IntroductiontoFlashMemory1节.ppt

Introduction to Flash Memory,2006. 11. 15. Mobile Embedded System Lab. Kiseok, Choi,Table of Contents,Stateless PC Flash Memory Basics NAND vs. NOR SLC vs. MLC NAND Flash Memory FTL (Flash Translation Layer) An FTL Design Based on Log Blocks The Log Block The Map Block SSD (Solid State Disk) State-of-the-art Technologies Research Issues Summary,Stateless PC,Definition : The PC that has no states, especially non-volatile states.,Stateless PC,,Stateless PC,State,What’s in the portable storage? Operating Systems File Systems Private data Applications,Current PC,,Remove non-volatile states,,,Stateless PC,Storage for the states Flash memory is a good solution for the physical storage. Why flash memory? Faster access Shock / Temperature resistance Smaller size Lighter weight Lower power Noiseless (0dB),Flash Memory Basics,Flash memory A non-volatile semiconductor memory device Key feature : To overwrite data, the memory cell should be erased first. Kinds of flash memory NOR Introduced by Intel in 1988 Randomly access data, like a computer’s main memory Use for executing program code NAND Introduced by Toshiba in 1989 Smaller and denser. → NAND is better at storing data. Faster erase and write time,NOR,NAND,NAND vs. NOR,,,,,,,Pros of NAND,,Pros of NOR,,Cons of NAND,,Cons of NOR,For Mass Storage,For Code Storage,,,1. Smaller cell size 2. Limited bad blocks allowed 3. Fast writing 4. Lower power consumption,1. Fast random (read) access,1. Slow random (read) access,1. Larger cell size 2. No bad blocks are allowed 3. Slow writing 4. Higher power consumption,We focus on the NAND flash memory!,NAND Flash Memory,Organization of NAND flash memory Small-block flash memory Each page is (512 + 16) bytes long 32 pages in each block Large-block flash memory Each page is (2048 + 64) bytes long 64 pages in each block,,,Page 0,Block 1,Page 1,Page m-1,Block n-1,Block 0,512,16,2048,64,Page layout for small-block flash memory,Page layout for large-block flash memory,,,Main Area,Spare Area,chip,,,NAND Flash Memory,Primitive operations of NAND flash memory Read page (chip #, block #, page #) ~20 us Write (program) page (chip #, block #, page #) ~200 us Erase block (chip #, block #) ~2 ms,FTL (Flash Translation Layer),,,,,+,Device Driver,,,,,,,Read,Write,Erase,File System,,,,,Read Sectors,Write Sectors,Mismatch!,,+,Device Driver,,,,,FTL,+,Read Sectors,Write Sectors,File System,,,,,Read Sectors,Write Sectors,,FTL (Flash Translation Layer),Definition Software layer that makes flash memory appear to the system like a disk drive Challenges in FTL Asymmetry in read and write speeds No overwrite is allowed without erasing,,FTL basics,Read request from upper layer No problem. Write request from upper layer There is a problem. Erase operation must be done first (the erase operation is performed in a block unit) to overwrite data.,An FTL Design Based on Log Blocks,Background 2 kinds of blocks Data block: block level managed block (most) Log block: page level managed block (a few) Temporary storage for small size writes to data blocks,,,,valid,,,valid,valid,,,valid,,,,,,,,Write!,Data is written to the log block,,Data Block,Log Block,An FTL Design Based on Log Blocks,Whenever the log block is full, We need to merge the data block and the log block.,An FTL Design Based on Log Blocks,Merge Operation Log Block Merge,,,,valid,valid,,,valid,valid,,valid,valid,,,valid,valid,,,,,,,,,,,,Free blocks,Data Block,Log Block,Data Block,Free Block,An FTL Design Based on Log Blocks,Merge Operation (cont’d) Log Block Switch,,,,,,,,,valid,valid,valid,valid,valid,valid,valid,valid,Free blocks,,Data Block,Data Block,Log Block,An FTL Design Based on Log Blocks,Where is the mapping information? Mapping information : logical address to physical address It is stored in map blocks.,Map Block,The Map Block Where to store the mapping information Previous scheme: each page/block in the associated spare area in the form of logical address tags  requiring scanning of the entire space of flash memory to collect logical address tags Map blocks: dedicated blocks to enable faster startup and on-demand fetching  using map blocks in a round robin manner Map Directory: the map of the mapping table in SRAM and is used to locate each portion of the mapping table stored in map blocks  the map directory is stored in a reserved area of the flash memory,SSD (Solid State Disk),The disk that uses the semiconductor as storage DRAM-based Flash-based P-ATA / S-ATA interface FTL NAND flash memory Target markets Enterprise server storage Mainstream PC storage,SSD (Solid State Disk),HYDRA project 2005.9 ~ Mtron + SNU 3.5 inch engineering sample already released,(PCMark 04),,State-of-the-art Technologies,In the perspective of the pure NAND Copy-back operation Using internal data copies, data transfer overhead can be removed (used for merge operations). Cache programming Besides the page register, there’s a cache register. Using double buffering, throughput can be increased.,State-of-the-art Tec。