Where we are survey

• Things are generally going well

• The HW problems are: 1) challenging – maybe need more guidance 2) sometimes unclear – maybe need explanation

• We need a consistent break in class: - one hour is max concentration time - need 15 min break

Announcements

• HW on Chap 6 6.3 & 6.4 Due Mon 10/30/06

• Project #1 Due 10-25-06 Wed 11/1/06 • Midterm #1 10-30-06 Covers Chaps 3 thru 6

Chapter 6External 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 used to be aluminium• Now glass

—Improved surface uniformity– Increases reliability

—Reduction in surface defects– Reduced read/write errors

—Lower flight heights (See later)—Better stiffness—Better shock/damage resistance

Disk Data Layout

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 start of track and sector

• Format 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

• (Rotational) latency—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

Timing of Disk I/O Transfer

RAID

• Redundant Array of Independent Disks (Redundant Array of Inexpensive Disks)• 6 levels in common use• Not a hierarchy• 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

Redundant Array of Independent Disks Levels

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

• 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 0, 1, 2

Data Mapping For RAID 0

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 3 & 4

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

RAID 5 & 6

RAID Comparison (1)

Raid Comparison (2)

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 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 Format

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

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-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 - what’s in a name?

• Digital Video Disk ?—Used to indicate a player for movies

– Only plays video disks

• Digital Versatile Disk ?—Used to indicate a computer drive

– Will read computer disks and play video disks

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

Three objectives had to be resolved to make the DVD a financially viable medium.

• First:.......  The linear velocity of a DVD must be held constant and be able to reproduce a vertical frame rate of 29.97 frames/second to meet RS-170A specifications for sync signals to maintain compatibility with the rest of the video world (at least in the case of creating a Video DVD). Consider that if the rpm is held constant, then the linear velocity will be quite different from the inner tracks compared to the outer tracks. Thus there must be a mechanism  to measure the linear velocity and accurately adjust the disk rpm to maintain a constant linear velocity.

DVD’s

• 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.  Consider that the track width on a DVD is only .74 um (microns) in width - which is much smaller than a single hair which is typically 50 microns in diameter. Approximately 67 DVD grooves would fit rather nicely in the width of a single human hair !  Add to that, the vagaries of rotating mechanical hardware, fluctuating power line voltages etc, and it became obvious this was not going to be an easy inexpensive task if conventional design approaches were taken.

DVD’s

• Third: .......  The real engineering "killer" was that the DVD player had to be made affordable if it ever was to be a viable product.  Building sophisticated tracking electro-mechanical mechanisms into each DVD player and have them remain compatible with high repeatable accuracy across different manufacturer's product lines and media offerings, was not an option. No way that was ever gonna work at an even semi - reasonable price tag.  Add to that the mechanism being jostled about in shipping etc, and it was a real engineering challenge.  So some clever engineers dreamt up a system whereby each "blank" DVD was to have what is known as  pre-grooves.  Thus a blank DVD disk isn't really blank at all. The disk already is pressed with the track grooves accurately pre-cut and encoded with a constant bit rate frequency.

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.

Magnetic Tape

• Serial access• Slow• Very cheap• Backup and archive