chapter 7 external memory computer architecture chapter seven: external memory

Chapter 7 External Memory

Computer Architecture

Chapter Seven: External Memory

Types of External Memory

• Magnetic Disk

– RAID (Redundant Array of Independent disks)

– Removable

• Optical

– CD-ROM

– CD-Recordable (CD-R)

– CD-R/W

– DVD

• Magnetic Tape


Magnetic Disk• A disk - a circular platter constructed of nonmagnetic material.

- called substrate. • Disk substrate coated with magnetizable material• Substrate used – aluminium, glass substrate (more recently)• Benefits of glass substrate:-

– Improved magnetic film surface uniformity• Increases reliability

– Reduction in surface defects• Reduced read/write errors

– Lower fly heights– Better stiffness– Greater ability to withstand shock and damage.


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

– Current through coil produces magnetic 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-one head for read/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

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Read and Write Mechanisms

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Inductive Write MR Read

Data Organization and Formatting

• Concentric rings or tracks– Gaps between adjacent tracks-avoid misalignment

of head/magnetic fields interference.– Reduce gap to increase capacity– Same number of bits per track (variable packing

density)– Constant angular velocity (CAV)

• Tracks divided into sectors• Minimum block size is one sector• May have more than one sector per block


Disk Data Layout


Disk Velocity• Bit 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 velocity (CAV)

– 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

• Can use zones to increase capacity– Each zone has fixed bits per track– More complex circuitry

Disk Layout Methods Diagram

Overcome the waste space for the outer tracks.

Same data size for short inner and long outer tracks.


Finding Sectors

• Must be able to identify – start of track.– start and end of each sector

• Disk Formatting– Means to control data recorded on disk– Additional information not available to user,

only for the disk drive.– Marks tracks and sectors


Example of Winchester Disk FormatSeagate ST506


Physical Characteristics

• Head - Fixed (rare) – one head / track - movable with respect to the platter

• Disk - Removable (remove/replace with another) (e.g Floppy disk, pen drives) - Fixed (permanently mounted in a disk

drive) (e.g : hard disk)

• Magnetizable coating - Single or double (usually) sided of platter

• Platter - Single or multiple (stacked vertically)• Head mechanism

– Contact (Floppy)– Fixed gap-air gap– Flying (Winchester)


Fixed/Movable Head Disk

• Fixed head– One read write head per track– Heads mounted on fixed rigid arm

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


Removable or Not

• Removable disk– Can be removed from drive and replaced

with another disk– Provides unlimited storage capacity– Easy data transfer between systems

• Non-removable disk– Permanently mounted in the drive


Multiple Platter

• One head per side

• Heads are joined and aligned

• Aligned tracks on each platter form cylinders

• Data is striped by cylinder– reduces head movement– Increases speed (transfer rate)


Multiple Platters Chapter Seven: External Memory

Toshiba HDD2189 2.5" hard disk drive platters, head and actuator arm. near 1:1 macro.

http://upload.wikimedia.org/wikipedia/commons/5/5a/Hard_disk_platters_and_head.jpg

Tracks and Cylinders


Floppy Disk

• 8”, 5.25”, 3.5”

• Small capacity– Up to 1.44Mbyte (2.88M never popular)

• Slow

• Universal

• Cheap

• Obsolete


Winchester Hard Disk

• Developed by IBM in Winchester (USA)

• Sealed unit

• One or more platters (disks)

• Heads fly on boundary layer of air as disk spins

• Very small head to disk gap

• Getting more robust


Winchester Hard Disk

• Universal

• Cheap

• Fastest external storage

• Getting larger all the time– 250 Gigabyte now easily available


Speed

• Seek time– Moving head to correct track

• (Rotational) latency– Waiting for data to rotate under head

• Access time = Seek + Latency

• Transfer rate


Disk Performance Parameters

• When operating, disk rotating at constant speed.• To read/write, the head must be positioned at the

desired track and at the beginning at the desired sector on track.

• Time taken to position the head at the track – seek time.

• Time taken for the beginning of the sector to reach the head – rotational delay / latency

• Time taken to get into position to read or write -access time.

• The read/write operation performed as the sector moves under the head.

• Time required for data transfer – transfer time


Timing of Disk I/O Transfer


Inside a Hard Disk


SCSI-Small computer sys interface

RAID (INTRODUCTION)

• The demand for better performance from secondary storage device introduced the development of RAID systems.

• To match the super fast processor.• RAIDS system treat multiple disk drives as a single

entity.• Data recording, distributed on different drives which

appear as a single drive to the application program.• Data stripping is the common technique used in

RAID systems.• A single file is split into multiple strips that are written

on multiple disk drives.


RAID (INTRODUCTION)• Initially - Redundant Array of Inexpensive Disks• Then - Redundant Array of Independent Disks• 7 levels (0 to 6)• Not a hierarchy relationship

Share three common characteristics:-

1) Set of physical disks viewed as single logical drive by O/S

2) Data distributed across physical drives

3) Can use redundant capacity to store parity information

Advantages : • Performance improvement due to parallelism by multiple disk

drives. • Fault tolerance due to replication of data.


RAID 0• No redundancy• Data striped across all disks• Round Robin striping• Increase 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 Disks• Data is striped across disks• 2 copies of each stripe on separate disks• Read from either• Write to both• Recovery is simple

– Swap faulty disk & re-mirror– No down time

• Expensive


RAID 2• Disks are synchronized• Very small stripes

– Often single byte/word

• Error correction calculated across corresponding bits on disks

• Multiple parity disks store Hamming code error correction in corresponding positions

• Lots of redundancy– Expensive– Not used


RAID 3

• Similar to RAID 2• Only one redundant disk, no matter how

large the array• Simple parity bit for each set of

corresponding bits• Data on failed drive can be reconstructed

from surviving data and parity info• Very high transfer rates


RAID 4

• Each disk operates independently

• Good for high I/O request rate

• Large stripes

• Bit by bit parity calculated across stripes on each disk

• Parity stored on parity disk


RAID 5

• Like RAID 4

• Parity striped across all disks

• Round robin allocation for parity stripe

• Avoids RAID 4 bottleneck at parity disk

• Commonly used in network servers


RAID 6

• Two parity calculations

• Stored in separate blocks on different disks

• User requirement of N disks needs N+2

• High data availability– Three disks need to fail for data loss– Significant write penalty because each

write affects two parity blocks.


Data Mapping For RAID 0 Chapter Seven: External Memory

RAID 0, 1, 2


RAID 3 & 4


RAID 5 & 6


RAID Levels (summary)RAID level

Principle Merits Demerits

0 Data stripping and block interleave: different blocks of a file are written on different disk drives.

High performance at low cost

No redundancy

1 Disk mirroring; each disk drive has a mirror disk drive; every byte is written in both drives.

•Good redundancy

•Performance improvement

Expensive

2 Data stripping with bit interleave and hamming codes for error detection and correction; each bit in a byte is written in a different disk drive; separate drive for hamming codes.

•High performance due to parallel storage/read•No duplication of data.

Expensive




3 Data stripping with bit interleave and parity; each bit in a byte written in a different disk drive; separate drive for parity bit.

•High performance•No duplication of data•Good reliability

Expensive

4 Data stripping with block interleave and parity; each block of file written in a different disk drive; separate drive for parity.

•Multiple simultaneous operations on different drives.

•Good reliability

Expensive




5 Data stripping with block interleave and distributed check; data writing similar to level 4 but parity bits are distributed in all disk drives

•Same as previous

expensive

6 P + Q redundancy; uses Reed-Solomon codes; two different error codes (P and Q) are generated and stored (distributed) in two drive with block interleave data striping similar to level 5.

•High redundancy; up to two disk failures are tolerable

Expensive and additional delays.


Optical Storage (Introduction)• Information in magnetic disk is recorded in concentric

tracks.• With CAV system, the number of bits per track is constant.• An increase in intensity is achieved with multiple zoned

recording -the surface is divided into a number of zones with zones farther from the center containing more bits than zones closer to the center.

• This magnetic disk increases capacity, but still not optimum.• Then CDs and CD ROMs (optical storage device)

introduced but not organize information in concentric tracks.

• Contains a single spiral track beginning near the center and spiraling out to the outer edge of the disk.

• The disk rotates at constant linear velocity (CLV) during read operation


CAV vs CLV

Concentric tracks Spiral tracks

Optical Storage CD-ROM

• CD (Compact Disk) Originally for audio.• CDROM (Compact Disk Read Only Memory)• Digitally recorded information (computer data or

music)• 650Mbytes giving over 70 minutes audio• Polycarbonate coated with highly reflective

coat, usually aluminium or gold• Constant packing density• Constant linear velocity (CLV)


Optical Storage CD-ROM

• Data stored as microscopic pits• Read by reflecting laser-shines through the clear

polycarbonate while motor spins.• The intensity of the laser changes as it encounters

a pit (rough surface-light scatters and low intensity light reflected back to the source).

• Lands-the area between pits. A smooth surface-higher intensity.

• The intensity changes by lands and pits detected by photo sensor and converted into digital signal (0 or 1)


CD Operation


CD-ROM Drive Speeds

• Audio is single speed– Constant linear velocity (CLV)– 1.2 ms-1

– Track (spiral) is 5.27km long– Gives 4391 seconds = 73.2 minutes

• Other speeds are quoted as multiples• e.g. 24x• Quoted figure is maximum drive can

achieve


CD-ROM Format2 bytes of all 0s + 10 bytes of all 1s

• Data on CDROM are organized as a seq. of block• Sync – indicate the beginning of a block• Header – block address and mode byte

– Mode 0=blank data field– Mode 1=2048 byte data+error correction– Mode 2=2336 byte data

• Data – user data• Auxiliary – additional user data in mode 2, mode 1 288 bytes error

correcting codes (ECC)


Inside a CD player


Random Access on CD-ROM

• Difficult

• Move head to rough position

• Set correct speed

• Read address

• Adjust to required location


CD-ROM for & against

• Large capacity (?)

• Easy to mass produce

• Removable

• Robust

• Expensive for small runs

• Slow

• Read only


Other Optical Storage• CD-R (CD-Recordable)

– Also known as WORM-Write Once, Read Many– Now affordable– Compatible with CD-ROM drives– Dye layer change reflectivity activated by high intensity laser

• CD-RW (CD-Rewritable)– Erasable– Getting cheaper– Mostly CD-ROM drive compatible– Phase change

• Material has two different reflectivities in different phase states


DVD – (Digital Versatile Disk)

• Read Only and writeable like CDs

• Bits are packed more closely

• Spiral narrower than the CD one

• Two layers

• Two sides (17 GB)


DVD - technology

• Multi-layer

• Very high capacity (4.7G per layer)

• Full length movie on single disk– Using MPEG (moving pictures experts

group) compression

• Finally standardized


DVD – Writable

• Loads of trouble with standards

• First generation DVD drives may not read first generation DVD-W disks

• First generation DVD drives may not read CD-RW disks


CD and DVD


Magnetic Tape

• Serial access

• Slow

• Very cheap

• Backup and archive


chapter 7 external memory computer architecture chapter seven: external memory

Documents

disk substrate

single readwrite head

head track movable

platter disk removable

disk drive e

sectors addressablemove

fixed bits

g floppy disk