[Drbd-dev] Another drbd race

[Philipp Reisner]

> > 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.
>
> lets call it "Outdated"
>
> my idea is that a crashed Secondary will come up as !Primary|Connected, so
> it can assume it is outdated. (similar to the choice about wfc-degr...)
>
> we can only possibly lose write transaction in the very moment we
> promote a Secondary to Primary. until we do that, and the harddisk where
> the transactions have been written to is still physically intact, the
> data is still there, though maybe not available.
>
> we can try to make sure that we never promote a Secondary that possibly
> (or knowingly) is outdated.
>
> see below.
>

Let us assume that we have two boxes (N1 and N2) and that tese
two boxes are connected by two networks (net and cnet [ clinets'-net ]).

Net is used by DRBD, while heartbeat uses both, net and cnet

I know that you are talking about fencing by STONITH, but DRBD is
not limited to that. Here comes my understanding of how fencing
(other tan STONITH) could work with DRBD-0.8 :

 N1  net   N2
 P/S ---  S/P     everything up and running.
 P/? - -  S/?     network breaks ; N1 freezes IO
 P/? - -  S/?     N1 fences N2:
                  In the Stonith case: turn off N2.
                  In the "smart" case: 
                  N1 asks N2 to fence itself from the storage via cnet.
                  HB calls "drbdadm fence r0" on N2.
                  N2 replies to N1 that fencins is done via cnet.
                  N1 calls "drbdadm peer-dead r0".
 P/D - -  S/?     N1 thaws IO

N2 got the the "Outdated" flag set in its meta-data, by the "fence" 
command. I am not sure if it should be called "fence", other ideas:
"considered-dead","die","fence","outdate". What do you think ?

My question is:
 Is it planed that heartbeat will be able to perform this kind of fencing ?

-Philipp
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