[Drbd-dev] How Locking in GFS works...

[Lars Ellenberg]

/ 2004-10-04 16:17:21 +0200
\ Philipp Reisner:
> On Monday 04 October 2004 16:09, Philipp Reisner wrote:
> > On Monday 04 October 2004 15:49, Lars Marowsky-Bree wrote:
> > > On 2004-10-04T15:26:15, Philipp Reisner <philipp.reisner at linbit.com> 
> wrote:
> > > > If everything works (esp. the locking of the shared disk fs) no.
> > > >
> > > > But just consider that the locking of the shared disk FS on
> > > > top of us is broken, and that it issues a write request to
> > > > the same block number on both nodes.
> > > >
> > > > Then each node would write its copy first and the peers
> > > > version of the data at second to that block number.
> > > >
> > > > => We would have different data in this block on our
> > > >    two copies. - And we would event know about it!
> > >
> > > You would know the moment the replicated write from the remote end came
> > > in, no?
> > >
> > > "Oh my, this is dirty locally too and unacked. We better arbitate now;
> > > ie one side wins and the other one is silently discarded."
> >
> > This is what I like about mailinglists. This is a new idea, that
> > certainly needs to be considered.
> >
> > Hmm, I just tooks a sheet of paper and drew a view diagrams of it.
> >
> > It works as long as writing the block takes longer than transmitting
> > the block.
> >
> > The scheme simply fails if transmitting takes longer than writing.
> >
> 
> No. It works... I will write a text describing it.

I think for two nodes (and drbd will stay that way for some time),
the easiest to implement would be "solution one" anyways.
but, I may be wrong. and, it involves additional latency,
even though it does not need an additional comm step (we can take the
write ack of one node as the "submit now locally" for the other.
or it involves one additional comm step (the extra "submit now" packet),
and still introduce additional latency.

but yes, I think a consistent arbitration
would do the trick much cheaper.

though for the (N>2)-node case I'd like to see your paper first ;)

	lge