On Thu, Dec 17, 2009 at 4:35 AM, Majed B. [off-list ref] 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).

right - but I am not sure if data loss has occurred, where data is the
data being stored on the raid, not the raid metadata.

My guess is I need to copy the raid superblock from one of the other
disks (say sdb), find the byets that identify the disk and change from
sdb to sda.

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

root@dhcp128:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
[raid4] [raid10]
unused devices: <none>

Is the array currently running degraded or is it suspended?

um, not running, not sure I would call it suspended.

What happened to the spare disk assigned?

I don't understand.

Did it finish resyncing
before you installed grub on the wrong disk?

I think so.

I am fairly sure I could assemble the array before I installed grub.

On Thu, Dec 17, 2009 at 8:21 AM, Majed B. [off-list ref] wrote:

quoted

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" [off-list ref] 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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