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External MemoryChapter 3 (C)Chapter 3 (C)

CS.216 Computer Architecture and OrganizationCS.216 Computer Architecture and Organization

External Memory

• Magnetic disks

• Compact Disk

• Tape

Magnetic Disks (1/9)

–The disk is a metal or plastic platter coated with magnetizable material

–Data is recorded onto and later read from the disk using a conducting coil, the head

–Data is organized into concentric rings, called tracks, on the platter

Magnetic Disks (2/9)

– Tracks are separated by gaps– Disk rotates at a constant speed -- constant angu

lar velocity• Number of data bits per track is a constant• Data density is higher on the inner tracks

– Logical data transfer unit is the sector• Sectors are identified on each track during the f

ormatting process

Magnetic Disks (3/9)

Magnetic Disks (4/9)

Gap1 Id Gap2 Data Gap3 Gap1 Id Gap2 Data Gap3

TrackSyncByte

Head Sector CRC SyncByte

Data CRC

Track & Sector Format

Magnetic Disks (5/9)

Disk Cylinder

Magnetic Disks (6/9)– Disk characteristics

• Single vs. multiple platters per drive (each platter has its own read/write head)

• Fixed vs. movable head– Fixed head has a head per track– Movable head uses one head per platter

• Removable vs. nonremovable platters– Removable platter can be removed from disk drive for storage of tr

ansfer to another machine

Magnetic Disks (9/9)

•Data accessing times– Seek time -- position the head over the correct t

rack– Rotational latency -- wait for the desired sector

to come under the head– Access time -- seek time plus rotational latency– Block transfer time -- time to read the block (se

ctor) off of the disk and transfer it to main memory

RAID Technology (1/11)

– Disk drive performance has not kept pace with improvements in other parts of the system

– Limited in many cases by the electromechanical transport means

– Capacity of a high performance disk drive can be duplicated by operating many (much cheaper) disks in parallel with simultaneous access

RAID Technology (2/11)

– Data is distributed across all disks

– With many parallel disks operating as if they were a single unit, redundancy techniques can be used to guard against data loss in the unit (due to aggregate failure rate being higher)

– “RAID” developed at Berkeley -- Redundant Array of Independent Disks• Six levels: 0 -- 5

RAID Technology (3/11)

RAID Technology (4/11)– RAID 0

• No redundancy techniques are used• Data is distributed over all disks in the array• Data is divided into strips for actual storage

– Similar in operation to interleaved memory data storage

• Can be used to support high data transfer rates by having block transfer size be in multiples of the strip

• Can support low response time by having the block transfer size equal a strip -- support multiple strip transfers in parallel

RAID Technology (5/11)

RAID Technology (6/11)

– RAID 1• All disks are mirrored -- duplicated

– Data is stored on a disk and its mirror– Read from either the disk or its mirror – Write must be done to both the disk and mirror

• Fault recovery is easy -- use the data on the mirror

• System is expensive!

RAID Technology (7/11)

– RAID 2• All disks are used for every access -- disk

s are synchronized together• Data strips are small (byte)• Error correcting code computed across all

disks and stored on additional disks• Uses fewer disks than RAID 1 but still exp

ensive– Number of additional disks is proportional to log of number of data di

sks

RAID Technology (8/11)

– RAID 3• Like RAID 2 however only a single redunda

nt disk is used -- the parity drive• Parity bit is computed for the set of individu

al bits in the same position on all disks• If a drive fails, parity information on the red

undant disks can be used to calculate the data from the failed disk “on the fly”

RAID Technology (9/11)– RAID 4

• Access is to individual strips rather than to all disks at once (RAID 3)

• Bit-by-bit parity is calculated across corresponding strips on each disk

• Parity bits stored in the redundant disk• Write penalty

– For every write to a strip, the parity strip must also be recalculated and written

– Thus 1 logical write equals 2 physical disk accesses– The parity drive is always written to and can thus be a bottleneck

• Write-Write Concurrent Access is not possible

RAID Technology (10/11)

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Write Penalty RAID 4 & 5

RAID Technology (11/11)

–Raid 5• Parity information is distributed on dat

a disks in a round-robin scheme

• No parity disk needed

• Write-Write Concurrent Access may be possible

Optical Disks (1/5)

– Advent of CDs in the early 1980s revolutionized the audio and computer industries

– Basic operation• CD is operated using constant linear velocity• Essentially one long track spiraled onto the disk• Track passes under the disk’s head at a constant rat

e -- requires the disk to change rotational speed based on what part of the track you are on

• To write to the disk, – Mechanical pressing by a Master Disk– a laser is used to burn pits into the track -- write once (CD-R)!

Optical Disks (2/5)– Basic operation (cont.)

• During reads, a low power laser illuminates the track and its pits

– In the track, pits reflect light differently than no pits thus allowing you to store 1s and 0s

Optical Disks (3/5)

–Master disk is made using the laser– Master is used to “press” copies in a mass production mechanical style– Cheaper than production of information on magnetic disks

– Capacity 650Mbytes giving over 70 minutes audio

– Only economical for production of large quantities of disks

– Disks are removable and thus archival– Slower than magnetic disks

Optical Disks (4/5)

– WORMs -- Write Once, Read Many disks• User can produce CD ROMs in limited quantities• Specially prepared disk is written to using a mediu

m power laser• Can be read many times just like a normal CD RO

M• Permits archival storage of user information, distrib

ution of large amounts of information by a user

Optical Disks (5/5)

– Erasable optical disk• Combines laser and magnetic technology to p

ermit information storage• Laser heats an area that can then have an e-fi

eld orientation changed to alter information storage

• “State of the e-field” can be detected using polarized light during reads

Magnetic Tape (1/2)– The first kind of secondary memory

– Still widely used• Very cheap• Very slow

– Sequential access• Data is organized as records with physic

al air gaps between records• One words is stored across the width of t

he tape and read using multiple read/write heads

Magnetic Tape (2/2)

Summary• Goal of the memory hierarchy is to produce a

memory system that has an average access time of roughly the L1 memory and an average cost per bit roughly equal to the lowest level in the hierarchy

• Range of performance spans 10 orders of magnitude!

• Components / levels discussed– Cache– Main memory– Secondary memory

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