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Bug#624343: linux-image-2.6.38-2-amd64: frequent message "bio too big device md0 (248 > 240)" in kern.log

On Mon, 02 May 2011 11:11:25 +0200, David Brown <david@westcontrol.com> wrote:
> This is not directly related to your issues here, but it is possible to 
> make a 1-disk raid1 set so that you are not normally degraded.  When you 
> want to do the backup, you can grow the raid1 set with the usb disk, 
> want for the resync, then fail it and remove it, then "grow" the raid1 
> back to 1 disk.  That way you don't feel you are always living in a 
> degraded state.

Hi, David.  I appreciate the concern, but I am not at all concerned
about "living in a degraded state".  I'm far more concerned about data
loss and the fact that this bug has seemingly revealed that some
commonly held assumptions and uses of software raid are wrong, with
potentially far-reaching affects.

I also don't see how the setup you're describing will avoid this bug.
If this bug is triggered by having a layer between md and the filesystem
and then changing the raid configuration by adding or removing a disk,
then I don't see how there's a difference between hot-adding to a
degraded array and growing a single-disk raid1.  In fact, I would
suspect that your suggestion would be more problematic because it
involves *two* raid reconfigurations (grow and then shrink) rather than
one (hot-add) to achieve the same result.  I imagine that each raid
reconfiguration could potentially triggering the bug.  But I still don't
have a clear understanding of what is going on here to be sure.


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