[CentOS] Question about RAID 5 array rebuild with mdadm

[Mark Hennessy]

Thanks for answering my e-mail!!

On Apr 17, 2008, at 1:50 PM, Ross S. W. Walker wrote:

> Mark Hennessy wrote:
>>
>> ok, now it says that the array is recovering and will take about 10
>> hours to rebulid.
>> /dev/sd[c-i] say that they are "active	sync" and
>> /dev/sdj says it's a
>> spare that's rebuilding.
>> But now I scroll back in my history and see that oops, the chunk size
>> is WRONG.  Not only that, but I don't stop the array until the  
>> rebuild
>> is at around 8%
>
> Well, now I think it's all messed up.
>
>> Ok, I stop the array and rebuild with
>> mdadm --create /dev/md2 --assume-clean --level=5 --chunk --raid-
>> devices=8 /dev/sdc /dev/sdd /dev/sde /dev/sdf /dev/sdg /dev/sdh /dev/
>> sdi /dev/sdj
>>
>> Now it says it's going to take another 10 hours to rebuild.

> It's truly hosed now.
>

I was thinking that too, but I waited until the drive was about 5%  
recovered and mounted it read-only.  It mounted successfully.  I was  
able to cat log files stored there as well as do full listings of  
tarballs there without interruption.  I went ahead and copied a bunch  
of important things off of that array onto another one and received no  
complaints from the OS.

What did I miss?  I just want to learn and to understand.  Perhaps  
there is documentation that I didn't find via Google and Wikipedia  
that would explain in more detail how this works that you could direct  
me to.

Thanks for your kind assistance!

>> How likely are my data irretrievable/gone and at what step would it
>> have happened if so?
>
> I hope you have backups cause your going to need them.

What's the likelihood of data corruption despite the fs being  
browsable and the files accessible like I describe?

>
> If only you posted to the list BEFORE you tried to recover it without
> knowing what to do.

Agreed (strongly).

>
> -Ross
>
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