[Drbd-dev] Another drbd race

Philipp Reisner philipp.reisner at linbit.com
Tue Sep 7 11:39:29 CEST 2004

On Saturday 04 September 2004 12:00, Lars Ellenberg wrote:
> On Sat, Sep 04, 2004 at 11:48:14AM +0200, Lars Marowsky-Bree wrote:
> > Hi,
> >
> > lge and I have yesterday discussed a 'new' drbd race condition and also
> > touched on its resolution.
> >
> > Scope: in a split-brain, drbd might confirm write to the clients and
> > might on a subsequent failover lose the transactions which _have been
> > confirmed_. This is not acceptable.
> >
> > Sequence:
> >
> > Step N1 Link N2
> > 1 P ok S
> > 2 P breaks S node1 notices, goes into stand alone,
> >     stops waiting for N2 to confirm.
> > 3 P broken S S notices, initiates fencing
> > 4 x broken P N2 becomes primary
> >
> > Writes which have been done in between step 2-4 will have been confirmed
> > to the higher layers, but are not actually available on N2. This is data
> > loss; N2 is still consistent, but lost confirmed transaction.
> >
> > Partially, this is solved by the Oracle-requested "only ever confirm if
> > committed to both nodes", but of course then if it's not a broken link,
> > but N2 really went down, we'd be blocking on N1 forever, which we don't
> > want to do for HA.
> >
> > So, here's the new sequence to solve this:
> >
> > Step N1 Link N2
> > 1 P ok S
> > 2 P(blk) ok X P blocks waiting for acks; heartbeat
> >     notices that it has lost N2, and initiates
> >     fencing.
> > 3 P(blk) ok fenced heartbeat tells drbd on N1 that yes, we
> >     know it's dead, we fenced it, no point
> >     waiting.
> > 4 P ok fenced Cluster proceeds to run.
> >
> > Now, in this super-safe mode, if now N1 also fails after step 3 but
> > before N2 comes back up and is resynced, we need to make sure that N2
> > does refuse to become primary itself. This will probably require
> > additional magic in the cluster manager to handle correctly, but N2
> > needs an additional flag to prevent this from happening by accident.
> >
> > Lars?
> I think we can do this detection already with the combination of the
> Consistent and Connected as well as HaveBeenPrimary flag. Only the logic
> needs to be built in.

I do not want to "misuse" the Consistent Bit for this.

!Consistent  .... means that we are in the middle of a sync.
                   = data is not usable at all.
 Fenced      .... our data is 100% okay, but not the latest copy.

> Most likely right after connection loss the Primary should blocks for a
> configurable (default: infinity?) amount of time before giving end_io
> events back to the upper layer.
> We then need to be able to tell it to resume operation (we can do this,
> as soon as we took precautions to prevent the Secondary to become
> Primary without being forced or resynced before).
> Or, if the cluster decides to do so, the Secondary has time to STONITH
> the Primary (while that is still blocking) and take over.
> I want to include a timeout, so the cluster manager don't need to
> know about "peer is dead" notification, it only needs to know about

I see. Makes sense, but on the other hand STONITH (more genral:
FENCING)  might fail, as LMB points out in one of the other mails.

-> We should probabely _not_ offer a timeout here, as soon as
   "on-disconnect freeze_io;" is set, it is freezed forever.
   Or it gets a "drbdadm resume-io r0" from the cluster manager.

> Maybe we want to introduce this functionality as a new wire protocoll,
> or only in proto C.

I see it controled by the 

"on-disconnect freeze_io;" option.

For N2 we need a "drbdadm fence-off r0" command and for N1 we need 
a "drbdadm resume-io r0".

* The fenced bit gets cleard when the resync is finished.
* A node refuses to become primary when the fenced bit is set.
* "drbdadm -- --do-what-I-say primary r0" overrules (and cleares?) 
  the fenced bit

To be defined: What should we do at node startup with the fenced bit.
               (At least display it at the user-dialog)

: Dipl-Ing Philipp Reisner                      Tel +43-1-8178292-50 :
: LINBIT Information Technologies GmbH          Fax +43-1-8178292-82 :
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