Re: reconstruct raid superblock

[Michael Evans <mjevans1983@gmail.com>]

On Thu, Dec 17, 2009 at 2:35 AM, Majed B. <majedb@gmail.com> wrote:
> I have misread the information you've provided, so allow me to correct myself:
>
> You're running a RAID6 array, with 2 disks lost/failed. Any disk loss
> after that will cause data loss since you have no redundancy (2 disks
> died).
>
> I believe it's still possible to reassemble the array, but you only
> need to remove the MBR. See this page for information:
> http://www.cyberciti.biz/faq/linux-how-to-uninstall-grub/
> dd if=/dev/null of=/dev/sdX bs=446 count=1
>
> Before proceeding, provide the output of cat /proc/mdstat
> Is the array currently running degraded or is it suspended?
> What happened to the spare disk assigned? Did it finish resyncing
> before you installed grub on the wrong disk?
>
> On Thu, Dec 17, 2009 at 8:21 AM, Majed B. <majedb@gmail.com> wrote:
>> If your other disks are sane and you are able to run a degraded array,  then
>> you can remove grub using dd then re-add the disk to the array.
>>
>> To clear the first 1MB of the disk:
>> dd if=/dev/zero of=/dev/sdx bs=1M count=1
>> Replace sdx with the disk name that has grub.
>>
>> On Dec 17, 2009 6:53 AM, "Carl Karsten" <carl@personnelware.com> wrote:
>>
>> I took over a box that had 1 ide boot drive, 6 sata raid drives (4
>> internal, 2 external.)  I believe the 2 externals were redundant, so
>> could be removed.  so I did, and mkfs-ed them.  then I installed
>> ubuntu to the ide, and installed grub to sda, which turns out to be
>> the first sata.  which would be fine if the raid was on sda1, but it
>> is on sda, and now the raid wont' assemble.  no surprise, and I do
>> have a backup of the data spread across 5 external drives.  but before
>> I  abandon the array, I am wondering if I can fix it by recreating
>> mdadm's metatdata on sda, given I have sd[bcd] to work with.
>>
>> any suggestions?
>>
>> root@dhcp128:~# mdadm --examine /dev/sd[abcd]
>> mdadm: No md superblock detected on /dev/sda.
>> /dev/sdb:
>>          Magic : a92b4efc
>>        Version : 00.90.00
>>           UUID : 8d0cf436:3fc2d2ef:93d71b24:b036cc6b
>>  Creation Time : Wed Mar 25 21:04:08 2009
>>     Raid Level : raid6
>>  Used Dev Size : 1465137408 (1397.26 GiB 1500.30 GB)
>>     Array Size : 5860549632 (5589.06 GiB 6001.20 GB)
>>   Raid Devices : 6
>>  Total Devices : 6
>> Preferred Minor : 0
>>
>>    Update Time : Tue Mar 31 23:08:02 2009
>>          State : clean
>>  Active Devices : 5
>> Working Devices : 6
>>  Failed Devices : 1
>>  Spare Devices : 1
>>       Checksum : a4fbb93a - correct
>>         Events : 8430
>>
>>     Chunk Size : 64K
>>
>>      Number   Major   Minor   RaidDevice State
>> this     6       8       16        6      spare   /dev/sdb
>>
>>   0     0       8        0        0      active sync   /dev/sda
>>   1     1       8       64        1      active sync   /dev/sde
>>   2     2       8       32        2      active sync   /dev/sdc
>>   3     3       8       48        3      active sync   /dev/sdd
>>   4     4       0        0        4      faulty removed
>>   5     5       8       80        5      active sync
>>   6     6       8       16        6      spare   /dev/sdb
>> /dev/sdc:
>>          Magic : a92b4efc
>>        Version : 00.90.00
>>           UUID : 8d0cf436:3fc2d2ef:93d71b24:b036cc6b
>>  Creation Time : Wed Mar 25 21:04:08 2009
>>     Raid Level : raid6
>>  Used Dev Size : 1465137408 (1397.26 GiB 1500.30 GB)
>>     Array Size : 5860549632 (5589.06 GiB 6001.20 GB)
>>   Raid Devices : 6
>>  Total Devices : 4
>> Preferred Minor : 0
>>
>>    Update Time : Sun Jul 12 11:31:47 2009
>>          State : clean
>>  Active Devices : 4
>> Working Devices : 4
>>  Failed Devices : 2
>>  Spare Devices : 0
>>       Checksum : a59452db - correct
>>         Events : 580158
>>
>>     Chunk Size : 64K
>>
>>      Number   Major   Minor   RaidDevice State
>> this     2       8       32        2      active sync   /dev/sdc
>>
>>   0     0       8        0        0      active sync   /dev/sda
>>   1     1       0        0        1      faulty removed
>>   2     2       8       32        2      active sync   /dev/sdc
>>   3     3       8       48        3      active sync   /dev/sdd
>>   4     4       0        0        4      faulty removed
>>   5     5       8       96        5      active sync
>> /dev/sdd:
>>          Magic : a92b4efc
>>        Version : 00.90.00
>>           UUID : 8d0cf436:3fc2d2ef:93d71b24:b036cc6b
>>  Creation Time : Wed Mar 25 21:04:08 2009
>>     Raid Level : raid6
>>  Used Dev Size : 1465137408 (1397.26 GiB 1500.30 GB)
>>     Array Size : 5860549632 (5589.06 GiB 6001.20 GB)
>>   Raid Devices : 6
>>  Total Devices : 4
>> Preferred Minor : 0
>>
>>    Update Time : Sun Jul 12 11:31:47 2009
>>          State : clean
>>  Active Devices : 4
>> Working Devices : 4
>>  Failed Devices : 2
>>  Spare Devices : 0
>>       Checksum : a59452ed - correct
>>         Events : 580158
>>
>>     Chunk Size : 64K
>>
>>      Number   Major   Minor   RaidDevice State
>> this     3       8       48        3      active sync   /dev/sdd
>>
>>   0     0       8        0        0      active sync   /dev/sda
>>   1     1       0        0        1      faulty removed
>>   2     2       8       32        2      active sync   /dev/sdc
>>   3     3       8       48        3      active sync   /dev/sdd
>>   4     4       0        0        4      faulty removed
>>   5     5       8       96        5      active sync
>>
>> --
>> Carl K
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>
>
>
> --
>       Majed B.
> --
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>

He's using v0.9 superblocks; even if the replaced device was at 99%
complete it would still have to restart from 0% when re-added.
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