Re: Recover array after I panicked

[Andreas Klauer <Andreas.Klauer@metamorpher.de>]

On Sun, Apr 23, 2017 at 10:06:15PM +0800, Brad Campbell wrote:
> Nobody seems to have mentioned the reshape issue.

Good point.

If it was mid-reshape you need two sets of overlays, 
create two RAIDs (one for each configuration), and 
then find the point where it converges.

> If my reading of the code is correct (and my memory
> is any good), simply adding a disk to a raid5 on a 
> recent enough kernel should make the resync go backwards.

Doesn't it cut the offset by half and grow forwards...?

With growing a disk that should give you a segment where 
data is identical for both 5-disk and 6-disk RAID-5. 
And that's where you join them using dmsetup linear.

Before:

/dev/loop0:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : 4611f41b:0464e815:8b6f9cfe:b29c56fd
           Name : EIS:42  (local to host EIS)
  Creation Time : Sun Apr 23 16:44:59 2017
     Raid Level : raid5
   Raid Devices : 5

 Avail Dev Size : 11720783024 (5588.90 GiB 6001.04 GB)
     Array Size : 23441565696 (22355.62 GiB 24004.16 GB)
  Used Dev Size : 11720782848 (5588.90 GiB 6001.04 GB)
    Data Offset : 262144 sectors
   Super Offset : 8 sectors
   Unused Space : before=262064 sectors, after=176 sectors
          State : clean
    Device UUID : acd8d9fd:7b7cf9a0:f63369d1:907ffa66

Internal Bitmap : 8 sectors from superblock
    Update Time : Sun Apr 23 16:44:59 2017
  Bad Block Log : 512 entries available at offset 32 sectors
       Checksum : f89bdc5 - correct
         Events : 2

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 0
   Array State : AAAAA ('A' == active, '.' == missing, 'R' == replacing)

After/During grow:

/dev/loop0:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x45
     Array UUID : 4611f41b:0464e815:8b6f9cfe:b29c56fd
           Name : EIS:42  (local to host EIS)
  Creation Time : Sun Apr 23 16:44:59 2017
     Raid Level : raid5
   Raid Devices : 6

 Avail Dev Size : 11720783024 (5588.90 GiB 6001.04 GB)
     Array Size : 29301957120 (27944.52 GiB 30005.20 GB)
  Used Dev Size : 11720782848 (5588.90 GiB 6001.04 GB)
    Data Offset : 262144 sectors
|     New Offset : 257024 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : acd8d9fd:7b7cf9a0:f63369d1:907ffa66

Internal Bitmap : 8 sectors from superblock
|  Reshape pos'n : 1472000 (1437.50 MiB 1507.33 MB)
|  Delta Devices : 1 (5->6)

    Update Time : Sun Apr 23 16:45:38 2017
  Bad Block Log : 512 entries available at offset 32 sectors
       Checksum : fbd9a55 - correct
         Events : 30

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 0
   Array State : AAAAAA ('A' == active, '.' == missing, 'R' == replacing)

Basically you have to know the New Offset 
(search first 128M of your drives for filesystem headers, that should be it)
and then guess the Reshape pos'n by comparing raw data at offset X 
(find non-zero data at identical offsets for both raid sets)

Regards
Andreas Klauer