1. Chapter 5 External Memory

2. Contents
• Magnetic Disk
• RAID
• Optical Memory
• Magnetic Type 22

3. Magnetic Disk Data Organization and Formatting
Physical Characteristics
Disk Performance Parameters 22

4. Magnetic Disk
Magnetic disk
Disk is a circular platter constructed of metal or plastic coated with a magnetizable material
Data are recorded on and later retrieved from the disk via a conducting coil named head 22

5. Data Organization & Formatting
Magnetic disk
Tracks
Organization of data on the platter in a concentric set of ring
Adjacent tracks are separated by gaps
Density in the bits per linear inch, increases in moving from the outermost track to the innermost track 26

6. Data Organization & Formatting
Magnetic disk
Data are transferred to and from the disk in blocks
Data are stored in block-size regions known as sectors
There are typically between 10 and 100 sectors per track, and these may be of either fixed or variable length 26

7. Disk Data Layout
Magnetic disk 31

8. Winchester Disk Format
Magnetic disk
Gap1
Gap1 Id
Gap2
Data
Gap3 Id
Gap2
Data
Gap3
Sync
Byte
Track
Sync
Byte
Head
Sector
CRC
Data
CRC

9. Physical Characteristics
Magnetic disk
Head Motion
Fixed head (one per track)
Disk Portablilty
Nonremovable disk
Sides
single sided
double sided
Platters
Single platter
Multiple platter
Head Mechanism
Contact (floppy)
Aerodynamic gap(Winchester)

10. Physical Characteristics
Magnetic disk
Fixed or Movable head
Fixed head
One read write head per track
Heads mounted on fixed ridged arm
Movable head
One read write head per side
Mounted on a movable arm

11. Physical Characteristics
Magnetic disk
Removable or Not
Removable disk
Can be removed from drive and replaced with another disk
Provides unlimited storage capacity
Easy data transfer between systems
Nonremovable disk
Permanently mounted in the drive

12. Fixed-and Movable-Head Disks
Magnetic disk

13. Multiple Platter One head per side Heads are joined and aligned
Magnetic disk
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)

14. Multiple-Platter Disk
Magnetic disk

15. Floppy Disk 8”, 5.25”, 3.5” Small capacity Slow, flexible platter
Magnetic disk
8”, 5.25”, 3.5”
Small capacity
Up to 1.44Mbyte (2.88M never popular)
Slow, flexible platter
Universal
Cheap

16. Winchester Hard Disk Developed by IBM in Winchester (USA) Sealed unit
Magnetic 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
Universal
Cheap
Fastest external storage
Getting larger all the time
Multiple Gigabyte now usual

17. Disk Performance Parameters
Magnetic disk
When the disk drive is operating, the disk is rotating at constant speed
To read or write, the head must be positioned at the desired track and at the beginning of the desired sector on that track
Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed-head system
Seek time
time it takes to position the head at the track is known as seek time

18. Disk Performance Parameters
Magnetic disk
Rotational delay
The time it takes for the beginning of the sector to reach the head
Access time
Sum of the seek time, if any, and the rotational delay
A technique known as rotational positional sensing(RPS) is used
When the seek command has been issued, the channel is released to handle other I/O operations
the seek is completed, the device determines when the data will rotate under the head

19. Timing of a Disk I/O Transfer
Magnetic disk

20. Disk Performance Parameters
Magnetic disk
Seek Time
Time it takes to position the head at the track
Moving head to correct track
Approximate seek time formula
Ts = m x n + s
Ts = estimated seek time
n = number of track traversed
m = constant that depends on the disk drive
s = startup time 33

21. Disk Performance Parameters
Magnetic disk
Rotational Delay
Time it takes for the beginning of the sector to reach the head
Waiting for data to rotate under head
Transfer Time
T = b/(rN)
T = transfer time
B = number of bytes to be transferred
N = number of bytes on a track
R = rotation speed, in revolutions per second

22. Disk Performance Parameters
Magnetic disk
Access time = Seek + Rotational delay
Time it takes to get into position to read or write
Total average access time
Ta = Ts + 1/2r + b/rN
(Ts : average seek time)

23. RAID Rate in performance improvement
Magnetic disk
Rate in performance improvement
Secondary storage << processor, main memory
Disk storage system is the main focus in improving overall computer system performance
Redundant Array of Independent Disks (RAID)
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 Inexpensive Disks
6 levels in common use
Not a hierarchy

24. RAID Levels
Magnetic disk

25. RAID Level 0 It does not include redundancy to improve performance
Magnetic disk
It does not include redundancy to improve performance
Data are striped across the available disks
A set of logically consecutive strips that maps exactly one strip to each array member is referred to as stripe
If a single I/O request consists of multiple logically contiguous strips then up to n strips for that request can be handled in parallel, greatly reducing the I/O transfer time

26. RAID 0(Nonredundant)
Magnetic disk

27. Data Mapping for RAID Level 0 Array
Magnetic disk

28. RAID Level 1 Mirrored Disks Data is striped across disks
Magnetic disk
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

29. RAID 1(Mirrored)
Magnetic disk

30. RAID Level 2 Disks are synchronized Very small stripes
Magnetic disk
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

31. RAID 2
Magnetic disk

32. RAID Level 3 Similar to RAID 2
Magnetic disk
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

33. RAID 3
Magnetic disk

34. RAID Level 4 Each disk operates independently
Magnetic disk
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

35. RAID 4(Block-Level Parity)
Magnetic disk

36. RAID Level 5 Like RAID 4 Parity striped across all disks
Magnetic disk
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
N.B. DOES NOT MEAN 5 DISKS!!!!!

37. RAID 5
Magnetic disk

38. RAID Level 6
Magnetic disk
Two different parity calculations carried out and stored in separate blocks on different disks
User data required N disks consists of N_2 disks.
P, Q : two different data check algorithms
Regenerate data even if two disks containing user data fail.
Advantage if RAID 6
High data availability

39. RAID 6(Dual Redundancy)
Magnetic disk

40. Optical Memory CD-ROM WORM Erasable Optical Disk
Digital Video Disk (DVD)
Magneto-Optical Disks

41. CD-ROM Originally for audio 650Mbytes giving over 70 minutes audio
Optical memory
Originally for audio
650Mbytes giving over 70 minutes audio
Polycarbonate coated with highly reflective coat, usually aluminum
Data stored as pits
Read by reflecting laser
Constant packing density
Constant linear velocity

42. CD-ROM Audio is single speed Other speeds are quoted as multiples
Optical memory
Audio is single speed
Constant linier 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
The quoted figure is the maximum the drive can achieve

43. Optical Disk Products
Optical memory

44. Disk Layout Method
Optical memory
(a) Constant angular velocity

45. Disk Layout Method
Optical memory
(b) Constant linear velocity

46. CD-ROM Format Sync : beginning of a block
Optical memory
Sync : beginning of a block
Header : block address and mode of 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. 00 FF
x 10
Min
Sec
Sector
Mode
Data
Layered
ECC
12 byte
Sync
4 byte Id
2048 byte
288 byte
2352 byte

47. CD ROM Advantage & Disadvantage
Optical memory
Three major advantages of CD-ROM :
Storage capacity is much greater on the optical disk
Optical disk can be mass replicated inexpensively
Optical disk is removable, allowing the disk itself to be used for archival storage
Disadvantages of CD-ROM :
Read only and cannot be updated
Access time much longer than that of a magnetic disk drive, as much as half a second

48. WORM Write-once read-many CD
Optical memory
Write-once read-many CD
More expensive disk controller than CD-ROM
CD-writable

49. Erasable Optical Disk Can be repeatedly written and overwritten.
Optical memory
Can be repeatedly written and overwritten.
Can be used secondary storage
CD-RW
Advantages
High capacity
Portability
Reliability

50. Digital Video Disk Used to indicate a player for movies Multi-layer
Optical memory
Used to indicate a player for movies
Only plays video disks
Multi-layer
Very high capacity (4.7G per layer)
Full length movie on single disk
Using MPEG compression
Finally standardized (honest!)
Movies carry regional coding
Players only play correct region films
Can be “fixed”

51. Magnetic Tape Serial access Slow Very cheap Backup and archive

52. Nine-Track Magnetic Disk Format
Magnetic tape