation

Information is retrieved from a CD or CD-ROM by a low-powered laser laser housed in an optical-disk player, or drive unitoptical-disk player, or drive unit. The laser shines through the clear protective coating while a motor spins the disk past it. The intensityintensity of the reflected light of the laser changes as it encounters a pit. This change is detected by a photosensor photosensor and converted into a digital signaldigital signal.

CD-ROM Drive Speeds

Audio is single speed Constant Constant linearlinear velocity velocity, 1.2 ms-1

The length of track (spiral) is 5.27km The standard maximum playing time of

an audio compact disk, 4391 seconds (73.2 minutes)

Audio CD can hold 650Mbytes giving over Audio CD can hold 650Mbytes giving over 70 minutes audio.70 minutes audio.

Other speeds are quoted as multiples

CD-ROM Format

Data on the CD-ROM is organized as Data on the CD-ROM is organized as a sequent of a sequent of blocksblocks.. Sync: the sync field identifies the beginning of a block.Sync: the sync field identifies the beginning of a block. Header: the header contains the block address and the mode byte.Header: the header contains the block address and the mode byte.

Mode 0=blank data fieldMode 0=blank data field Mode 1=2048 byte dataMode 1=2048 byte data ++ error correctionerror correction Mode 2=2336 byte dataMode 2=2336 byte data

Auxiliary: Additional user data in mode2. In mode 1, this is a 288-Auxiliary: Additional user data in mode2. In mode 1, this is a 288-byte error correcting code.byte error correcting code.

(b) Constant Linear Velocity(b) Constant Linear Velocity(a) Constant Angular Velocity(a) Constant Angular Velocity

Figure 5.8 Comparison of Disk Layout MethodsFigure 5.8 Comparison of Disk Layout Methods

Constant angular velocityConstant angular velocity (CAVCAV) DiskBit near centre of rotating disk passes fixed point slower

than bit on outside of disk Increase spacing between bits in different tracks Rotate disk at constant angular velocityconstant angular velocity (CAVCAV)

The information can be scanned as the same rate by rotating the disk at a fixed speed.

Gives pie shaped sectors and concentric tracks Individual tracks and sectors addressable Move head to given track and wait for given sector Waste of space on outer tracks

Lower data density

CD-ROMCD-ROM Can use Can use zoneszones to increase capacity to increase capacity

Each zone has fixed bits per trackEach zone has fixed bits per track There are scanned at the same rate by There are scanned at the same rate by

rotating the disk a variable speed.rotating the disk a variable speed. The pits are read by the laser at the The pits are read by the laser at the constant constant

linear velocitylinear velocity ( (CLVCLV)) More complex circuitryMore complex circuitry

constant linear velocity (CLV)constant linear velocity (CLV)

Disk Layout Methods Diagram

Random Access on CD-ROM

Difficult Move head to rough position Set correct speed Read address Adjust to required location

CD-ROM for & against

Advantages Large capacity Easy to mass produce Removable Robust

Disadvantages Expensive for small runs Longer access time Read only

Other Optical Storage

CD-Recordable (CD-R)CD-Recordable (CD-R) WORMWORM (Write-once read-many)

Subsequently written once with a laser beam of modest intensity.

Constant angular velocity at the sacrifice of some capacity it provides a permanent record of large volumes of user

data.

Now affordable Compatible with CD-ROM drives

CD-RWCD-RW / Erasable Optical Disk / Erasable Optical Disk Erasable Getting cheaper Mostly CD-ROM drive compatible Phase changePhase change

Material has two different reflectivities in different phase states

The primary disadvantagedisadvantage of phase change optical disks is the material eventually and permanently loses its desirable properties.

advantages of erasable optical diskadvantages of erasable optical disk• High capacity• Portability• Reliability

constant angular velocity

DVD - what’s in a name?

Digital Video DiskDigital Video Disk Key highlights of DVD

A standard DVD holds 4.7 Gbytes per layer, with a dual-layer single-sided DVD holding 8.5 Gbytes

DVD uses a form of video compressionvideo compression known as MPEGMPEG for high-quality full-screen pictures

A single layer DVD can hold a two-hour, thirteen-minute movie, while a dual layer can hold a movie greater than four hours long.

Digital Versatile DiskDigital Versatile Disk Used to indicate a computer drive

Will read computer disks and play video disks

DVD - technology

Multi-layerVery high capacity (4.7G per layer)Full length movie on single disk

Using MPEG compression

Finally standardized (honest!)Movies carry regional codingregional codingPlayers only play correct region filmsCan be “fixed”

DVD – Writable

Loads of trouble with standardsstandardsFirst generation DVD drives may not read

first generation DVD-W disksFirst generation DVD drives may not read

CD-RW disksWait for it to settle down before buying!

Magneto-Optical Disks

A magneto-optical (MO) disk drive uses an opticaloptical laserlaser to enhance the capacities of a conventional magnetic disk system. The disk is coated with a material whose polarity

can be altered only at high temperatures. Information is written onto the disk by using the laser to heat a tiny spot on its surface and then applying a magnetic field.

The read operation is purely opticalAdvantages

Longevity Lower cost per megabyte than magnetic storage

The The recording technologyrecording technology Magnetic- Magnetic-Optical Disks is fundamentally magnetic, Optical Disks is fundamentally magnetic, however an optical laser is used to heat a however an optical laser is used to heat a spot to high temperatures to alter the spot to high temperatures to alter the material magnetic field’s polarity. The read material magnetic field’s polarity. The read operation of M-O disk is purely optical by operation of M-O disk is purely optical by detecting the degree of rotation of the detecting the degree of rotation of the polarized light. polarized light.

磁光存储器比纯光学存储器的优点是可以重复写入磁光存储器比纯光学存储器的优点是可以重复写入次数多；比磁盘的优点每兆字节成本便宜。次数多；比磁盘的优点每兆字节成本便宜。

CD and DVD

Magnetic Tape

Serial access If the tape head is positioned at record 1, then

to read recode N, it is necessary to read physical record 1 through N-1, one at a time.

SlowVery cheapBackup and archive

Digital Audio Tape (DAT)

Uses rotating head (like video)High capacity on small tape

4Gbyte uncompressed 8Gbyte compressed

Backup of PC/network servers

Problems:Problems:

5.4 5.55.4 5.5