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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 02:04:18 +0100, Ben Hutchings <ben@decadent.org.uk> wrote:
> On Sun, 2011-05-01 at 20:42 -0400, Daniel Kahn Gillmor wrote:
> So far as I'm aware, the RAID may stop working, but without loss of data
> that's already on disk.

What exactly does "RAID may stop working mean"?  Do you mean that this
bug will be triggered?  The raid will refuse to do further syncs?  Or do
you mean something else?

> > How is an admin to know which I/O capabilities to check before adding a
> > device to a RAID array?  When is it acceptable to mix I/O capabilities?
> >  Can a RAID array which is not currently being used as a backing store
> > for a filesystem be assembled of unlike disks?  What if it is then
> > (later) used as a backing store for a filesystem?
> [...]
> I think the answers are:
> - Not easily
> - When the RAID does not have another device on top

This is very upsetting to me, if it's true.  It completely undermines
all of my assumptions about how software raid works.

Are you really saying that md with mixed disks is not possible/supported
when the md device has *any* other device on top of it?  This is a in
fact a *very* common setup.  *ALL* of my raid devices have other devices
on top of them (lvm at least).  In fact, the debian installer supports
putting dm and/or lvm on top of md on mixed disks.  If what you're
saying is true then the debian installer is in big trouble.


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