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    SubjectRe: [patch] ext2/3: document conditions when reliable operation is possible
    Pavel Machek wrote:

    > Ok, can you help? Having a piece of MD documentation explaining the
    > "powerfail nukes entire stripe" and how current filesystems do not
    > deal with that would be nice, along with description when exactly that
    > happens.

    Except of course for the inconvenient detail that a power
    failure on a degraded RAID 5 array does *NOT* nuke the
    entire stripe.

    A 5-disk RAID 5 array will have 4 data blocks and 1 parity
    block in each stripe. A degraded array will have either
    4 data blocks or 3 data blocks and 1 parity block in the
    stripe.

    If we are dealing with a parity-less stripe, we cannot
    lose any data due to RAID 5, because each of the 4 data
    blocks has a disk block available. We could still lose
    a data write due to a power failure, but this could also
    happen with the RAID 5 array still intact.

    If we are dealing with a 3-data, 1-parity stripe, then
    3 of the 4 data blocks have an available disk block and
    will not be lost (if they make it to disk). The only
    block that maintains on all 3 data blocks and the parity
    block being correct is the block that does not currently
    have a disk to be written to.

    In short, if a stripe is not written completely on a
    degraded RAID 5 array, you can lose:
    1) the blocks that were not written (duh)
    2) the block that doesn't have a disk

    The first part of this loss is also true in a non-degraded
    RAID 5 array. The fact that the array is degraded really
    does not add much additional data loss here and you certainly
    will not lose the entire stripe like you suggest.

    --
    All rights reversed.


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