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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 Fri, 29 Apr 2011 05:39:40 +0100, Ben Hutchings <ben@decadent.org.uk> wrote:
> On Wed, 2011-04-27 at 09:19 -0700, Jameson Graef Rollins wrote:
> > I run what I imagine is a fairly unusual disk setup on my laptop,
> > consisting of:
> > 
> >   ssd -> raid1 -> dm-crypt -> lvm -> ext4
> > 
> > I use the raid1 as a backup.  The raid1 operates normally in degraded
> > mode.  For backups I then hot-add a usb hdd, let the raid1 sync, and
> > then fail/remove the external hdd. 
> Well, this is not expected to work.  Possibly the hot-addition of a disk
> with different bio restrictions should be rejected.  But I'm not sure,
> because it is safe to do that if there is no mounted filesystem or
> stacking device on top of the RAID.

Hi, Ben.  Can you explain why this is not expected to work?  Which part
exactly is not expected to work and why?

> I would recommend using filesystem-level backup (e.g. dirvish or
> backuppc).  Aside from this bug, if the SSD fails during a RAID resync
> you will be left with an inconsistent and therefore useless 'backup'.

I appreciate your recommendation, but it doesn't really have anything to
do with this bug report.  Unless I am doing something that is
*expressly* not supposed to work, then it should work, and if it doesn't
then it's either a bug or a documentation failure (ie. if this setup is
not supposed to work then it should be clearly documented somewhere what
exactly the problem is).

> The block layer correctly returns an error after logging this message.
> If it's due to a read operation, the error should be propagated up to
> the application that tried to read.  If it's due to a write operation, I
> would expect the error to result in the RAID becoming desynchronised.
> In some cases it might be propagated to the application that tried to
> write.

Can you say what is "correct" about the returned error?  That's what I'm
still not understanding.  Why is there an error and what is it coming


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