Read “engineering” here as “Engineering & Computer Science”

Chapter 6

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—DVD-R—DVD-RW

• Magnetic Tape

Magnetic Disk

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

• Substrate originally was aluminium Is now glass

—Improved surface uniformity– Increases reliability

—Reduction in surface defects– Reduced read/write errors

—Lower flight heights (head rides on air gap)—Better stiffness—Better shock/damage resistance

Disk Data Layout - Platter

Tracks and Cylinders

Multiple Platters

Disk Layout Methods Diagram

Physical Characteristics of Disk Systems

Inductive Write MR Read

Typical Hard Disk Drive Parameters

Formating

• Must be able to identify position of data: start of track and sector

• Formatting disk:—Additional information not available to user—Marks tracks and sectors

Winchester Disk Format (Seagate ST506)

30 fixed-length sectors per track

Speed

• Seek time—Time to position head at track

• Latency (Rotational) —Time for head to rotate to beginning of sector

• Access time - Seek time + Latency time

• Transfer rate - The rate at which data can be transferred after access T = b / N * 1/r Transfer time = bytes transferred / bytes/track * sec/revolution

Note: How does organization on disk (e.g. random vs sequential) effect total time?

Timing of Disk I/O Transfer

RAID – Goals: Speed, Reliability, Standardization

• Redundant Array of Independent Disks (or Redundant Array of Inexpensive Disks ?)

• Set of physical disks viewed as single logical drive by O/S• Data distributed across physical drives• Can use redundant capacity to store parity information • 7 “levels” of RAID organization (not a hierarchy)

— 0 not really a RAID organization (no redundancy)— 1, 3 used for high transfer rate— 5, 6 used for high transaction rate— 2, 4 not commercially available

• Requirements for high transfer rate:—High transfer rate alone entire path between host

memory to disk drives—Application must make I/O requests that drive disks

efficiently

Redundant Array of Independent Disks

RAID 0, 1, 2

Data Mapping For RAID 0

RAID 0

• No redundancy (Not “really” RAID)

• 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 (not used)

• 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— Very Expensive— Not used commercially

RAID 3 & 4

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• Not very expensive or complex

RAID 4 (not used)

• 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

• Good for high request rates rather than high transfer rates

• Every write impacts the parity disk so it becomes a bottleneck.

• Not used commercially

RAID 5 & 6

RAID 5

• Very similar to 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 (two parity calculations)

Redundant Array of Independent Disks

RAID Comparison (1)

Raid Comparison (2)

Types of External Memory

• Magnetic Disk—RAID (Redundant Array of Independent Disks)—Removable

• Optical—CD-ROM—CD-Recordable (CD-R)—CD-R/W—DVD—DVD-R—DVD-RW

• Magnetic Tape

Optical Products

Optical Storage CD-ROM

• Originally for audio• 650Mbytes giving over 70 minutes audio• Polycarbonate coated with highly

reflective coat, usually aluminium• Data stored as pits• Read by reflecting laser• Constant packing density• Constant linear velocity

CD Construction

CD Layout

Size Perspective

CD reader

CD-ROM Drive Speeds

• Audio is single speed—Constant linear velocity—1.2 m/sec—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

• Note: CD-ROM has option of error correction

(not on CD)

CD-ROM Format

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

Random Access on CD-ROM

• Difficult

• Process:—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-Recordable (CD-R)—WORM (Write once, read many)—Now affordable—Compatible with CD-ROM drives

• CD-RW—Erasable—Getting cheaper—Mostly CD-ROM drive compatible—Phase change

– Material has two different reflectivities in different phase states

DVD - technology

• Multi-layer

• Very high capacity (4.7G per layer)

• Full length movie on single disk—Using MPEG compression

CD vs DVD

DVD’s

Two objectives had to be resolved to make the DVDs viable.

• The linear velocity of a DVD must be held constant and be able to reproduce a vertical frame rate of 29.97 frames/second

• Every DVD player had to have absolute tracking accuracy to insure the extremely narrow laser beam would scan exactly in the middle of the track where the data was recorded. 

• The solution: The disk is pressed with the track grooves accurately pre-cut and encoded with a constant bit rate frequency. Thus a blank DVD disk isn't really blank at all.

DVD-R

• The pre-grooves in the case of  DVD-R and DVD-RW discs, are not perfect spirals. Instead, the groove is modulated with a constant frequency of 140.6 kHz, known also as the wobble frequency (since the groove actually wobbles !)  Much like a lateral cut phonograph groove, groove wobbling means that the grooves wander back and forth in sinusoidal fashion at a fixed amplitude. This constant frequency allows accurate tracking by the laser as well as provides a highly accurate timing signal to which the write clock frequency is derived.

• Between the grooves are the pre-pits.  The pre-pits contain the sector addressing information.

DVD+R

• The +R format pre-groove also uses a wobble frequency, but at a much higher frequency 817kHz. Instead of pre-pits, the R+ formats convey the sector addressing information by frequency modulation of the wobble frequency.

Types of External Memory

• Magnetic Disk—RAID (Redundant Array of Independent Disks)—Removable

• Optical—CD-ROM—CD-Recordable (CD-R)—CD-R/W—DVD—DVD-R—DVD-RW

• Magnetic Tape

Magnetic Tape

• Serial access

• Slow

• Very cheap

• Used for backup and archive