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    UBI: Unsorted Block Images · 801c135c
    Artem B. Bityutskiy 提交于
    UBI (Latin: "where?") manages multiple logical volumes on a single
    flash device, specifically supporting NAND flash devices. UBI provides
    a flexible partitioning concept which still allows for wear-levelling
    across the whole flash device.
    
    In a sense, UBI may be compared to the Logical Volume Manager
    (LVM). Whereas LVM maps logical sector numbers to physical HDD sector
    numbers, UBI maps logical eraseblocks to physical eraseblocks.
    
    More information may be found at
    http://www.linux-mtd.infradead.org/doc/ubi.html
    
    Partitioning/Re-partitioning
    
      An UBI volume occupies a certain number of erase blocks. This is
      limited by a configured maximum volume size, which could also be
      viewed as the partition size. Each individual UBI volume's size can
      be changed independently of the other UBI volumes, provided that the
      sum of all volume sizes doesn't exceed a certain limit.
    
      UBI supports dynamic volumes and static volumes. Static volumes are
      read-only and their contents are protected by CRC check sums.
    
    Bad eraseblocks handling
    
      UBI transparently handles bad eraseblocks. When a physical
      eraseblock becomes bad, it is substituted by a good physical
      eraseblock, and the user does not even notice this.
    
    Scrubbing
    
      On a NAND flash bit flips can occur on any write operation,
      sometimes also on read. If bit flips persist on the device, at first
      they can still be corrected by ECC, but once they accumulate,
      correction will become impossible. Thus it is best to actively scrub
      the affected eraseblock, by first copying it to a free eraseblock
      and then erasing the original. The UBI layer performs this type of
      scrubbing under the covers, transparently to the UBI volume users.
    
    Erase Counts
    
      UBI maintains an erase count header per eraseblock. This frees
      higher-level layers (like file systems) from doing this and allows
      for centralized erase count management instead. The erase counts are
      used by the wear-levelling algorithm in the UBI layer. The algorithm
      itself is exchangeable.
    
    Booting from NAND
    
      For booting directly from NAND flash the hardware must at least be
      capable of fetching and executing a small portion of the NAND
      flash. Some NAND flash controllers have this kind of support. They
      usually limit the window to a few kilobytes in erase block 0. This
      "initial program loader" (IPL) must then contain sufficient logic to
      load and execute the next boot phase.
    
      Due to bad eraseblocks, which may be randomly scattered over the
      flash device, it is problematic to store the "secondary program
      loader" (SPL) statically. Also, due to bit-flips it may become
      corrupted over time. UBI allows to solve this problem gracefully by
      storing the SPL in a small static UBI volume.
    
    UBI volumes vs. static partitions
    
      UBI volumes are still very similar to static MTD partitions:
    
        * both consist of eraseblocks (logical eraseblocks in case of UBI
          volumes, and physical eraseblocks in case of static partitions;
        * both support three basic operations - read, write, erase.
    
      But UBI volumes have the following advantages over traditional
      static MTD partitions:
    
        * there are no eraseblock wear-leveling constraints in case of UBI
          volumes, so the user should not care about this;
        * there are no bit-flips and bad eraseblocks in case of UBI volumes.
    
      So, UBI volumes may be considered as flash devices with relaxed
      restrictions.
    
    Where can it be found?
    
      Documentation, kernel code and applications can be found in the MTD
      gits.
    
    What are the applications for?
    
      The applications help to create binary flash images for two purposes: pfi
      files (partial flash images) for in-system update of UBI volumes, and plain
      binary images, with or without OOB data in case of NAND, for a manufacturing
      step. Furthermore some tools are/and will be created that allow flash content
      analysis after a system has crashed..
    
    Who did UBI?
    
      The original ideas, where UBI is based on, were developed by Andreas
      Arnez, Frank Haverkamp and Thomas Gleixner. Josh W. Boyer and some others
      were involved too. The implementation of the kernel layer was done by Artem
      B. Bityutskiy. The user-space applications and tools were written by Oliver
      Lohmann with contributions from Frank Haverkamp, Andreas Arnez, and Artem.
      Joern Engel contributed a patch which modifies JFFS2 so that it can be run on
      a UBI volume. Thomas Gleixner did modifications to the NAND layer. Alexander
      Schmidt made some testing work as well as core functionality improvements.
    Signed-off-by: NArtem B. Bityutskiy <dedekind@linutronix.de>
    Signed-off-by: NFrank Haverkamp <haver@vnet.ibm.com>
    801c135c
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