Re: [PATCH v3 0/4] dm-replicator: introduce new remote replication target

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Dec 09 2009 at  1:12pm -0500,
James Bottomley <James.Bottomley@HansenPartnership.com> wrote:

> On Wed, 2009-12-09 at 05:39 -0500, Christoph Hellwig wrote:
> > On Tue, Dec 08, 2009 at 12:42:27PM -0600, James Bottomley wrote:
> > > Could I just echo Lars' statement.  With the upstream inclusion of drbd,
> > > dm-replicator becomes a *third* replication system asking to be in
> > > kernel.  It is definitely a kernel policy question of whether we want
> > > three separate replicators, and so should be Cc'd to lkml so that people
> > > interested in that can weigh in.
> > 
> > And unliley the previous two this one actually offers the benefit of
> > beeing integrated with our major block device management framework.
> 
> md/nbd *is* integrated with a major block management framework.  The
> fact that it's md not dm reflects the fact that it leverages the md
> raid1 framework to perform the replication and merely uses nbd as a
> remote block transmission pipe.  I'd submit this is the correct way to
> do things.

Yes and no...

As someone who producticized md+nbd for a previous proprietary employer
(standing on the shoulders of the work that was done by steeleye) I can
say that md+nbd can provide the core plumbing but you need quite a bit
of higher-level tools integration to make it _really_ approachable for
the enterprise.  command line and UI interface and backend DB to store
all relations, etc... And all sorts of nasty corner cases (e.g. split
brain and double failure scenarios) are left as an exercise to the
md+nbd user.

> The problem now is that the md raid framework isn't integrated into dm,
> but I think someone else is looking at that ...
> 
> > Interesting that the question comes up now after I was shot down for it
> > in the drbd discussion.
> 
> So the value add of drbd over md/nbd is symmetric active.  I think that
> could be pulled into the md raid infrastructure as well, but someone has
> to figure out how.

md+nbd really isn't the way forward here IMHO..  if anything I think we
need to focus on melding drbd and dm-replicator into the DM framework.

A single system management tool-chain and interface is increasingly
important in the enterprise.

Mike