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    SubjectCalculate logical RAID-6 position from physical position
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    I have had a three-disk failure on a RAID 6 array. On the third disk, I
    can mostly recover it with dd_rescue, from which I know exactly which
    blocks were affected. From this, how can I calculate the logical positions
    that will be affected when I reassemble the RAID?

    This is 2.6.26-bpo.1-686-bigmem (Debian 2.6.26-4~bpo40+1)

    I'm looking for a general formula, but my specific scenario is:

    /dev/md0:
    Version : 01.00.03
    Creation Time : Sun Sep 30 09:14:07 2007
    Raid Level : raid6
    Array Size : 6349009472 (6054.89 GiB 6501.39 GB)
    Device Size : 976770688 (465.76 GiB 500.11 GB)
    Raid Devices : 15
    Total Devices : 13
    Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Sat Nov 15 02:32:54 2008
    State : clean, degraded
    Active Devices : 13
    Working Devices : 13
    Failed Devices : 0
    Spare Devices : 0

    Chunk Size : 64K

    Name : 'fargo':0
    UUID : 64107957:2bad471d:603d9582:796e97f5
    Events : 3279764

    Number Major Minor RaidDevice State
    0 8 128 0 active sync /dev/sdi
    1 8 240 1 active sync /dev/sdp
    2 8 80 2 active sync /dev/sdf
    3 8 160 3 active sync /dev/sdk
    4 0 0 4 removed
    5 8 176 5 active sync /dev/sdl
    15 8 224 6 active sync /dev/sdo
    7 8 96 7 active sync /dev/sdg
    8 8 64 8 active sync /dev/sde
    9 8 192 9 active sync /dev/sdm
    18 8 48 10 active sync /dev/sdd
    16 8 32 11 active sync /dev/sdc
    12 0 0 12 removed
    13 8 112 13 active sync /dev/sdh
    17 8 208 14 active sync /dev/sdn


    After some time, position 17 (/dev/sdn) was the disk that failed. The
    failure is 152 blocks from 5741236.0k to 5741311.5k. This is what I want
    to map to the logical layout.


    Wesley Leggette


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