Re: 4 partition raid 5 with 2 disks active and 2 spare, how to force?

Thanks Michael, I am clear about the problem of why the multiple failure would cause me to lose data. Which is why I wanted to consult this mailing list before proceeding. 

Could you tell me how to keep the array read-only?  and mark one or both of these spares as active forcibly? and Also, once I am able to use these spares as active and the data is not consistent in a particular stripe, how does the kernel resolve the inconsistency (as in what data does it use, the one based on the data stripes or the one based on the parity?) this one is just academic interest since it'll be difficult to figure out which is the right data anyways.

Thanks,
Anshuman

On 25-Mar-2010, at 5:07 PM, Michael Evans wrote:

On Thu, Mar 25, 2010 at 2:30 AM, Anshuman Aggarwal
[off-list ref] wrote:

quoted

All, thanks in advance...particularly Neil.

My raid5 setup has 4 partitions, 2 of which are showing up as spare and 2 as active. The mdadm --assemble --force gives me the following error:
2 active devices and 2 spare cannot start device

it is a raid 5, with superblock 1.2, 4 devices in the order sda1, sdb5, sdc5, sdd5. I have lvm2 on top of this with other devices ...so as you all know data is irreplaceable blah blah.

I know that this device has not been written to for a while, so the data can be considered intact (hopefully all) if I can get the device to start up...but I'm not sure of the best way to coax the kernel to assemble it. Relevant information follows:

=== This device is working fine ===
mdadm --examine  -e1.2 /dev/sdb5
/dev/sdb5:
         Magic : a92b4efc
       Version : 1.2
   Feature Map : 0x1
    Array UUID : 42c56ea0:2484f566:387adc6c:b3f6a014
          Name : GATEWAY:127  (local to host GATEWAY)
 Creation Time : Sat Aug 22 09:44:21 2009
    Raid Level : raid5
  Raid Devices : 4

 Avail Dev Size : 586099060 (279.47 GiB 300.08 GB)
    Array Size : 1758296832 (838.42 GiB 900.25 GB)
 Used Dev Size : 586098944 (279.47 GiB 300.08 GB)
   Data Offset : 272 sectors
  Super Offset : 8 sectors
         State : clean
   Device UUID : f8ebb9f8:b447f894:d8b0b59f:ca8e98eb

Internal Bitmap : 2 sectors from superblock
   Update Time : Fri Mar 19 00:56:15 2010
      Checksum : 1005cfbc - correct
        Events : 3796145

        Layout : left-symmetric
    Chunk Size : 64K

  Device Role : Active device 2
  Array State : .AA. ('A' == active, '.' == missing)

=== This device is marked spare, can be marked active (IMHO) ===
mdadm --examine  -e1.2 /dev/sdd5
/dev/sdd5:
         Magic : a92b4efc
       Version : 1.2
   Feature Map : 0x1
    Array UUID : 42c56ea0:2484f566:387adc6c:b3f6a014
          Name : GATEWAY:127  (local to host GATEWAY)
 Creation Time : Sat Aug 22 09:44:21 2009
    Raid Level : raid5
  Raid Devices : 4

 Avail Dev Size : 586099060 (279.47 GiB 300.08 GB)
    Array Size : 1758296832 (838.42 GiB 900.25 GB)
 Used Dev Size : 586098944 (279.47 GiB 300.08 GB)
   Data Offset : 272 sectors
  Super Offset : 8 sectors
         State : clean
   Device UUID : 763a832f:1a9a7ea8:ce90d4a3:32e8ae54

Internal Bitmap : 2 sectors from superblock
   Update Time : Fri Mar 19 00:56:15 2010
      Checksum : c78aab46 - correct
        Events : 3796145

        Layout : left-symmetric
    Chunk Size : 64K

  Device Role : spare
  Array State : .AA. ('A' == active, '.' == missing)


=== This is the completely failed device (needs replacement)    ===
mdadm --examine  -e1.2 /dev/sda1
[HANGS!!]



I already have the replacement drive available as sde5 but want to be able to reconstruct as much as possible)

Thanks again,
Anshuman Aggarwal--
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You have a raid 5 array.

(drives then data+parity per drive as an example)
1234

123P
45P6
7P89
...

You are missing two drives, meaning you lack parity and 1 data stripe
and have NO parity to recover it with.

It's like seeing:

.23.
.5P.
.P8.

and expecting to somehow recover the missing data when it is no longer
within the clean information.

Your only hope is to assemble the array in read only mode with the
other devices, if they can still even be read.  In that case you might
at least be able to recover nearly all of your data; hopefully any
missing areas are in unimportant files or non-allocated space.

At this point you should be EXTREMELY CAREFUL, and DO NOTHING, without
having a good solid plan in place.  Rushing /WILL/ cause you to loose
data that might still potentially be recovered.