[DRBD-user] Performance regression with DRBD 8.3.12 and newer

[Matthias Hensler]

On Mon, Jun 11, 2012 at 06:35:18PM +0200, Matthias Hensler wrote:
> [...]
> I checked the changelog for 8.3.12, but nothing obviously struck me.
> Also diffing the sourcetrees 8.3.11->8.3.12 I did not find any
> obvious.

Let me follow up on this myself. As suggested on IRC I tried to build
drbd from source, just to take the elrepo packages from the equation.

So I started with DRBD 8.3.13, and as expected I had a low performance.

Then I tried 8.3.11, and I also had a low performance (although 8.3.11
from elrepo worked fine).

That left me puzzled for a while, since I examined the elrepo packages
more closely. As it seemed, all working drbd versions where build on
2.6.32-71, while all broken versions where build on 2.6.32-220.


So, I installed the old el6 2.6.32-71 kernel (took me a while to find
it, since it was removed from nearly all archives) and its devel
package, booted into that kernel and build two new versions from source:
8.3.11 and 8.3.13. Then I booted back to 2.6.32-220.

First try with my selfcompiled 8.3.11 modules: everything is fine.
Second try with my selfcompiled 8.3.13 modules: still everything is
fine.

Indeed, the problem lies within the kernel version used to build the
drbd.ko module. I double checked by using all userland tools from 8.3.13
elrepo build together with my drbd.ko build on 2.6.32-71 (but run from
2.6.32-220).

Just to be clear: all tests were made with kernel 2.6.32-220, and the
userland version does not matter.

drbd.ko              | 8.3.11 | 8.3.13
---------------------+--------+-------
build on 2.6.32-71   | good   | good
build on 2.6.32-220  | bad    | bad


So, how to debug this further? I would suspect looking at the symbols of
both modules might give a clue?

Regards,
Matthias
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