1. Flash memory Yi-Chang Li
Dept. Computer Science and Information Engineering
National Taiwan University
Advisor: Prof. Chia-Lin Yang

2. Outline OS support for flash Design on heterogeneous flash (SLC+MLC)
Efficient evaluation method for wear-leveling 2

3. Outline OS support for flash Design on heterogeneous flash (SLC+MLC)
Efficient evaluation method for wear-leveling 3

4. OS Support for Flash
Windows Vista provides two supports for flash memory
Support for hybrid drive
ReadyDrive
Support for flash plug in device
ReadyBoost
Reference: Windows PC Accelerators white paper

5. Support for Hybrid Disk - ReadyDrive
A feature to support the use of hybrid disk
Buffer write requests on the flash
Allows the disk to stay spun down longer and save power
Prefetch data to dedicate space on flash
Requirements on hybrid disk:
At least 50 MB of nonvolatile flash storage (NV cache) capacity
NV caches must perform at
4 MB/s for random 4K reads and writes
16 MB/s for 64K sequential reads
8 MB/s for 64K sequential writes
Recommendations on hybrid disk
256 MB to 1 GB of NV cache capacity; more is better
Wear-leveling algorithms to ensure longevity of the NV cache
Reference: Windows PC Accelerators white paper

6. Support for USB Flash - ReadyBoost
A feature to expand main memory size by plugging in a flash drive
Requirements on flash storage
USB flash drives must use USB 2.0 standard
Contain at least 230 MB of free capacity
Perform at 2.5 MB/s for random 4K reads and 1.75 MB/s for random 512K writes
要再看懂…
Reference: Windows PC Accelerators white paper

7. Outline OS support for flash Design on heterogeneous flash (SLC+MLC)
SLC, MLC, dual-mode flash cell
Related works
Our current progress
Efficient wear-leveling testing method 7

8. Outline OS support for flash Design on heterogeneous flash (SLC+MLC)
SLC, MLC, dual-mode flash cell
Related works
Our current progress
Efficient wear-leveling testing method 8

9. What Is SLC / MLC / Dual-Mode Flash Cell
SLC ( Single-Level Cell )
Store one bit of data in each cell
MLC ( Multi-Level Cell )
Store more than a single bit of information in each cell
Dual-mode flash cell
A flash cell can be configured to SLC or MLC mode
MLC-mode
MLC-
mode
SLC- mode
faster access

10. Comparison of SLC and MLC
NAND – SLC
NAND - MLC
Page Read
25 us
50~60 us
Page Write
200 us
600~800 us
Erase Latency
1.5 ms
1.5~3.3 ms
Block Endurance
100K
10K
Active Power
3.3 V / 15 mA
Stand-by Power
3.3 V / 10 uA
Cost per GB (mid 2007)
$10.37
$6.81
16GB (2Gx8)
$11.9
$2.22 2x
3x~4x
Performance
1x~2x
Life time
10x
Cost, (Capacity) 10

11. Design A Hybrid Flash Device
Design goal of a hybrid flash device
Performance close to SLC &
Cost close to MLC
General approach
Combine a small SLC and a large MLC
Maintain performance by data placement policy

12. Issues for Data Allocation
Performance & Energy
Allocate frequently accessed data (read + write) to SLC
Issue: How much should be allocated to SLC?
GC in SLC might hurt performance & energy
Endurance
Allocate frequent writes to SLC
Erasure count between SLC and MLC should be balanced

13. Outline OS support for flash Efficient wear-leveling testing method
Design on heterogeneous flash (SLC+MLC)
SLC, MLC, dual-mode flash cell
Related works
Our current progress 13

14. Related Works
Hybrid Solid-State Disks: Combining Heterogeneous NAND Flash in Large SSDs (ASPDAC’08)
Improving NAND Flash Based Disk Caches ( ISCA’08) 14

15. Hybrid Solid-State Disks
Add a small SLC to improve average response time
20 GB MLC MB SLC, achieve 17% throughput and 14 % energy consumption improvement
Management of the additional SLC
Sequential write & Garbage collection
Phase out any valid data found in victim block to MLC
Free block
Used block
Direction of write & GC 15 15

16. Hybrid Solid-State Disks
Data placement policy
Hot data  SLC, cold data  MLC
Hot-cold identification: request size
Wear-leveling between SLC and MLC
If the wearingSLC > 10 * wearingMLC
Reduce write on SLC
1. Reject writes to data that do not already exist in the SLC flash
2. Decrease the GC threshold in SLC
Ultra Mobile PC
with following applications:
1. Web browsers,
2. clients,
3. movie players,
4. FTP clients,
5. office suites,
6. games

17. NAND Flash Based Disk Cache
Use a dual-mode NAND flash that stores 2 bits per cell MLC and is capable of switching from MLC to SLC mode
A page/block of dual-mode flash
When to switch from MLC to SLC?
Reduce cell density from MLC to SLC mode to enhance cache reliability
Migrate data on frequently read MLC page to a new empty SLC page 17

18. What’s Missing in These Works ?
For Hybrid SSDs
No concern about frequently read data
For both
No concern of the GC effect

19. Outline OS support for flash Design on heterogeneous flash (SLC+MLC)
SLC, MLC, dual-mode flash cell
Related works
Our current progress
Efficient wear-leveling testing method 19

20. Our Current Progress An analytic model for data placement
Utilize dual-mode flash cell in SSDs
Decide a SLC/MLC partition to optimize for performance or energy consumption

21. An Analytic Model for Data Placement
Refer to an analytic model when allocating a newly arriving data
SLC
Analytic model
write
MLC

22. An Analytic Model to Decide Data Placement
Factors considered in the analytic model
Average execution time
Different operation latency between SLC and MLC
How does characteristic of this data affects GC in SLC/MLC?
Frequently written data might increase GC frequency
Infrequently written data might increase GC overhead
ex.
Flash endurance
Erasure counts between SLC/MLC should be balanced 22

23. Utilize Dual-Mode Flash Cell in SSDs
Effects in different SLC/MLC partition:
Larger SLC
Better performance
Less energy consumption from shorter latency
Higher endurance level
Larger MLC
Larger capacity, less GC frequency
Less energy consumption from less GC frequency
We can adjust SLC/MLC partition to
Optimize for system performance
Optimize for energy consumption 23

24. Outline OS support for flash Design on heterogeneous flash (SLC+MLC)
Efficient evaluation method for wear-leveling 24

25. Efficient evaluation method for wear leveling
Evaluation metrics for wear-leveling
Standard deviation of erasure count
Maximum erasure count of flash block
However, we are not able to obtain erasure counts from a flash product
Existing wear-leveling evaluation method on flash products runs benchmarks on a flash until a worn-out block occurs
Waste a new flash drive
Take a long time

26. Observations An interesting phenomenon
More writes to a flash  Higher write speed
Possible reason: More writes  Thinner tunnel oxide
A correlation between write speed and erase count exists
Control Gate
Floating Gate
Substrate
Drain
Source
Tunnel Oxide
Architecture of a Flash Cell

27. Hypothesis Erasure count ↑  Write speed ↑
Erasure count = fErase-Write(write-speed)
With the same fErase-Write, good wear-leveling means
Smaller write speed deviation of blocks
Slower maximal write speed

28. How To Verify These Hypotheses?
Hypothesis 1: Erasure count ↑  Write speed ↑
With a programmable flash controller
Issue “erase(block0) + write(block0)” until block 0 is worn out
Measure write speed for each write
Test 2~ 3 kinds of flash chips
Hypothesis 2:
With the same fErase-Write, good wear-leveling
-> Smaller write deviation of blocks &
Slower maximal write speed
Implement two wear-leveling methods on flash controller to observe if this hypothesis is valid

29. Overview of Evaluation Method
Step 1: Create TABLEErase-Write for the target flash drive
Since different flash drives have different fErase-Write
Step 2: Run a commonly used benchmark on the target flash until weal-leveling takes effects
Step 3: Sequentially write all logical pages of the flash drive
Mapping write speed to erasure count by TABLEErase-Write
Step 4: Evaluate the quality of the wear-leveling algorithm in the target flash drive based on
Max (erasure countEstimated)
Standard deviation (erasure countEstimated)
TABLEErase-Write of flash A
Erasure count
Write speed W0 1 W1 2 W2

30. Step 1. Build TABLEErase-Write
Process of building the table:
Sequentially write all logical pages of the flash drive n times
In the ith round
Erasure count ≈ i -1
Record average write speeds of all pages writes except the ones affected by garbage collection
Measure write speed of each page write
Ignore the ones below average
Recalculate average write speed of remaining page write speed
Fill the erasure count and average write speed into the table
Write pattern used to build the table: all zeros
Question: how many entries (n) do we need?
i = 2 1
Free Page
Valid Page
Invalid Page
TABLEErase-Write of flash A
average
Erasure count
Write speed W0 1 W1 2 W2
Write Speed
Logical Page Address

31. Step 2. Run A Commonly Used Benchmark
Write zeros to addresses of the benchmark
Question: How long?

32. Step 3. Erasure Count Estimation
Sequentially write all logical pages of the flash drive
Record write speeds of each page write except the ones affected by garbage collection
Measure write speed of each page write
Ignore the ones slower than W0
Index TABLEErase-Write to obtain erasure counts

33. Step 4 Evaluate the following two metrics
Max (erasure countEstimated)
Standard deviation (erasure countEstimated)
Example X axes: Address of logic pages, Y axes: Erasure countEstimated
Good Wear-leveling
standard deviation
max
Poor Wear-leveling

34. Help from 鈺創

35. End
Thank you! 35

36. Backup slides

37. Hybrid SSDs – Performance Result
configuration
Page size/
Block size
Mapping unit size
SLC
c1, c2, c3
2KB / 128KB
2KB
MLC c1 c2
4KB / 256KB
16KB c3
4KB / 512KB
64KB
Ultra Mobile PC
with following applications:
1. Web browsers,
2. clients,
3. movie players,
4. FTP clients,
5. office suites,
6. games
Response time of request i in conventional SSDs
Response time of request i in hybrid SSDs 37 37