1. Chapter4: Memory
External Memory

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
Magnetic Tape
Magnetic disks – most important component of external memory range from PC to mainframe and super computer
RAID disk technology used to achieve great performance and higher availability
Optical storage technology become increasingly important for all types of computer systems. CD and DVD are becoming increasingly important

3. Disk substrate coated with magnetizable material (iron oxide…rust)
Magnetic Disk
Disk substrate coated with magnetizable material (iron oxide…rust)
Substrate used to be aluminium or aluminium alloy material
Now glass
Improved surface uniformity
Increases reliability
Reduction in surface defects
Reduced read/write errors
Lower flight heights
Better stiffness
Reduce disk dynamics
Better shock/damage resistance
A disk is a circular platter constructed of nonmagnetic material called substrate
Traditionally, substrate has been an aluminium or aluminium alloy material
More recently, glass substrate have been reduced
5 advantages of using glass

4. Read and Write Mechanisms
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
Used in floppy disk systems
Read (contemporary):
Separate read head, close to write head
Consists of partially shielded magnetoresistive (MR) sensor
Electrical resistance depends on direction of magnetic field
High frequency operation
Higher storage density and speed
Write: electric pulses sent to write head, and the resulting magnetic pattern recorded on surface below with different pattern for + and – current
Read : exploit the fact that magnetic field moving relative to a coil produces electrical current in the coil. When surface passes under the head, it generates current of the same polarity as the one already recorded
Read contemporary: by passing current thru MR sensor, resistance changes are detected as voltage signals. MR Design allows high frequency operation, which equates to greater storage densities and operating speeds.

5. Inductive Write MR Read
Inductive Write/MR Read Head
Current induces magnetic field across the gap, in turn magnetizes small area of recording medium
Reversing current direction reverses the direction of magnetization on the recording medium

6. Data Organization and Formatting
Concentric rings or tracks
Gaps between tracks
Reduce gap to increase capacity
Same number of bits per track (variable packing density)
Constant angular velocity
Tracks divided into sectors
Minimum block size is one sector
May have more than one sector per block
Head is relatively small device capable of reading from and writing to a portion of a platter rotating beneath it.

7. Disk Data Layout
To avoid imposing unreasonable precision requirement on the system
Error due to misalignment or simply interference of magnetic field
Hundreds of sectors per track. May be fixed or variable length. Most contemporary systems, fixed-length are used
Disk data layout
Adjacent tracks are separated by gaps. This to prevent/minimizes error
Data are transferred to and from the disk in sectors

8. 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 (density)
Each zone has fixed bits per track
More complex circuitry (allows for greater overall storage capacity)
Density = in bits per linear inch
Some way must found to compensate for the variation in speed so that the head can read all the bits at the same rate
Information can then be scanned at the same rate by rotating the disk at the fixed speed, known as CAV
Constant = fixed speed
Zones = multi zone

9. Disk Layout Methods Diagram
The layout of a disk using CAV (b) The nature of multiple zone recording

10. Must be able to identify start of track and sector Format disk
Finding Sectors
Must be able to identify start of track and sector
Format disk
Additional information not available to user
Marks tracks and sectors

11. Winchester Disk Format Seagate ST506
Each track contains 30 fixed length sectors of 600 bytes each
Each sector holds 512 bytes of data plus control infos useful to the disk controller
ID field used to locate a particular sector & SYNCH byte delimits the beginning of the field
Track# identifies a track on a surface & Head# identifies a head because this disk has multiple surfaces
ID and data field, each contain an error detecting code
ID field, unique identifier/address
SYNCH byte, a special bit pattern

12. Physical Characteristics
Head motion
Fixed (rare) or movable head
Disk portability
Removable or fixed
Sided
Single or double (usually) sided
Platter
Single or multiple platter
Head mechanism
Contact (Floppy)
Fixed gap
Flying gap (Winchester)

13. Fixed/Movable Head Disk
Fixed head
One read-write head per track
Heads mounted on fixed ridged arm, extends across all the tracks
Movable head
One read write head per side
Mounted on a movable arm
Arm can be extended/retracted for positioning above the track
Fixed head – very rare today

14. Sided/Double Sided
Most disks: magnetizable coating applied to both side
Only cheap disk use single-sided

15. Removable or Not Removable disk Nonremovable disk
Can be removed from drive and replaced with another disk
Provides unlimited storage capacity
Easy data transfer between systems
Eg: floppy disk, ZIP cartridge
Nonremovable disk
Permanently mounted in the drive
Eg: hard disk in a PC

16. Heads are joined and aligned
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)

17. Multiple Platters 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)

18. Tracks and Cylinders
Cylinder: set of all the tracks in the same relative position on the platter
All the shaded tracks are part of one Cylinder
Head mechanism- physical contact with medium during read/write operation

19. Floppy Disk 8”, 5.25”, 3.5” Small capacity Slow Universal Cheap
Up to 1.44Mbyte (2.88M never popular)
Slow
Universal
Cheap
Obsolete?

20. Winchester Hard Disk (1)
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

21. Winchester Hard Disk (2)
Universal
Cheap
Fastest external storage
Getting larger all the time
250 Gigabyte now easily available

22. Access time = Seek + Latency Transfer rate
Speed
Seek time
Moving head to correct track
(Rotational) latency
Waiting for data to rotate under head
Access time = Seek + Latency
Transfer rate
Time to/from the disk depends on the rotation speed of the disk
T = b/rN (b= no. Of bytes to be transferred
r = rotation speed
N = no of bytes on a track)

23. Timing of Disk I/O Transfer
Timing of a Disk I/O transfer
disk I/O operation depends on the computer system, OS, nature of I/O channel and disk controller hardware

24. Assignment#2 – RAID What is RAID?
RAID has a total of seven levels. Briefly explain for each of them.
Due date: 31Oct2011