Degraded raid handling

[Phillip Susi]

Heinz Mauelshagen wrote:
>>>> If the mirror IS activated by dmraid with one disk missing, does dmraid 
>>>> update the metadata to indicate that the set is inconsistent?
>>> Not yet. You'ld need to use the BIOS management util to do that for now.
>> Whoa, that sounds like a data loosing bug.  If dmraid activates the 
>> mirror in a degraded state, then the missing disk will become out of 
>> sync.  Without marking the disk as inconsistent, if the other disk 
>> reappears when you reboot, the bios will activate the set thinking it is 
>> consistent when it really is not, which can lead to incorrect operation 
>> and data loss.
> 
> In that case you would have removed the drive and used the setup util
> to fix things already. But yes, if you don't, you can get a problem.

It could happen that the bios sees both disks just fine, but for 
whatever reason, the kernel only sees one.  In that case, dmraid will 
activate the set with just the one disk, but without updating the 
metadata to indicate this has been done, the set will be corrupted at 
the next boot.  Not good...