Re: Recovery after failed chunk size change

[NeilBrown <nfbrown@novell.com>]

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On Fri, Apr 01 2016, Benjamin Meier wrote:

> Hi there,
>
> I tried to do a chunk size change from 4096k to 64k on a 7-disk RAID6 
> array. I am using Debian Jessie with kernel 3.16 and mdadm 3.3.2. After 
> I initiated the change the process staled immediately. I could watch it 
> in /proc/mdadm that there has not been any progress at all. The 
> backup-file hasn't been touched for days now.
>
> So I decided to backup all data from that device in case it isn't 
> starting at next reboot. Accidentally the system was restarted before 
> the backup was finished. And now the array is not assembling any more, 
> even with the correct --backup-file. I get "mdadm: Failed to restore 
> critical section for reshape, sorry.".
>
> So the first question is: How can I access the data again? I think there 
> is no damage at this time- I appended an output from --examine at the 
> end of this message. All seven drives giving me the same output in all 
> relevant topics. Especially "Chunk Size", "New Chunksize" and "Reshape 
> pos'n" is all the same.
> What is the best way now that I do not damage any data?

 mdadm --assemble --force --update=revert-reshape --invalid-backup
 --backup-file=/whatever /dev/md/TA /dev/list-of-devices

using mdadm 3.4.

>
> Second question: Is the problem with the level change a known bug?

Yes, this has been happening to a few people.  The shape doesn't really
start properly.
If someone can provide a recipe for how to reproduce the problem
(e.g. using loop-back devices) I'll happily look into fixing it, or
identifying which kernel it is already fixed in.

Neilbrown


>
> Thanks for reading!
>
> --
> /dev/disk/by-partlabel/hyper_TA_1:
>            Magic : a92b4efc
>          Version : 1.2
>      Feature Map : 0x5
>             Name : hyper:TA  (local to host hyper)
>       Raid Level : raid6
>     Raid Devices : 7
>
>   Avail Dev Size : 3434725376 (1637.80 GiB 1758.58 GB)
>       Array Size : 8586813440 (8189.02 GiB 8792.90 GB)
>      Data Offset : 147456 sectors
>     Super Offset : 8 sectors
>     Unused Space : before=147368 sectors, after=0 sectors
>            State : clean
>
> Internal Bitmap : 8 sectors from superblock
>    Reshape pos'n : 0
>    New Chunksize : 64K
>
>      Update Time : Thu Mar 31 17:57:01 2016
>    Bad Block Log : 512 entries available at offset 72 sectors
>         Checksum : e7172c1f - correct
>           Events : 527046
>
>           Layout : left-symmetric
>       Chunk Size : 4096K
>
>     Device Role : Active device 5
>     Array State : AAAAAAA ('A' == active, '.' == missing, 'R' == replacing)
>
> --
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