mdadm, superblocks, spooky behavior...

[dsr-mzpnVDyJpH4k7aNtvndDlA at public.gmane.org (Dan Ritter)]

On Mon, Aug 03, 2009 at 04:57:36PM -0400, Kent Borg wrote:
> Some mdadm guru, please help me!
> 
> I am confused about mdadm.  I have a raid0 array that mdadm won't
> assemble, but if I make loopbacks of the components I *can* assemble my
> array from that.
> 
> 
> I have an entry in /etc/mdadm/mdadm.conf for my array and when I do:
> 
>   # mdadm --assemble --scan
>   mdadm: no devices found for /dev/md25
> 
> Let's risk blowing away my data (after a backup):
> 
>   # mdadm --create /dev/md25 --level=0 --raid-devices=4 /dev/md16
> /dev/md17 /dev/md18 /dev/md19
>   mdadm: Cannot open /dev/md16: Device or resource busy
>   mdadm: Cannot open /dev/md17: Device or resource busy
>   mdadm: Cannot open /dev/md18: Device or resource busy
>   mdadm: Cannot open /dev/md19: Device or resource busy
>   mdadm: create aborted
> 
> Look at one of the components with "mdadm --misc --examine /dev/md19"
> and it knows it is part of md25.  Each of the four components claims the
> same UUID for md25.
> 
> 
> Some background:  I have a pair of disks that I chopped up into a bunch
> of matching partitions, and I have each pair in a raid1 array.  Most of
> those are used directly, but I wanted a bigger partition; that is why I
> am gluing them into a raid0 array.  And because there is raid1
> underneath the raid0, that striping isn't as dangerous as it looks.

You almost certainly want raid10. Not raid01. 

mdadm knows how to do raid10.

mdadm --create /dev/md25 --level=raid10 --bitmap internal -n 4 \
/dev/partition1 /dev/partition2 /dev/partition3 /dev/partition4

or as appropriate.

-dsr-


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