1. Understanding Modern Flash Memory Systems
Thomas McCormick
Chief Engineer/Technologist, Swissbit
ESC Minneapolis

2. Flash Memory Systems
Flash memory systems are everywhere

3. Motivation Why do we care? Complicated sub-system Wear-Item
Boot code – Failure is non-functional system

4. Floating Gate Transistor
Limited P/E Cycles

5. Flash Memory: NOR & NAND
Byte Addressable
Reduced Scalability
Costly
Low Density
NAND:
Block Addressable
Scalability
Cost Effective
High Density

6. Flash Memory: SLC, MLC, & TLC

7. Flash Memory: Endurance

8. Flash Memory: Retention

9. Challenges (NAND) Erase-then-program
Course-grained structures (block & page)
Errors
Endurance (MLC)
Retention (MLC)
Problem: How do we design for a long lifetime?
Solution: Managed flash memory system with well designed host

10. Interface
Sector (LBA)
(Wikipedia – Creative Commons)

11. FTL Flash Translation Layer (FTL)
FTL Designs: Block, Hybrid, Page (~2012)

12. Sequential Writes

13. Random Writes

14. Garbage Collection (GC)
Page-mode FTL is more efficient (deferral)

15. Dynamic Wear-Leveling

16. Static Wear-Leveling
Flash for code & data will wear equally (retention)

17. Flash Systems: Failure Modes
Erase Failures (hard, managed)
Program Failures (hard, managed)
Retention Failures (soft, additional work)
Retention -> UECC (AVOID!!!!)

18. Retention Management: Active (1)
Active – Engages when data is transferred
Error Detection & Correction (EDC)
Read-Retry

19. Retention Management: Active (2)
ECC (Hamming, R-S, BCH, LPDC)

20. Retention Management: Active (3)
Read-Retry

21. Retention Management: Passive
Passive – Background
Refresh
Refresh & Mark Bad

22. Flash System Lifetime WAF = f (workloadnature) WAF ~1 (Sequential)
WAF >> 1 (Random)

23. WAF (1) WAFRandom = f(Over-ProvisioningTrue, FTL Overhead)
McCormick, Validating Analytic Write Amplification Models
2016 Flash Memory Summit
Over-ProvisioningTrue = (Physical Size – User Data Size)/(User Data Size)
Configuration
IDEMA Capacities
Extended (SLC)
32 GB, 64 GB, 128 GB, …
Standard (MLC)
30 GB, 60 GB, 120 GB, …
Enterprise
25 GB, 50 GB, 100 GB, …

24. WAF (2)
[McCormick, FMS2016]

25. WAF Reduction Options: Page-mode FTL (2x) [McCormick, FMS 2015]
Increased over-provisioning (2x – 4x)
Sequential writes
Large, contiguous files (up to 5x)
File-system: FAT, ext (no journaling)
Key Point: Managed drive (MLC) can approach older drive (SLC)
Ex: 2x * 2x * 5x = 20 -> 33.3 (100KSLC/3KMLC)

26. Monitoring Endurance Host writes and flash writes -> WAF
P/E Cycles & Rating
Lifetime (%)
Standard (SMART Interface)
Proprietary (Vendor)
Host writes and flash writes -> WAF

27. Lifetime Estimation
Sample & Extrapolate

28. Conclusions Managed flash memory system (NAND) Code & data
Retention = f(EnduranceUtilized)
EnduranceUtilized = f(Workload, WAF)
SLC -> MLC: 33x reduction
Minimize WAF: Page FTL, OP, sequential
Monitor

29. Speaker/Author Details
Tom McCormick – Chief Engineer/Technologist
Swissbit

30. Thank You!
Questions?
@ESC_Conf
ESC Minneapolis