[DRBD-user] Protocol D

Lars Ellenberg lars.ellenberg at linbit.com
Wed Oct 9 21:05:40 CEST 2013

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

> > On Wednesday 28 August 2013 00:07:35 Walter Robert Ditzler wrote:
> > > Hi DRBD List
> > >
> > > I would like to know if there is any plan to start a closer support together
> > > with XEN and DRBD related to Protocol D in the near feature?

> On 28.08.2013, at 15:28, Eugene Istomin <e.istomin at edss.ee> wrote:
> > Sirs,
> > would be really nice if you merge Remus Protocol D
> > (git://aramis.nss.cs.ubc.ca/drbd-8.3-remus) with DRBD mainline, I
> > can help you with testing.

I understand that some people would very much like
to have this working and supported.

It has not been merged back then,
because we see potential of data (silent) data corruption.

In fact I remember that I had the impression this,
as implemented, this cannot possibly work,
theoretically, *in general*.

It may *appear* to work for some time.
I don't remember the details, but I think the coordination of in-memory
and on-storage checkpoints was just racy, and you will simply get lucky
"most of the time", and even if you won't be so lucky, it is very much
possible that you won't notice the inconsistencies immediately.
Which is known as silent data corruption, as is one of the worst things
to happen to your data, ever.

Given different priorities on our side right now,
I don't see us working on understanding and fixing these races
(if they actually exist). Not nearterm, anyways.

Seeing that I don't even quite remember the technical details,
I may be wrong in my assessment.  Maybe it does work as is,
and it may even be the coolest thing that happened to
the combination of virtualization + HA, ever.

Feel free to try and convince me of that,
or reprioritize DRBD's next top feature,
so we would make this work, if technically sensibly possible.

But as things are now,
if you use this "remus + DRBD protocol D",
and it breaks, you get to keep the pieces.

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

DRBD® and LINBIT® are registered trademarks of LINBIT, Austria.
please don't Cc me, but send to list   --   I'm subscribed

More information about the drbd-user mailing list