[Drbd-dev] DRBD8: Split-brain if primary and syncTarget

[Philipp Reisner]

Am Donnerstag, 8. März 2007 23:21 schrieb Montrose, Ernest:
> Hi all,
>
> We are seeing an issue with split brain if one node is syncing as
> syncTarget while being Primary.
> two node A and B.
> * make B primary and the syncTarget
> * Start a sync.
> * ifdown eth1 to break communication
> * ifup eth1.
> * then on the node in standalone "drbdadm connect"
> We get a split-brain.
>
> I think the  problem is that if we are primary and we lose contact from
> the other side we generate a new current UUID which causes a Split-Brain
> next time we connect.
> This only happens if we are the sync target and we are primary. Perhaps
> we should not generate a UUID if we were syncing when the disconnect
> happen. Below is a possible patch for this in after_state_ch():

Hi Ernest,

I think the current behaviour is correct.

* When a node is SyncTarget it actually exposes the data of the sync
  source node to its applications. (And the applications can potentially 
  see the data when the SyncTarget node is primary.)

* When you disconnect such a node, it has to fall back to its local
  data set. == suddenly the applications see a different data set,
  and of course the apps might continue to modify this data set...

* Wen you reconnect this, you have a split brain situation. But you 
  might let the automatic-split-brain resolving handler solve the
  situation. Use some after-sb-?pri settings, and an rr-conflict of
  "violently" E.g.:

  after-sb-0pri discard-least-changes
  after-sb-1pri violently-as0p
  after-sb-2pri violently-as0p
  rr-conflict   violently

  Then the resync should continue. Since the "violently" allows DRBD
  to change the data set again, that is seen on the Primary node.

-Phil
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