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    SubjectRe: O_DIRECT to md raid 6 is slow
    On 20/08/2012 02:01, NeilBrown wrote:
    > On Sun, 19 Aug 2012 18:34:28 -0500 Stan Hoeppner <stan@hardwarefreak.com>
    > wrote:
    >
    >
    > Since we are trying to set the record straight....
    >
    >> md/RAID6 must read all devices in a RMW cycle.
    >
    > md/RAID6 must read all data devices (i.e. not parity devices) which it is not
    > going to write to, in an RWM cycle (which the code actually calls RCW -
    > reconstruct-write).
    >
    >>
    >> md/RAID5 takes a shortcut for single block writes, and must only read
    >> one drive for the RMW cycle.
    >
    > md/RAID5 uses an alternate mechanism when the number of data blocks that need
    > to be written is less than half the number of data blocks in a stripe. In
    > this alternate mechansim (which the code calls RMW - read-modify-write),
    > md/RAID5 reads all the blocks that it is about to write to, plus the parity
    > block. It then computes the new parity and writes it out along with the new
    > data.
    >

    I've learned something here too - I thought this mechanism was only used
    for a single block write. Thanks for the correction, Neil.

    If you (or anyone else) are ever interested in implementing the same
    thing in raid6, the maths is not actually too bad (now that I've thought
    about it). (I understand the theory here, but I'm afraid I don't have
    the experience with kernel programming to do the implementation.)

    To change a few data blocks, you need to read in the old data blocks
    (Da, Db, etc.) and the old parities (P, Q).

    Calculate the xor differences Xa = Da + D'a, Xb = Db + D'b, etc.

    The new P parity is P' = P + Xa + Xb +...

    The new Q parity is Q' = P + (g^a).Xa + (g^b).Xb + ...
    The power series there is just the normal raid6 Q-parity calculation
    with most entries set to 0, and the Xa, Xb, etc. in the appropriate spots.

    If the raid6 Q-parity function already has short-cuts for handling zero
    entries (I haven't looked, but the mechanism might be in place to
    slightly speed up dual-failure recovery), then all the blocks are in place.




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