Chapter 6 External Memory

1
Chapter 6External 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
• DVD-R
• DVD-RW
• Magnetic Tape

3
Magnetic Disk

• Disk substrate coated with magnetizable material
  (iron oxiderust)
• 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

4
Disk Data Layout
5
Tracks and Cylinders
6
Multiple Platters
7
Disk Layout Methods Diagram
8
Physical Characteristics of Disk Systems
9
Inductive Write MR Read
10
Typical Hard Disk Drive Parameters
11
Formating

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

12
Winchester Disk Format (Seagate ST506)
30 fixed-length sectors per track
13
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

14
Timing of Disk I/O Transfer
15
RAID Goals Speed, Reliability, Standardization

• Redundant Array of Independent Disks
• (Redundant Array of Inexpensive Disks)
• 6 (of 7) levels in common use
• 0, 1, 3 used for high transfer rate
• 4, 5, 6 used for high transaction rate
• 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

16
Redundant Array of Independent Disks Levels
17
RAID 0, 1, 2
18
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

19
Data Mapping For RAID 0
20
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

21
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

22
RAID 3 4
23
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

24
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

25
RAID 5 6
26
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

27
RAID 6

• Two parity calculations
• Stored in separate blocks on different disks
• User requirement of N disks needs N2
• High data availability
• Three disks need to fail for data loss
• Significant write penalty

28
RAID Comparison (1)
29
Raid Comparison (2)
30
Optical Products
31
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

32
CD Construction
33
CD Layout
34
Size Perspective
35
CD reader
36
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

37
CD-ROM Format

• Mode 0blank data field
• Mode 12048 byte dataerror correction
• Mode 22336 byte data

38
Random Access on CD-ROM

• Difficult
• Move head to rough position
• Set correct speed
• Read address
• Adjust to required location

39
CD-ROM for against

• Large capacity (?)
• Easy to mass produce
• Removable
• Robust
• Expensive for small runs
• Slow
• Read only

40
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

41
DVD - whats 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

42
DVD - technology

• Multi-layer
• Very high capacity (4.7G per layer)
• Full length movie on single disk
• Using MPEG compression

43
CD vs DVD
44
DVDs

• 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.

45
DVDs

• 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.

46
DVDs

• 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.

47
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.

48
DVDR

• 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.

49
Magnetic Tape

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