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    Subject[RFC][PATCH 0/7] UBI checkpointing support
    The following patch set implements checkpointing support for
    UBI. Checkpointing is an optional feature which stores the physical to
    logical eraseblock relations in a checkpointing superblock to reduce
    the initialization time of UBI. The current init time of UBI is
    proportional to the number of physical erase blocks on the FLASH
    device. With checkpointing enabled the scan time is limited to a fixed
    number of blocks.

    Checkpointing does not affect any of the existing UBI robustness
    features and in fact the checkpointing code falls back into scanning
    mode when the checkpoint superblock(s) are corrupted.

    The checkpoints consist of two elements:

    1) A primary checkpoint block, which contains merily a pointer to the
    erase block(s) which hold the real checkpointing data.

    This primary block is guaranteed to be held within the first N
    eraseblocks of a device. N is momentarily set to 16, but it might
    be necessary to make this configurable in some way.

    2) The secondary checkpoint blocks, which contain the real
    checkpointing data (physical to logical eraseblock relations,
    erase counts, sequence numbers ...)

    Aside of that the checkpointing data contains a list of blocks
    which belong to the active working pool. The active working pool is
    a fixed number of blocks for shortterm, longterm and unknown
    storage time, which can be modified before the next checkpoint set
    is written to FLASH. These blocks need to be scanned in the
    conventional UBI scan mode.

    The reason for these pool blocks is to reduce the checkpoint
    updates to the necessary minimum to avoid accelerated device
    wearout in scenarios where data changes rapidly. The checkpoint
    data is updated whenever a working pool runs out of blocks.

    The number of pool blocks can be defined with a config option at
    the moment, but this could also be done at runtime via sysfs. In
    case of a change the checkpointing data would be reconstructed.

    So the checkpoint scan consists of the following steps:

    1) Find the primary checkpoint block by scanning the start of the

    2) Read the real checkpoint data and construct the UBI device info

    3) Scan the pool blocks.

    All these operations scan a limited number of erase blocks which makes
    the UBI init O(1) and independent of the device size.

    The checkpoint functionality is fully compatible with existing UBI
    deployments. If no checkpoint blocks can be found then the device is
    scanned and the checkpoint blocks are created from the scanned

    Aside of review and testing it needs to be decided, whether the number
    of pool blocks should be deduced from the device size (number of
    physical eraseblocks) or made configurable at compile or runtime.

    Thanks to the folks at CELF who sponsored this work!

    [RFC][PATCH 1/7] MTD: UBI: Add checkpoint on-chip layout
    [RFC][PATCH 2/7] MTD: UBI: Add checkpoint struct to ubi_device
    [RFC][PATCH 3/7] MTD: UBI: Export next_sqnum()
    [RFC][PATCH 4/7] MTD: UBI: Make wl subsystem checkpoint aware
    [RFC][PATCH 5/7] MTD: UBI: Make process_eb() checkpoint aware
    [RFC][PATCH 6/7] MTD: UBI: Implement checkpointing support
    [RFC][PATCH 7/7] MTD: UBI: wire up checkpointing

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