[Drbd-dev] Problems with code to disallow connection when peer has mismatched UUID

[Graham, Simon]

Attached is a proposed patch to address this problem that does the
following:

1. When connection is lost, a new UUID is not created on the primary if
the local disk is inconsistent
2. Allow connection to be established if local disk is inconsistent even
if the remote is using
   a different UUID - our data is useless and cannot have been changed
anyway. The following handshake
   will cause the appropriate resync.

Although only one of these is actually required to fix my problem, I
thought it was better to be consistent
and change both.

Simon

> -----Original Message-----
> From: drbd-dev-bounces at linbit.com [mailto:drbd-dev-bounces at linbit.com]
> On Behalf Of Graham, Simon
> Sent: Thursday, September 11, 2008 6:33 AM
> To: drbd-dev at linbit.com
> Subject: RE: [Drbd-dev] Problems with code to disallow connection when
> peerhasmismatched UUID
> 
> Thanks Lars,
> 
> > > I'm not sure what the right answer is here, but I have a couple of
> > > observations:
> > > 1. When we lose connection and are Primary/Inconsistent, should we
> > > really create a new current UUID? The local data is bad.
> >
> > I don't think we should.
> 
> OK - I'll look at implementing this.
> 
> > that piece of code is to avoid "time warps" of content.
> > the specific scenario this should protect against is:
> >
> > Primary, Connected
> >  connection breaks.
> > Primary  ---?
> >  primary generates new "current uuid",
> >  and continues to write
> >  local disk breaks
> > Primary, Diskless, ---?
> >  connection heals
> >
> > now, if we allow this connection and accept the data of the
> secondary,
> > we just jumped back in time.
> 
> Ah OK -- would it perhaps then be OK to check for local disk state <=
> Negotiating instead of < Outdated:
> 
> 	if (mdev->state.conn < Connected &&
> 	    mdev->state.disk <= Negotiating &&
> 	    mdev->state.role == Primary &&
> 	    (mdev->ed_uuid & ~((u64)1)) != (p_uuid[Current] &
> ~((u64)1))) {
> 		ERR("Can only connect to data with current
> UUID=%016llX\n",
> 		    (unsigned long long)mdev->ed_uuid);
> 		drbd_force_state(mdev,NS(conn,Disconnecting));
> 		return FALSE;
> 	}
> 
> This would handle the case we see where we KNOW the local disk does
not
> have good data... (and we knew before the connection was lost, so we
> would NOT have actually been able to change the local copy of the data
> on disk).
> 
> Simon
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