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									Flash Memory based Storage


       Thursday April 5, 2007

           Youngjae Kim

Disk Drive vs. Flash Memory

 Read / Write                          Read / Program / Erase

 (+) Lost cost per bit                 (+) Random Access
                                       (+) Non-volatile
 (-) Mechanical movement (SPM & VCM)   (+) Low Power Consumption (2W)
 (-) High power consumption (10-15W)
 (-) Heavy weight compared to flash    (-) Erase before Write
                                       (-) Erasing operation in the unit of block
                                          (not page)
                                       (-) Maximum # of erase operations per cell
                                       (-) High cost per bit
MOS (Metal-Oxide Semiconductor) Memory Hierarchy

History of Flash Memory

NOR and NAND Flash Array

      (a) NOR              (b) NAND

NOR and NAND Flash Array

NAND Flash Memory – Program/Erase

• F-N tunneling
   – Give a higher voltage and electrons are trapped through gate into
     floating gate transistor.

    Flash Memory Comparison

•    NOR (Code Executable in Place like Memory)
      –   Fast read and slow write
•    NAND (Data-storage)
      –   Fast write and lower cost

                           Flash Type              Performance               Application

                                            -High Random Access       Program Storage
           Code                             -Byte Programming         -Cellular Phone
          Storage                           Acceptable:               -DVD, Set TOP Box for
                                            -Slow Programming         BIOS
                                            -Slow Erasing
                                            -High Sped Programming    Small form factor
                                            -High Speed Erasing       -Digital Still Camera
            File     NAND
                                            -High Speed Serial Read   -Silicon Audio, PDA
          Storage    -Samsung/Thoshiba
                                            Acceptable:               -Mass storage as Silicon
                                            -Slow Random Access       Disk-Drive

NAND Flash Non-Volatile Flash Cards

• Various Standard Memory Cards

Functional Block Diagram for SAMSUNG K9K8G08U0M NAND Flash

Array Organization for SAMSUNG K9K8G08U0M NAND Flash

              •   Block: Erasing Unit
              •   Page: Addressable Unit

NAND Flash Technology

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. 740-745, September, 2003


• Flash Memory Technology
   – NAND vs. NOR

• Block Mapping Schemes
   – Emulating Disk with Flash Memory

• Garbage Collection

• Hybrid Hard Drives
   – Window Vista

    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   2. Copy the rest of   3. Obsolete original
                         in free block      Pages (1,2,3,4)       block                  15
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

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

    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
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

Data Structure for Mapping

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).

      HOST                         Flash Device

             File System    IDE,
             Block Device                               Flash Memory
                Driver                ROM         RAM

Page-Level Mapping

• Logical Sector Number to Physical Sector Number

• Limitation
   – Large SRAM => High Cost

Block-Level Mapping

• Logical Block Number to Physical Block Number + Offset


• Limitation
   – Involving extra flash memory operation with write requests

Hybrid Approach (Page + Block)

   Write Trace   1   2   3   4   4     3   4

   Page          1 2 3 4     4 3 4

   Block         1 2 3 4     1 2 3 4       1 2 3 4   1 2 3 4

   Block         1 2 3 4             3 4         4

   Log-Block     1 2 3 4     4 3 4

    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

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.

 Wear-Centric Reclamation (1/3) [Lofgren et al. 2000, 2003]

Flash                                            •   Using an erase counter of
                         Least Worn-Out Unit         erase unit
Most Worn-Out Unit             Counter               1. When the most worn-out unit is
   (Reclaimed)                                          reclaimed, its counter is
                             Sector: Free               compared to that of the least
     Counter                 Sector: Valid              worn-out unit.
   Sector: Valid             Sector: Valid           2. If greater than threshold
   Sector: Free      2                                  (e.g.,15,000),
                             Sector: Valid              the contents of the most worn out
  Sector: Invalid                                       unit are copied to the least worn
                             Spare Unit      1
   Sector: Valid                                        out unit.
                               Counter                  And the most worn-out unit
           3                 Sector: Free
                                                        becomes spare.
   Spare Unit                                        3. Otherwise, just keep going as it
                             Sector: Free               does
                             Sector: Free
                             Sector: Free        •   Wear-leveling
                                                     – Moving static blocks to worn-out
                                                     – Usually sector with static data is
                                                       least-worn out unit
 Wear-Centric Reclamation (2/3) [Jou and Jeppesen III 1996]

Flash                                              •   Using the wear (number of erasure)
                                                       –   The valid contents of erase unit
Reclaimed Unit 0        Reclaimed Unit 1                   reclaimed are copied to another unit.
  Sector: Free            Sector: Free         5       –   But, the unit is not erased
  Sector: Valid           Sector: Valid                    immediately
  Sector: Valid           Sector: Valid
                                                       –   It’s marked as erasure unit and added
                                                           to queue of erase candidate (RAM)
  Sector: Valid           Sector: Valid                –   The queue is sorted by wear
                   1                       3           –   Whenever system needs a free unit
   Free Unit               Free Unit
                                                           and the unit with least wear is
  Sector: Free            Sector: Free                     erased
  Sector: Free            Sector: Free
  Sector: Free            Sector: Free
  Sector: Free            Sector: Free
            U0     U1

  Priority queue sorted by wear

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 1000th reclamation, a unit containing only valid data is
   – 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.)

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
   – Weight: the age of the block, time since last invalidation
       • Large weight → the remaining valid data are relatively static.

                               cost X weight

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.

                           Cold Blocks           Hot Blocks


                               2   Free Blocks

Hybrid Hard Disk (HDD)

• Seagate’s ReadyDrive
   – HDD Prototype of Samsung & Seagate for Labtop (WinHEC conf.
   – 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
           ,1697,1966806,00.asp (May 24, 2006)
Seagate’s 5400 RPM Hybrid Hard Drive

• 160 GB of regular perpendicular (PMR) space
• 256 KB flash memory
• It is for Windows Vista in Q1 2007.

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
       • 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.


      •   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
eNVy [ASPLOS’94]

               <Diagram of eNVy in a Host>

             <Diagram of eNVy Architecture>

eNVy [ASPLOS’94]

                         <Copy on Write: Atomic operation>

• 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
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

Non-volatile Memory File Systems

                           • JFFS2 (Journaling
                             Flash File System)
                              – Built-in linux kernel after
                              – JFFS1 (1999) → JFFS2
                                (2001) → JFFS3 (on-

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

   – 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)


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