Re: How to recover after md crash during reshape?

[Neil Brown <nfbrown@novell.com>]

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Phil Turmel <philip@turmel.org> writes:

> Good morning Neil,
>
> On 10/20/2015 09:35 PM, Neil Brown wrote:
>
>> Nothing dumb about that - you don't need a --backup option.
>> If you did, mdadm would have complained.
>> 
>> You only need --backup when the size of the array is unchanged or
>> decreasing.
>
>> mdadm reads the first few stripes and stores them somewhere in each of
>> the spares.  md (in the kernel) then reads those stripes again and
>> writes them out in the new configuration.  It appears that one of the
>> writes failed, others might have succeeded.  This may not have corrupted
>> anything (the first few blocks are in the same position for both the old
>> and new layout) but it might have done.
>
>> If you do want to look for the backup, it is around about the middle of
>> the device and has some metadata which contains the string
>> "md_backup_data-1".  If you find that, you are close to getting the
>> backup data back.
>
> Hmmm.  This feature has advanced beyond my last look at the code.  I was
> under the impression the backup option was only optional when mdadm
> could move the data offset.  Does this new algorithm apply to v0.90
> metadata, a v3.2 kernel, and v3.2.5 mdadm?
>

It isn't a new algorithm, it is the original algorithm.

In mdadm-2.4-pre1 (march 2006), you couldn't specify a backup file, but
you could grow a raid5 to more devices.
That was changed by a patch with comment:

    Allow resize to backup to a file.
    
    To support resizing an array without a spare, mdadm now understands
      --backup-file=
    which should point to a file for storing a backup of critical data.
    This can be given to --grow which will create the file, or
    --assemble which will restore from the file if needed.
    
The backup-file was subsequently used to support in-place reshapes and
array shrinking.

NeilBrown

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