Interrupted reshape -- mangled backup ?

I have a Raid5 array with 4 devices that I wanted to see if I could get a better performance out of, so i tried changing the chunk size from 64K to something bigger. (famous last words) . I got into some other trouble and thought I needed a reboot. On reboot I several times managed to mount and specify the device with my backup file during initramfs, but the reshape stopped every time once the system was at initialized.

This is under debian sqeeze with a 3.2.0-0.bpo.3-686-pae kernel from backports. I installed mdadm from backports to get the latest version of that as well, and tried rebooting with --freeze-reshape. Suspect that I mixed up my initrd.img-files and started without --freeze-reshape the first time after installing the new mdadm. Now mdadm says it can not find a backup in my backup file. Opening up the backup in emacs, it seems to contain only NULs. Can't be right, can it? I have been mounting the backup under a directory under /dev/, on the assumption that the mount wold survive past the initramfs stage.

My bumbling has been happening with a current, correct, /etc/mdadm/mdadm.conf containigng:
--------
DEVICE /dev/sdh /dev/sde /dev/sdc /dev/sdd
CREATE owner=root group=disk mode=0660 auto=yes
HOMEHOST <system>
ARRAY /dev/md1 level=raid5 num-devices=4 UUID=583001c4:650dcf0c:404aaa6f:7fc38959 spare-group=main
-------
The show-stopper happened with an initramfs and a script in /scripts/local-top/mdadm along the lines of:
-------
/sbin/mdadm --assemble -f --backup-file=/dev/bak/md1-backup /dev/md1 --run --auto=yes /dev/sdh /dev/sde /dev/sdc /dev/sdd
-------

At times I have also had to use the env-variable MDADM_GROW_ALLOW_OLD=1

Below is the output of mdadm -Evvvvs:
--------

/dev/sdh:

          Magic : a92b4efc
        Version : 0.91.00
           UUID : 583001c4:650dcf0c:404aaa6f:7fc38959
  Creation Time : Wed Dec  3 19:45:33 2008
     Raid Level : raid5
  Used Dev Size : 976762496 (931.51 GiB 1000.20 GB)
     Array Size : 2930287488 (2794.54 GiB 3000.61 GB)

Raid Devices : 4
Total Devices : 4
Preferred Minor : 1

Reshape pos'n : 2368561152 (2258.84 GiB 2425.41 GB) New Chunksize : 131072

Update Time : Wed Oct 17 02:15:53 2012
State : active
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Checksum : 14da0760 - correct
Events : 778795

Layout : left-symmetric
Chunk Size : 64K

Number Major Minor RaidDevice State
this 0 8 112 0 active sync /dev/sdh

   0     0       8      112        0      active sync   /dev/sdh
   1     1       8       48        1      active sync   /dev/sdd
   2     2       8       32        2      active sync   /dev/sdc
   3     3       8       64        3      active sync   /dev/sde
/dev/sde:
          Magic : a92b4efc
        Version : 0.91.00
           UUID : 583001c4:650dcf0c:404aaa6f:7fc38959
  Creation Time : Wed Dec  3 19:45:33 2008
     Raid Level : raid5
  Used Dev Size : 976762496 (931.51 GiB 1000.20 GB)
     Array Size : 2930287488 (2794.54 GiB 3000.61 GB)

Raid Devices : 4
Total Devices : 4
Preferred Minor : 1

Reshape pos'n : 2368561152 (2258.84 GiB 2425.41 GB) New Chunksize : 131072

Update Time : Wed Oct 17 02:15:53 2012
State : active
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Checksum : 14da0736 - correct
Events : 778795

Layout : left-symmetric
Chunk Size : 64K

Number Major Minor RaidDevice State
this 3 8 64 3 active sync /dev/sde

   0     0       8      112        0      active sync   /dev/sdh
   1     1       8       48        1      active sync   /dev/sdd
   2     2       8       32        2      active sync   /dev/sdc
   3     3       8       64        3      active sync   /dev/sde
/dev/sdc:
          Magic : a92b4efc
        Version : 0.91.00
           UUID : 583001c4:650dcf0c:404aaa6f:7fc38959
  Creation Time : Wed Dec  3 19:45:33 2008
     Raid Level : raid5
  Used Dev Size : 976762496 (931.51 GiB 1000.20 GB)
     Array Size : 2930287488 (2794.54 GiB 3000.61 GB)

Raid Devices : 4
Total Devices : 4
Preferred Minor : 1

Reshape pos'n : 2368561152 (2258.84 GiB 2425.41 GB) New Chunksize : 131072

Update Time : Wed Oct 17 02:15:53 2012
State : active
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Checksum : 14da0714 - correct
Events : 778795

Layout : left-symmetric
Chunk Size : 64K

Number Major Minor RaidDevice State
this 2 8 32 2 active sync /dev/sdc

   0     0       8      112        0      active sync   /dev/sdh
   1     1       8       48        1      active sync   /dev/sdd
   2     2       8       32        2      active sync   /dev/sdc
   3     3       8       64        3      active sync   /dev/sde
/dev/sdd:
          Magic : a92b4efc
        Version : 0.91.00
           UUID : 583001c4:650dcf0c:404aaa6f:7fc38959
  Creation Time : Wed Dec  3 19:45:33 2008
     Raid Level : raid5
  Used Dev Size : 976762496 (931.51 GiB 1000.20 GB)
     Array Size : 2930287488 (2794.54 GiB 3000.61 GB)

Raid Devices : 4
Total Devices : 4
Preferred Minor : 1

Reshape pos'n : 2368561152 (2258.84 GiB 2425.41 GB) New Chunksize : 131072

Update Time : Wed Oct 17 02:15:53 2012
State : active
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Checksum : 14da0722 - correct
Events : 778795

Layout : left-symmetric
Chunk Size : 64K

Number Major Minor RaidDevice State
this 1 8 48 1 active sync /dev/sdd

   0     0       8      112        0      active sync   /dev/sdh
   1     1       8       48        1      active sync   /dev/sdd
   2     2       8       32        2      active sync   /dev/sdc
   3     3       8       64        3      active sync   /dev/sde

---------------------------

I guess the moral of all this is that if you want to use mdadm you should pay attention and not be in too much of a hurry :-/ .
I'm just hoping that I can get my system back. This raid contains my entire system, and will take a LOT of work to recreate. Mail, calendars ... . Backups are a couple of weeks old ...

So worst-case you can do that again, but insert a "sleep 365d" immediately after the "mdadm --assemble" is run, so the system never completely initialises. Then just wait for the reshape to finish.

When mdadm assembles and array that needs to keep growing it will for a background process to continue monitoring the reshape process. Presumably that background process is getting killed. I don't know why.

The backup file could certainly contain lots of nuls, but it shouldn't be *all* nulls. At least there should be a header at the start which describes which area of the device is contained in the backup.

You can continue without a backup. You still need to specify a backup file, but if you add "--invalid-backup", it will continue even if the backup file doesn't contain anything useful.
If the machine was shutdown by a crash during reshape you might suffer corruption. If it was a clean shutdown you won't.

--freeze-reshape is intended to be the way to handle this, with
--grow --continue
once you are fully up and running, but I don't think that works correctly for 'native' metadata yet - it was implemented with IMSM metadata in mind.

NeilBrown