flash memory

Embedded System Software (Flash Memory)Fall, 2010Outlinen Introductionn NAND and NOR flashl Comparisonl Featuresn NAND flash memory –additional features n Flash softwarel FTL (Flash Translation Layer)l Dedicated flash file systemIntroductionn Embedded storage currently uses flash technology, which provides high-density nonvolatile storagen The advantages of the flash have dramatically increased their usage in embedded systemsl Low powerl High density non-volatile storagel Reliabilityn Shock resistanceNAND and NOR flashn Flash devices come in two flavors:l NAND and NOR flashl Historyn1. Both of them came around the same timel NOR introduced by Intell NAND introduced by Toshiban2. NOR caught up fast with the embedded world becauseof its easy to usen But NAND flash became popular because embeddedsystem evolved to have large storageNAND and NOR flash -Comparisonn Access to datal NORn The data can be accessed at random like SRAMn Operationsl Read routinen Read the contents of the flashl Erase routinen Erase is the process of making all the bits on a flash 1n Erase on the NOR flash happens in terms of blocksl Write routinen Write is the process of converting a 1 to 0 on the flashn Once a bit is made 0, it cannot be written into until the blockis erasedn Access to datal NANDn Divides the storage into blocks, which are divided into pagesagainn Each page is divided into regular data and out-of-band datan Out-of-band data is used for storing metadata such as ECCdata and bad block informationl Small block flash memory (will disappear soon)n Block size: 16KB, page size 512 B, 16 B for spare datal Large block flash memoryn Block size: 128KB, page size 2048 B, 64 B for spare data n Unlike NOR which can access data randomly, the NAND readsand writes are done in terms of pages whereas erase happenin terms of blockn Interface to the boardl NORn These are connected like the normal SRAM device to theprocessor address and data busl NANDn Raw NAND access is done by connecting the data andcommand lines to the usually 8 IO lines on flash chipn Execution of codel NORn Code can be executed directly from NORl NANDn It needs to be copied to memory for executionn Performancel NORn Slow erase, slow write and very fast read l NANDn Fast erase, fast write and fast readCurrent (mA)Access time (4KB)Idle Active Read Write Erase NOR0.07400.023 ms 28 ms 1.2 s NAND0.05200.156 ms0.652 ms2ms MicroDrive2025021 ms21 ms N/An Bad blocksl NORn Are not expected to have bad blocksl NANDn Basically media storage devices at lower prices, so expect thatthey have bad blocksn Usagesl NORn Code executionn Pretty expensive and lesser memory densities and have arelatively shorter life span (100,000 erase cycles)l NANDn Mainly used for storage devices for embedded systemsn High densities at lower prices and have a longer life span(1,000,000 erase cycles)NAND flash memory –additional features n Flash transistor cell-Programming -erasure# of electrons on the floating gates affects the threshold voltage V thControl gateFloating gate Source drainErased cellControl gate Floating gateProgrammed cellControl gate----Floating gateSourcedrainSourcedrain11nSLC (Single level cell) vs. MLC (Multi-level cell) SLC10V0VthMLC110V100001VthMore precise charge placement or sensing is required for MLC12nComparison between two schemesSLC (Large) MLC 2 / 256 1, 2 50/800/1500 2 plane erase 1 4bit ECC 10KPage / Block Size (KB) Plane Size Read/Program/Erase(us) Multi-erase Number of partial programming ECC (512Bytes) Endurance Cycle2 / 128 1, 2 20/200/1500 2 plane erase 4 1bit ECC 100K13nPerformance improvementlCopy-backnAccelerate page-to-page copy operation. Increase the bandwidth between page register and NAND cell arraylMulti-planen14Flash softwarenErase before writelTo update data a location on a flash memory, it has to be first erased before the new data can be written itnErase unit is larger than the read and write unitlNORn nErase size: block Read/write size: single word Erase size: block Read/write size: pagelNANDn nnTherefore, Garbage collection is needed15How garbage collection works: Update L L L L E E E E E E E E D L L L L E E E E E E ELive Empty Dead# Empty pages: 8# Empty pages: 716UpdateD D L L L L E EE E E EUpdateD D D L L L L EE E E E# Empty pages: 6# Empty pages: 517D D D L L L L EE E E ED E D E ERASE D E L E L L L EE E E L E# Empty pages: 5# Empty pages: 818The number of empty pages is below a threshold;nObservationsl llData may be moved from one place to another. Since erase operation needs a significant time (2 ms), it is important to reduce the number of erase operations Therefore,n1. FTL (Flash Translation Layer) is usedlRedirects the location of updated data from one page to anothern2. Dedicated flash file system is used19nMTDlAbstract layer for memory-based hardware (e.g. flash)n nBetween file system and hardware driver Accommodate easy development of file systems Flash doesn't match the description of either block or character devices. They behave similar to block device, but have differences.llWhy?nnFor example, block devices don't distinguish between write and erase operations.20VFS (Virtual File System) File system especially JFFS developed YAFFS for flash memoryRaw MTD accessFATEXT2Block accessFTL (Flash Translation Layer)MTD (Memory Technology Device)Hardware driver21Flash Software - Flash Translation LayerApplications (mp3, DSC)Create/Open/Read/Write(file)Traditional File SystemRead/write(sector)111FTLRead/Program/Erase(page/block)HDD123NAND22nWhere is it located ?Host system Applications ApplicationsHost systemOperating SystemOperating SystemFile Systems FTL (Flash Translation Layer)File Systems MMC Host Device DriverNAND ControllerMMC driver / FTLNAND Flash Memory NANDNAND Flash Memory NAND Flash Memory MultiMediaCard23nThe key components of FTLlMapping tablen n nPage level mapping Block level mapping Hybrid mapping24Page level mappingLogical Page # Physical Page #Page # 2à5 … Page # Page # Page # … Page # Page # Page # … Page #Logical page number0 1 … 31 32 1 … 63 64 65 … 95B C D B’A1 2 3 4 5 6 7 8……Page Map tableFlash memoryLarge table size overhead25Block level mappingLogical block number Page #Logical Block #0 1 … 31 32 1 … 63 64 65 … 95Merge operationPhysical Block #Block # Block # … Block # Block # Block # … Block # Block # Block # … Block #AB C DA B’ C D1 2 3 4 5 6 7 8…Block Map tableFlash memoryA lot of write copy overhead26Hybrid mapping Frequently modified 1. Write Buffer Block (Log Block): Page Mapping 2. Data Block: Block Mapping Not (or seldom) modified27nAn example of hybrid mappingFree block Log block<address sequence> 5,7,7,5Logical Block #0 1 … 3145 6 7Physical Block #Block # Block # … Block #100Data block5 7 7 5145 6 7…64 65 … 95 Block # Block # … Block #…Block Map tableFlash memoryWhen there is no free page in a log block, the merge operation of data block and log block is initiated.28switch merge<address sequence> 4,5,6,7Logical Block #0 1 … 31Free blockLog blockLog block 4 5 6 7100Physical Block #Block # Block # … Block #Data block is erased145 6 7…64 65 … 95 Block # Block # … Block #…Block Map tableFlash memoryWhen there is no free page in a log block, the merge operation of data block and log block is initiated.29nWear levelingl lLimited lifetime of blocks A software technique to balance the erase counts of physical blocksDynamic Wear-leveling Static Wear-leveling Maintain erase counts in both blocks and RAM Maximize the lifetime Performance degradation RAM overhead30Feature Pros. Cons.Randomly select a block at run-time No performance overhead Excessive wear-out in the specific area depending on host patternFlash Software - Dedicated flash file systemnDedicated flash file systemlConsiderationsn nGarbage collection Wear-levelingl l l llFTL is not needed Commonly used file systems JFFS, JFFS2, YAFFS YAFFS is developed especially for NAND flash memory Basically log-structured file system31nJFFS2 (Journaling Flash File System 2)ll lA log-structured file system designed for use on flash devices in embedded systems JFFS2 was developed by Red Hat JFFS2 has been included in the official Linux kernel since the 2.4.10 release.nThe stable branch, supporting only NOR flash devices, is included in the current 2.4 series of kernels But may not consider the characteristics of NAND fully YAFFS (Yet Another Flash File System)lCurrently extended to support NAND flashn nlIt is upgraded from JFFS132nLog-structured file systemlLog (Node)nContains both inode information and datal l l l lIdentification of the file to which the log belongs Version that is unique for a log belonging to a particular file Metadata such as timestamp Data and data size Offset of the data in the filenA write to a file creates a log that records the offset in the file with actual data To rebuild the entire directory hierarchy and a complete map for each inode of the physical location As times proceeds, certain logs become obsolete and needs to be cleaned Efficient wear-leveling is possible33lThe entire medium is scanned at mount timenlGarbage collectionnnnHow log-structured file systems work (JFFS) ?Version : 1 File offset : 0 Length : 200 AAAAAAAAA AAAA …Write 200 bytes ‘A’ at offset zero in file K Write 200 bytes ‘B’ at offset 200 in file K Write 50 bytes ‘C’ at offset 175 in file KVersion : 2 File offset : 200 Length : 200 BBBBBBBB BBBB … Version : 3 File offset : 175 Length : 50 CCCCCCCCC CCCCC …34Flashmetadata data metadata data metadata data free space node v1RAMFile Knode v2node v#35Version : 1 File offset : 0 Length : 200 AAAAAAAAA AAAA … Version : 2 File offset : 200 Length : 200 BBBBBBBB BBBB … Version : 3 File offset : 175 Length : 50 CCCCCCCCC CCCCC …CCCCCCCCC CCCCC …AAAAAAAAA AAAA … BBBBBBBB BBBB …AAAAAAAAA AAAA … CCCCCCCCC CCCCC … BBBBBBBB BBBB …36nData organizationClean (Live) Free (Empty) dirty37How garbage collection works;ERASE38。