direct links of old sources may well be a few messages off.
On Wed, Nov 30, 2011 at 2:08 AM, John Lauro <john.lauro at covenanteyes.com> wrote: > Hmm, I just reread that article... it sounds like it's just paranoid about split brain. > > Wouldn't protocol C cause the I/O (or at least write I/O) to fail if the link went down? or at least have that as an option of how to configure drbd? From the manual for protocol C: Synchronous replication protocol. Local write operations on the primary node are considered completed only after both the local and the remote disk write have been confirmed. So split brain shouldn't be possible if network loss between the two active nodes, or else protocol C is being violated. You should have to manually tell drbd that the other node is down. For a very, very, very odd definition of high availability. Node dies, and now we should have to intervene for the other node to take over? Protocol C replication is synchronous, _unless disconnected_. Please reread the documentation about disconnected mode: http://www.drbd.org/users-guide-8.3/s-node-failure.html Note that in a shared-nothing environment, and that's what DRBD is, any one node has no way to tell network failure from remote node failure -- all it can tell is that the peer no longer responds. Now, in dual-Primary mode disconnection _automatically_ and by definition means DRBD split brain. You get data divergence, your data sets are no longer synchronous, or in Lars' words, "it blows up in your face". Unless you freeze, at the moment of disconnection, all I/O on the device, and then evict one node from the cluster by a forced power-down. This is what the "fencing resource-and-stonith" option is intended for, and in combination with a fence-peer handler that would _immediately_ kick the node from the cluster would prevent this split brain from happening. Of course, it also amounts to a cluster shoot-out every time you have the slightest hiccup on your replication network. And that only applies to the DRBD side of things, but as Lars said, you'd also need a cluster aware iSCSI target to make the "multipath iSCSI target on dual-Primary DRBD" scenario work. Insanely, some people do this dual-Primary-with-iSCSI-target dance to _avoid_ what they think is the complexity of a cluster manager, and forgo the cluster configuration altogether. You can't do that. Nothing would be there to coordinate your fencing, if you did. Cheers, Florian -- Need help with DRBD? http://www.hastexo.com/knowledge/drbd