1 Flash Memory based Storage (CSE598D) Thursday April 5, 2007 Youngjae Kim

2 Disk Drive vs. Flash Memory Read / Write (+) Lost cost per bit (-) Mechanical movement (SPM & VCM) (-) High power consumption (10-15W) (-) Heavy weight compared to flash Read / Program / Erase (+) Random Access (+) Non-volatile (+) Low Power Consumption (2W) (-) Erase before Write (-) Erasing operation in the unit of block (not page) (-) Maximum # of erase operations per cell (-) High cost per bit

3 MOS (Metal-Oxide Semiconductor) Memory Hierarchy

4 History of Flash Memory

5 NOR and NAND Flash Array (a) NOR (b) NAND

6 NOR and NAND Flash Array

7 NAND Flash Memory – Program/Erase F-N tunneling –Give a higher voltage and electrons are trapped through gate into floating gate transistor.

8 Flash Memory Comparison NOR (Code Executable in Place like Memory) –Fast read and slow write NAND (Data-storage) –Fast write and lower cost Flash TypePerformanceApplication Code Storage NOR -Intel/Sharp -AMD/Fujitsu/Toshiba Important: -High Random Access -Byte Programming Acceptable: -Slow Programming -Slow Erasing Program Storage -Cellular Phone -DVD, Set TOP Box for BIOS File Storage NAND -Samsung/Thoshiba Important: -High Sped Programming -High Speed Erasing -High Speed Serial Read Acceptable: -Slow Random Access Small form factor -Digital Still Camera -Silicon Audio, PDA -Mass storage as Silicon Disk-Drive

9 NAND Flash Non-Volatile Flash Cards Various Standard Memory Cards

10 Functional Block Diagram for SAMSUNG K9K8G08U0M NAND Flash

11 Array Organization for SAMSUNG K9K8G08U0M NAND Flash Block: Erasing Unit Page: Addressable Unit

12 NAND Flash Technology

13 Comparison for Different Memory Types Design and evaluation of the compressed flash translation layer for high-speed and large-scale flash memory storages Proc. SoC Design Conference, pp , September, 2003

14 Outline Flash Memory Technology –NAND vs. NOR Block Mapping Schemes –Emulating Disk with Flash Memory Garbage Collection Hybrid Hard Drives –Window Vista

15 NAND Type Flash Memory Operation –Read / Write Page unit (Size of a page = Size of a sector (512B) in hard drive) –Erase Block unit (A set of pages) Characteristics –Not in-place update Erase an entire erase block for in-place update of page Original block Free block 1. Update page 0 in free block 2. Copy the rest of Pages (1,2,3,4) 3. Obsolete original block

16 Block-Mapping Technique (1/2) Emulate Block device (Disk-Drive) with Flash Memory –In traditional disk drive, 1.File system calls a device drive, requesting block read/write 2.Device driver stores the data and retrieve it from flash device Problems in Simple Linear Mapping –Lifetime shortening of flash memory by the limit of write operations 100,000 – 1,000,000 per cell –High risk of data loss due to the size difference between file system data block and erase block unit of flash

17 Block-Mapping Technique (2/2) Maximum number of write operation –Some data block may be written much more than others No problem in hard drive Operation time to the cell get slow down => wear and burn out Data loss risk from size difference between data block and erase unit in flash –Example Copy an entire unit (128KB) into RAM and modify 4KB while erasing the entire unit and write back. But, power loss –(128KB + 4KB) data loss

18 Block-Mapping Idea (1/2) Maintain mapping table –Virtual Block # - Physical Flash Address (Sector) Update Process 1.Do not overwrite the sector, instead, write to another free sector 2.Update mapping table (+) Evenly distribute the wear of erase units (+) Fast write (because of not-erasing process) (+) Minimize data loss when power off (possibly revert to the previous state) Write Process 1.Search for a free/erased sector 2.Initially, all the bits for (Sector and Header) should be 1s. 3.Clear free/used bit 4.Write virtual block # into the header and then write data in the sector 5.Clear prevalid/valid bit 6.Clear valid/obsolete bit of previous sector

19 Block-Mapping Idea (2/2) Power off during write operation (Case 1) If the power off occurs before new sector is set to be valid then ignore the data written (Case 2) Even if new sector is set valid, but if the power is off before the previous sector becomes obsolete, then both of them are valid. Select any one according to their versioning numbers

20 Data Structure for Mapping

21 Flash Translation Layer Fully emulate magnetic disks with flash memory –Support random-access Two features of flash memory –Erase before write –Erase unit size (block) is not the same as read/write size (page). File System Block Device Driver HOST Flash Device Controller ROMRAM Flash Memory IDE, SCSI

22 Page-Level Mapping Logical Sector Number to Physical Sector Number Limitation –Large SRAM => High Cost

23 Block-Level Mapping Logical Block Number to Physical Block Number + Offset 2 3 Limitation –Involving extra flash memory operation with write requests

24 Hybrid Approach (Page + Block) Write Trace Page Block Replace Block Log-Block

25 Garbage Collection To make spare for new and update blocks –Obsolete sectors must be reclaimed. –To reclaim a sector is done by erasing an entire unit. (Reclamation operates on entire erase units.) Reclamation –In background (e.g., when CPU is idle) –On-demand (e.g., when no free sectors) –Goal Wear-leveling Efficient reclamation Reclaiming process 1.Select erase units for reuse 2.Copy valid sectors (within an erasing unit) 3.Update mapping table 4.Erase the reclaimed erase units and add them to sector-reserve

26 Wear Leveling Limitation –Maximum number of erases/writes per cell (10K – 1M) Reliability of cell decreases (e.g., bad block). Wear Leveling –To evenly distribute the cell usages over the cells –Wear-Leveling versus Efficiency They are contradictory. For, example, erasing unit containing STATIC data –For efficiency, it should not be reclaimed because any storage is not free up. –For wear-leveling, it should be reclaimed because it reduces the wear of other units.

27 Wear-Centric Reclamation (1/3) [Lofgren et al. 2000, 2003] Sector: Free Sector: Valid Counter Sector: Free Counter Sector: Valid Sector: Free Sector: Invalid Sector: Valid Counter Most Worn-Out Unit (Reclaimed) Least Worn-Out Unit Spare Unit 2 3 Using an erase counter of erase unit 1.When the most worn-out unit is reclaimed, its counter is compared to that of the least worn-out unit. 2.If greater than threshold (e.g.,15,000), the contents of the most worn out unit are copied to the least worn out unit. And the most worn-out unit becomes spare. 3.Otherwise, just keep going as it does Wear-leveling –Moving static blocks to worn-out sectors –Usually sector with static data is least-worn out unit 1 Flash

28 Wear-Centric Reclamation (2/3) [Jou and Jeppesen III 1996] Sector: Free Sector: Valid Sector: Free Reclaimed Unit 1 Free Unit 3 Sector: Free Sector: Valid Sector: Free Reclaimed Unit 0 1 Free Unit Priority queue sorted by wear U1U Using the wear (number of erasure) –The valid contents of erase unit reclaimed are copied to another unit. –But, the unit is not erased immediately –It’s marked as erasure unit and added to queue of erase candidate (RAM) –The queue is sorted by wear –Whenever system needs a free unit and the unit with least wear is erased Flash RAM

29 Other Wear-Centric Reclamations (3/3) Using erase latencies [Han 2000] –Erase latency increases with wear. –Erase times are used to rank erase unit by wear. –It avoids to store erase counters. Randomized wear-leveling [Woodhouse 2001] –Every 1000 th reclamation, a unit containing only valid data is selected. –Pros and Cons (+) Moving static data from units with little wear to units with more wear (-) Extreme wear imbalance can occur. (e.g., a little worn-out unit with invalid data many never be reclaimed.)

30 Wear-Leveling with Efficient Reclamation (1/2) Using a weighted benefit/cost [Kawaguchi et al. 1995] –Benefit: the amount of invalid space in the unit –Cost: the need to read the valid data and to write back elsewhere –Weight: the age of the block, time since last invalidation Large weight → the remaining valid data are relatively static. benefit cost X weight

31 Wear-Leveling with Efficient Reclamation (2/2) Using Hot block and Cold block [Kawaguchi et al. 1995] –Cold block: Block allocated with low wear level –Hot block: Block with high wear level –Observation Units with dynamic data tend to be almost empty upon reclamation. Static units do not need to be reclaimed at all. Flash Cold BlocksHot Blocks 1 Free Blocks 2

32 Hybrid Hard Disk (HDD) Seagate’s ReadyDrive –HDD Prototype of Samsung & Seagate for Labtop (WinHEC conf. 2006) –128 MB NAND Flash Memory in hard disk Store frequently accessed sectors of data for quick reads (e.g., FAT table) –Flash is used to make less frequent disk power down and up. –Advantages Reliability, Power-Efficient, and Improved Performance Experiment –Run Office Applications –Spun up every three and four minutes –10% power saving (May 24, 2006)

33 Seagate’s 5400 RPM Hybrid Hard Drive 160 GB of regular perpendicular (PMR) space 256 KB flash memory It is for Windows Vista in Q

34 NAND Flash: Possible to replace the existing hard disks? This is suitable for mobile device. –Mobile device (e.g., Portable Digital Player, Cell Phone, etc.) Most applications are media (audio, video, etc.) Reads are dominate rather than writes. How about for server disk? –High cost/Large capacity for NAND flash, compared to traditional disk 64GB flash disk (Samsung, 2006) vs. 300GB Seagate Cheetah 15K.5 –Low reliability of data because of wear-out Many writes could wear out the cells.

35 FLASHCACHE [HCSS’94] DRAM Management –LRU block replacement Flash Management –Segment = A set of blocks/Erasing unit –Segment list (Free/Clean/Dirty) –Segment replacement (FIFO or LRU) Disk Management –Power management by spin up/down

36 eNVy [ASPLOS’94]

37 eNVy [ASPLOS’94] Write operation –Flash memory: Copy-on-write operation –SRAM is used as a Write Buffer for fast writes on flash. –Page replacement on SRAM: FIFO Page-mapping (logical to physical addresses) on SRAM

38 NVCache [MASCOTS’06] To reduce the power consumption of disk NVCache –To reduce disk power consumption by combining adaptive disk spin-down algorithm –To extend spin-down periods by undertaking in NVCache

39 Non-volatile Memory File Systems JFFS2 (Journaling Flash File System) –Built-in linux kernel after 2.4.X –JFFS1 (1999) → JFFS2 (2001) → JFFS3 (on- going)

40 Non-volatile Memory File Systems Using NVRAM at file system level –Conquest file system [USENIX’02] Persistent RAM (a sort of NVRAM) NVRAM stores “metadata”, “small files”, “executables”, and “shared libraries”. –HeRMES file system [HotOS’01] Magnetic RAM (a sort of NVRAM) NVRAM stores “metadata” and “small data / a few of first blocks”. –(+) reduce metadata overhead for writes/reads to improve performance NVRAM is use for write cache. –(+) enhance write performance (by buffering and reordering writes)