1. Introduction to Flash Memories And Flash Translation Layer
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한국 외국어대학교 디지털정보공학과
System Software LAB

2. Storage Technologies Magnetic recording Optical recording
Electronic memories

3. Applications of Flash Memories
Flash memory is the major type of NVM
(more than 90% of NVM market)
mobile devices
flash disks
embedded systems

4. What is a flash memory cell?

6. Physics of Flash Memory
Cell array in a flash memory
A flash memory cell
(Floating gate)
A flash memory cell can store charge. And the charge level represents data.

7. How does a cell store a bit?

8. How does a cell store a bit?1
Inject electrons: Hot electron injection mechanism, or Fowler-Nordheim tunneling mechanism
Remove electrons: Fowler-Nordheim tunneling mechanism

9. Single-level cell and Multi-level cell
Single-level cell: Two levels  One bit 1
cell
cell
Multi-level cell: q levels  bits 1 2
q-1
cell
cell
cell
cell
Typical number of cell levels: 2, 4, 8, 16

10. How is a cell programmed?
Through multiple rounds of charge injection
Source: [Bandyopadhyay, Serrano, Hasler 2005]
Target level
A flash cell

11. Speed and physical limits
Speed of operations:
Read: Fast
Write: Slower (due to multiple rounds of programming)
Erase: Very slow
Physical limits:
Endurance. (In NOR, a block can stand about 100,000 to
1,000,000 erasures.
In NAND, it can stand 10,000 to 100,000 erasures.)
Physical size (e.g., 34nm).
Voltage.
Number of electrons.

12. NOR and NAND Flash Memories
NOR: Older, still used.
NAND: Newer, much more popular now.

13. What is a NOR flash memory
Cells form blocks. A block
has about 100,000 cells.
2. NOR is a random-access device. Every cell is directly addressable by the processor. That is, a cell can be individually read and programmed.
Word line
Bit line
Control gate
Floating gate
(-)
Oxide layer
Drain
Source
flash memory cell

14. What is a NOR flash memory
Block erasure!!!!!!

15. Block erasure
In NOR, the level of a cell can be increased individually and
multiple times. But to lower any cell level, the whole block
must be erased at the same time.
Block
block of cells

16. What is a NAND flash memory
Cells form blocks. Every block is an array. Every row is a page.
Block
page
page
Typically: 32 to 128 rows (pages)
page
page
Read and write:
A page as a unit.
Typically: 512 to 2048 cells
in a row (page)
Block erasure!!!!!!

17. Writing a page in NAND
A page can be written only once before the block is erased.
It is even recommended that the pages are written sequentially.
Page 1
Partial writing: A page is partitioned
into 4 parts, and we can write a part
at a time.
Page 2
…………
Page 64
Part 1
Part 2
Part 3
Part 4
A page
Note: This is logic partition.
Why?: Programming is not very accurate, especially with
multiple times of writing (for the same page, and
for the interference between pages).

18. A typical NAND page with spare bytes
64 Bytes of spare area
A page:
2KB of data
Metadata
ECC
Undefined bits

19. Comparison of NOR & NAND flash
Basic difference: Different ways to connect cells in a block.
Additional difference: Ways to inject charge, used voltages.
NOR: cells
are independent
NAND: Cells in the
same column are
connected (and disturb
each other).

20. Comparison of NOR & NAND flash
1. Lower density.
1. Higher density.
2. Random access.
2. Page access.
3. More reliable.
3. Less reliable, error-prone.
(Requires ECCs.)
4. Slower erase.
4. Faster erase.
5. Faster random read.
5. Faster streaming read.
6. Mainly to store code.
6. Mainly to store data.

21. Flash File System
Wear leveling
Garbage collection
Mapping

22. Wear leveling
Wear leveling: Let the blocks be erased about the same number of times.
Method: Write data in different places (instead of the same block).
How to know the block’s level of wearing out:
Count the number of erasures, or
Measure the performance of its cells (e.g., erase latencies), or
Other methods?
Alternative approach: Just use randomization (i.e., randomly use
the blocks, and hopefully, things will even out).

23. Wear leveling techniques
Simple case:
If all the data in a block are obsolete, just erase it.
Write in blocks that are less worn out.
What if the blocks contain both obsolete and valid data?
Page 1: valid
Page 2: obsolete
Page 3: obsolete
Page 4: obsolete
…………
Page 64: valid

24. Combining wear leveling with garbage collection
This happens when we want to re-use those blocks that contain
both valid and invalid (obsolete) data.
Approaches:
Use a cost/benefit ratio to decide which block to erase.
(Before erasing it, the valid data need to be moved first.)
(2) Store frequently-changing data together, and store data
that do not change much together. (Reason: After a while,
in a block containing frequently-changing data, most of the
data are probably already invalid.)
(3) Many heuristic approaches. (And many patents.)
Most important: Design it based on the application.

25. When garbage collection is done
Garbage collection (of blocks) can happen when:
As background work, i.e., when CPU is idle; or
(2) On demand, i.e., when there is not enough free space.

26. Mapping How to use flash memories to store data?
One approach: Treat the flash memory as a block device, much like disk sectors.
Advantage: Allow standard file systems to use flash.
Problems with a simple linear mapping from virtual blocks to flash-memory pages:
Some blocks can be erased too often.
Unable (or inefficient) to write data smaller than a flash block.
Solution: Wear leveling (that is, to move data around).
Mapping between virtual blocks and physical pages is needed.
The spare part in a page may have bits indicating if the page is free/used or
valid/obsolete.

27. Mapping Direct map Virtual blocks Physical pages Inverse map
Stored in RAM, or partially
in RAM and partially in flash.
Inverse map
Physical pages Virtual blocks
Stored in flash.
Flash Translation Layer (FTL):
A technique to
store some of the direct map in flash, and
reduce the cost of updating the maps stored in flash.

28. Flash file systems
Tens of flash file systems (FFS) have been designed.
Clearly, more of them will be designed……
A flash file system:
Is a data structure that represents a collection of mutable
random-access files in a hierarchical name space.
Provides the block mapping technique.
Does wear leveling and garbage collection.
Maybe design a different, more flash-specific file system?
Do the same for data structures, such as B-trees and R-trees.

29. More flash-specific file systems
Most of the flash-specific file systems use the same overall principle, that of a log-structured file system.
Why? It is easier to record the small changes (and write them down sequentially), than to rewrite the whole file.

30. What happens when the blocks
are read/erased/written
again and again…

31. Disturb mechanisms
Write/read disturb: When a cell is programmed (or read), the cells in
the same column/row are softly programmed.
For some MLC, it is even recommended that after reading the same page
1000 times, write the clean data back again.

32. Errors, Signal processing, and ECCs
When a block is erased, its quality goes down.
The rate of errors increases…
Types of errors:
Random errors.
Fixed-position errors (because the cells really become
defected).
Cells in the same column can become bad together.
Ways to correct errors:
Signal processing
ECCs (Hamming, BCH codes.)
(Reed-Solomn codes? LDPC codes? Under study.)

33. New Area in Information Theory:
Coding for Flash Memories

34. Summary of recent results
Rewriting codes:
Worst-case performance: [Jiang,Bohossian,Bruck,ISIT’07], [Bohossian,Jiang,Bruck,ISIT’07],
[Jiang,Bruck,ISIT’08], [Yaakobi,Siegel,Vardy,Wolf,Allerton’08],
[Jiang,Langberg,Schwartz,Bruck,ISIT’09],
[Mahdavifar,Siegel,Vardy,Wolf,Yaakobi,ISIT'09]
Expected performance: [Finucane,Liu,Mitzenmacher,Allerton’08], [Jiang,Langberg,Schwartz,Bruck,ISIT’09]
Rank modulation:
Rewriting: [Jiang,Mateescu,Schwartz,Bruck,ISIT’08]
Error-correction codes: [Jiang,Schwartz,Bruck,ISIT’08]
Sequences: [Jiang,Mateescu,Schwartz,Bruck,ISIT’08], [Wang,Jiang,Burck,ISIT’09]
Capacity: [Jiang,Bruck,ISITA’08], [Lastras-Montano,Franceschini,Mittelholzer,Sharma,ISITA'08],
[Lastras-Montano,Franceschini,Mittelholzer,Karidis,Wegman,ISIT'09], [Jiang,Li,PACRIM’09]
Error correcting/scrubbing codes: [Cassuto,Schwartz,Bohossian,Bruck,ISIT’07], [Jiang,ISIT’07],
[Jiang,Li,Wang,CWIT’09]
Data movement: [Jiang,Mateescu,Yaakobi,Bruck,Siegel,Vardy,Wolf,ISIT’09],
[Jiang,Langberg,Mateescu,Bruck,Allerton’09]

35. Rewriting codes WOM (write-once memory) code
Floating code: Joint coding of multiple variables
Example: 2 bits are stored in 3 cells with 4 levels. Every time one bit is changed.
How many rewrites can be supported?
0,1
1,1
1,1
1,0
0,1
0,0
1,0
0,0
Now use floating codes.

36. Floating codes cell levels data
Example: 2 bits are stored in 3 cells with 4 levels. Every time one bit is changed.
cell levels
data

37. Rewriting codes 3 writes 7 writes WOM (write-once memory) code
Floating code: Joint coding of multiple variables
Example: 2 bits are stored in 3 cells with 4 levels. Every time one bit is changed.
How many rewrites can be supported?
3 writes
7 writes
0,1
1,1
0,0
1,0
Now use floating codes.
0,1
1,1
0,0
1,0

38. Floating codes
When two binary variables are stored in n cells of q levels, an optimal
floating code can support rewrites.
When k variables of alphabet size L are stored in n cells of q levels, the
number of rewrites that a floating code can support is:
If n is large  rewrites
times better
If k,L are large  Roughly rewrites
No coding  Roughly rewrites

39. More general model for rewriting
Floating codes: Every rewrite changes one variable.
011
111
State transitions of data
001
101
Example: 3 binary variables
Hypercube
010
110
000
100
Buffer codes: Remember most recent data. [BJB’07]
State transitions of data: De Bruijn graph
More general: [JLSB’09]
Maximum degree:
The data change in a
bounded-degree graph.

40. Trajectory code for bounded-degree rewrite
Model: The state-transition diagram of the data has bounded
degree
This code is asymptotically optimal.
[Jiang, Langberg, Schwartz, Bruck, ISIT’09]

41. Rank Modulation
[J, Mateescu, Schwartz, Bruck, ISIT’08]

42. Cell Programming Noisy, monotonic Trend: more levels, smaller cells
Question: How to write data
reliably when cells cannot be
programmed reliably?
Challenges: overshoot, worst-case constraint.
Approach: adaptive cell-ensemble programming.
Rank modulation is such an approach.

43. Rank Modulation Analog cell levels induce permutations.
Example: 3 cells can induce 3!=6 permutations
Permutations represent data.
Method of programming: from low to high.
Advantage: no overshoot, adaptive coding.
123
132
213
231
312
321

44. Rewriting; Error correction
Rewrite: How to rewrite data in the rank modulation scheme?
Error correction: How to design error-correcting codes? What does error mean?

45. A few other topics: (3) Data movement
Block 1
Block 2
Block n
Empty block
No coding:
erasures are needed.
With coding:
erasures are needed.
[Jiang, Mateescu, Yaakobi, Bruck, Siegel, Vardy, Wolf, ISIT’09]

46. Summary of recent results
Rewriting codes:
Worst-case performance: [Jiang,Bohossian,Bruck,ISIT’07], [Bohossian,Jiang,Bruck,ISIT’07],
[Jiang,Bruck,ISIT’08], [Yaakobi,Siegel,Vardy,Wolf,Allerton’08],
[Jiang,Langberg,Schwartz,Bruck,ISIT’09],
[Mahdavifar,Siegel,Vardy,Wolf,Yaakobi,ISIT'09]
Expected performance: [Finucane,Liu,Mitzenmacher,Allerton’08], [Jiang,Langberg,Schwartz,Bruck,ISIT’09]
Rank modulation:
Rewriting: [Jiang,Mateescu,Schwartz,Bruck,ISIT’08]
Error-correction codes: [Jiang,Schwartz,Bruck,ISIT’08]
Sequences: [Jiang,Mateescu,Schwartz,Bruck,ISIT’08], [Wang,Jiang,Burck,ISIT’09]
Capacity: [Jiang,Bruck,ISITA’08], [Lastras-Montano,Franceschini,Mittelholzer,Sharma,ISITA'08],
[Lastras-Montano,Franceschini,Mittelholzer,Karidis,Wegman,ISIT'09], [Jiang,Li,PACRIM’09]
Error correcting/scrubbing codes: [Cassuto,Schwartz,Bohossian,Bruck,ISIT’07], [Jiang,ISIT’07],
[Jiang,Li,Wang,CWIT’09]
Data movement: [Jiang,Mateescu,Yaakobi,Bruck,Siegel,Vardy,Wolf,ISIT’09],
[Jiang,Langberg,Mateescu,Bruck,Allerton’09]