Re: [dm-devel] cmwq and dm-crypt devices?

[Alasdair G Kergon <agk redhat com>]

On Wed, Nov 03, 2010 at 01:33:47PM +0100, Andi Kleen wrote:
> Traditionally stacking didn't work very 
> well in the kernel due to the limited kernel stack overflows. 
> I don't think that's significantly different here.
 
Stacking in dm works reasonably well today and many people rely upon
that.

> Anyways stacking could be probably fixed, but it's also that 99.999999%

Please fix this regression the patch causes then or give some
fundamental arguments why we can't satisfy the two requirements
simultaneously: I just don't understand why it is necessary to cause a
regression in stacking/low memory support in order to provide the
performance boost in the way you desire.  The two things ought to be
independent of each other.

(And I remain sceptical about the suitability of the patch even with a
stack of just two or three layers as I don't see anything in the failed
tests that shows why similar failures couldn't in principle affect
people with thinner stacks.  The compromise I suggested was to amend the
patch to introduce the change only for non-stacked devices.)

Alasdair