Flash memory

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 http://www.microsoft.com/whdc/system/sysperf/perfaccel.mspx

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 http://www.microsoft.com/whdc/system/sysperf/perfaccel.mspx

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 Reference: Windows PC Accelerators white paper http://www.microsoft.com/whdc/system/sysperf/perfaccel.mspx

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 faster access SLC-mode MLC- mode

10. Comparison of SLC and MLC 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 • Issue: How much should be allocated 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 Free block Used block Direction of write & GC • Add a small SLC to improve average response time • 20 GB MLC + 256 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 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. email 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. Tunnel Oxide Control Gate Floating Gate Substrate Drain Source 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 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

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? 2 i = 1 Free Page Valid Page Invalid Page TABLEErase-Write of flash A average 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 Ultra Mobile PC with following applications: 1. Web browsers, 2. email 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