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

Matthias Hensler lists-drbd at wspse.de
Mon Jun 11 23:00:09 CEST 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Mon, Jun 11, 2012 at 10:31:16PM +0200, Florian Haas wrote:
> On 06/11/12 22:14, Matthias Hensler wrote:
> > 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?
> As a knee-jerk response based on a hunch -- you've been warned :) --,
> this could be related to the BIO_RW_BARRIER vs. FLUSH/FUA dance that the
> RHEL 6 kernel has been doing between the initial RHEL 6 release, and
> more recent updates (when they've been backporting the "let's kill
> barriers" upstream changes from post-2.6.32).


> Try configuring your disk section with no-disk-barrier, no-disk-flushes
> and no-md-flushes (in both configurations) and see if your kernel module
> change still makes a difference.

Just did that:

Using the drbd.ko build on 2.6.32-71 shows minor increase in
performance (108,5 MByte/s, so some 5% more or so).

Using the drbd.ko build on 2.6.32-220.17.1 now finally brings the
expected performance (same as with the 2.6.32-71 built).

> Of course, in production you should only use those options if you have
> no volatile caches involved in the I/O path.

Yes, that is clear. I did not plan to disable barriers, as the
bottleneck in my setup should be clearly the network.

> Not sure if this is useful, but I sure hope it is. :)

Well, what does that mean: are the modules build on 2.6.32-71 broken in
a way that they do not use barriers (and therefore dangerous to use), or
is everything fine with the 2.6.32-71 builds and just building on a
newer kernel produces broken modules?

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