On 02.02.2012 01:55, Daniel Kahn Gillmor wrote:
> hi folks--
>
> i was speaking with phcoder today on #grub, about getting messages like
> this from grub when a partition that is part of a Linux SW RAID set
> (with metadata 0.9x) is close to the end of its containing block device:
>
>    superfluous RAID member (2 found)
Try attached patch
> Here's phcoder's explanation of the problem:
>
>> 16:08<  phcoder>  if you have<  64KiB between end of disk and end of partition
>>                   the metadata is exactly in the same place for either if the
>>                   whole disks are raided or only partitions. And no field which
>>                   allows to distinguish those
>   [...]
>> 16:09<  phcoder>  thing is that mdraid format rounds it down to a 64K aligned
>>                   boundary
>> 16:10<  dkg0>  64KiB aligned to the parent disk, or to the partition itself?
>> 16:10<  phcoder>  to whatever the host of the raid is. For your error to occur
>>                   it has to be both
>> 16:10<  dkg0>  hm, i suppose if the partition itself starts evenly aligned with
>>                a 64KiB boundary, it'd be the same thing.
>
> It sounds like there might be some circumstances (e.g. a RAID0 set of
> sda and sdb, creating md0, and a partition table on top of md0) where it
> is legitimately difficult to decide the correct interpretation.
>
> However, i think there might be a reasonable heuristic that can be used
> in the case of RAID1 sets that would avoid the ambiguity (and the scary
> messages/warnings to the user:
>
> Select the smallest known block device that can completely enclose the
> RAID member.  The larger block device(s) should not be considered to be
> exporting that RAID.
>
> these heuristics would mean that RAID1 sets with 0.9x metadata on any
> sort of disklabel would only have their member components show up once
> during a scan, rather than treating them as a duplicate.
>
> phcoder mentioned that the RAID code was undergoing something of an
> overhaul.  perhaps these heuristics could play into that update?
>
> I'm not sure how to address it with RAID0 or RAID10, but if we could fix
> the RAID1 case, that would be a big win for a lot of users.
>
> Regards,
>
> 	--dkg
>
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-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko